Information

15.3: Flatworms, Nematodes, and Arthropods - Biology


The animal phyla of this and subsequent modules are triploblastic and have an embryonic mesoderm sandwiched between the ectoderm and endoderm. These phyla are also bilaterally symmetrical, meaning that a longitudinal section will divide them into right and left sides that are mirror images of each other. Associated with bilateralism is the beginning of cephalization, the evolution of a concentration of nervous tissues and sensory organs in the head of the organism, which is where the organism first encounters its environment.

The flatworms are acoelomate organisms that include free-living and parasitic forms. The nematodes, or roundworms, possess a pseudocoelom and consist of both free-living and parasitic forms. Finally, the arthropods, one of the most successful taxonomic groups on the planet, are coelomate organisms with a hard exoskeleton and jointed appendages. The nematodes and the arthropods belong to a clade with a common ancestor, called Ecdysozoa. The name comes from the word ecdysis, which refers to the periodic shedding, or molting, of the exoskeleton. The ecdysozoan phyla have a hard cuticle covering their bodies that must be periodically shed and replaced for them to increase in size.

Flatworms

The relationships among flatworms, or phylum Platyhelminthes, is being revised and the description here will follow the traditional groupings. Most flatworms are parasitic, including important parasites of humans. Flatworms have three embryonic germ layers that give rise to surfaces covering tissues, internal tissues, and the lining of the digestive system. The epidermal tissue is a single layer of cells or a layer of fused cells covering a layer of circular muscle above a layer of longitudinal muscle. The mesodermal tissues include support cells and secretory cells that secrete mucus and other materials to the surface. The flatworms are acoelomate, so their bodies contain no cavities or spaces between the outer surface and the inner digestive tract.

Physiological Processes of Flatworms

Free-living species of flatworms are predators or scavengers, whereas parasitic forms feed from the tissues of their hosts. Most flatworms have an incomplete digestive system with an opening, the “mouth,” that is also used to expel digestive system wastes. Some species also have an anal opening. The gut may be a simple sac or highly branched. Digestion is extracellular, with enzymes secreted into the space by cells lining the tract, and digested materials taken into the same cells by phagocytosis. One group, the cestodes, does not have a digestive system, because their parasitic lifestyle and the environment in which they live (suspended within the digestive cavity of their host) allows them to absorb nutrients directly across their body wall. Flatworms have an excretory system with a network of tubules throughout the body that open to the environment and nearby flame cells, whose cilia beat to direct waste fluids concentrated in the tubules out of the body. The system is responsible for regulation of dissolved salts and excretion of nitrogenous wastes. The nervous system consists of a pair of nerve cords running the length of the body with connections between them and a large ganglion or concentration of nerve cells at the anterior end of the worm; here, there may also be a concentration of photosensory and chemosensory cells (Figure 15.3.1).

Since there is no circulatory or respiratory system, gas and nutrient exchange is dependent on diffusion and intercellular junctions. This necessarily limits the thickness of the body in these organisms, constraining them to be “flat” worms. Most flatworm species are monoecious (hermaphroditic, possessing both sets of sex organs), and fertilization is typically internal. Asexual reproduction is common in some groups in which an entire organism can be regenerated from just a part of itself.

Diversity of Flatworms

Flatworms are traditionally divided into four classes: Turbellaria, Monogenea, Trematoda, and Cestoda (Figure 15.3.2). The turbellarians include mainly free-living marine species, although some species live in freshwater or moist terrestrial environments. The simple planarians found in freshwater ponds and aquaria are examples. The epidermal layer of the underside of turbellarians is ciliated, and this helps them move. Some turbellarians are capable of remarkable feats of regeneration in which they may regrow the body, even from a small fragment.

The monogeneans are external parasites mostly of fish with life cycles consisting of a free-swimming larva that attaches to a fish to begin transformation to the parasitic adult form. They have only one host during their life, typically of just one species. The worms may produce enzymes that digest the host tissues or graze on surface mucus and skin particles. Most monogeneans are hermaphroditic, but the sperm develop first, and it is typical for them to mate between individuals and not to self-fertilize.

The trematodes, or flukes, are internal parasites of mollusks and many other groups, including humans. Trematodes have complex life cycles that involve a primary host in which sexual reproduction occurs and one or more secondary hosts in which asexual reproduction occurs. The primary host is almost always a mollusk. Trematodes are responsible for serious human diseases including schistosomiasis, caused by a blood fluke (Schistosoma). The disease infects an estimated 200 million people in the tropics and leads to organ damage and chronic symptoms including fatigue. Infection occurs when a human enters the water, and a larva, released from the primary snail host, locates and penetrates the skin. The parasite infects various organs in the body and feeds on red blood cells before reproducing. Many of the eggs are released in feces and find their way into a waterway where they are able to reinfect the primary snail host.

The cestodes, or tapeworms, are also internal parasites, mainly of vertebrates. Tapeworms live in the intestinal tract of the primary host and remain fixed using a sucker on the anterior end, or scolex, of the tapeworm body. The remaining body of the tapeworm is made up of a long series of units called proglottids, each of which may contain an excretory system with flame cells, but will contain reproductive structures, both male and female. Tapeworms do not have a digestive system, they absorb nutrients from the food matter passing them in the host’s intestine. Proglottids are produced at the scolex and are pushed to the end of the tapeworm as new proglottids form, at which point, they are “mature” and all structures except fertilized eggs have degenerated. Most reproduction occurs by cross-fertilization. The proglottid detaches and is released in the feces of the host. The fertilized eggs are eaten by an intermediate host. The juvenile worms emerge and infect the intermediate host, taking up residence, usually in muscle tissue. When the muscle tissue is eaten by the primary host, the cycle is completed. There are several tapeworm parasites of humans that are acquired by eating uncooked or poorly cooked pork, beef, and fish.

Nematodes

The phylum Nematoda, or roundworms, includes more than 28,000 species with an estimated 16,000 parasitic species. The name Nematoda is derived from the Greek word “nemos,” which means “thread.” Nematodes are present in all habitats and are extremely common, although they are usually not visible (Figure 15.3.3).

Most nematodes look similar to each other: slender tubes, tapered at each end (Figure 15.3.3). Nematodes are pseudocoelomates and have a complete digestive system with a distinct mouth and anus.

The nematode body is encased in a cuticle, a flexible but tough exoskeleton, or external skeleton, which offers protection and support. The cuticle contains a carbohydrate-protein polymer called chitin. The cuticle also lines the pharynx and rectum. Although the exoskeleton provides protection, it restricts growth, and therefore must be continually shed and replaced as the animal increases in size.

A nematode’s mouth opens at the anterior end with three or six lips and, in some species, teeth in the form of cuticular extensions. There may also be a sharp stylet that can protrude from the mouth to stab prey or pierce plant or animal cells. The mouth leads to a muscular pharynx and intestine, leading to the rectum and anal opening at the posterior end.

Physiological Processes of Nematodes

In nematodes, the excretory system is not specialized. Nitrogenous wastes are removed by diffusion. In marine nematodes, regulation of water and salt is achieved by specialized glands that remove unwanted ions while maintaining internal body fluid concentrations.

Most nematodes have four nerve cords that run along the length of the body on the top, bottom, and sides. The nerve cords fuse in a ring around the pharynx, to form a head ganglion or “brain” of the worm, as well as at the posterior end to form the tail ganglion. Beneath the epidermis lies a layer of longitudinal muscles that permits only side-to-side, wave-like undulation of the body.

CONCEPT IN ACTION

View this video to see nematodes move about and feed on bacteria.

Nematodes employ a diversity of sexual reproductive strategies depending on the species; they may be monoecious, dioecious (separate sexes), or may reproduce asexually by parthenogenesis. Caenorhabditis elegans is nearly unique among animals in having both self-fertilizing hermaphrodites and a male sex that can mate with the hermaphrodite.

Arthropoda

The name “arthropoda” means “jointed legs,” which aptly describes each of the enormous number of species belonging to this phylum. Arthropoda dominate the animal kingdom with an estimated 85 percent of known species, with many still undiscovered or undescribed. The principal characteristics of all the animals in this phylum are functional segmentation of the body and the presence of jointed appendages (Figure 15.3.4). As members of Ecdysozoa, arthropods also have an exoskeleton made principally of chitin. Arthropoda is the largest phylum in the animal world in terms of numbers of species, and insects form the single largest group within this phylum. Arthropods are true coelomate animals and exhibit prostostomic development.

Physiological Processes of Arthropods

A unique feature of arthropods is the presence of a segmented body with fusion of certain sets of segments to give rise to functional segments. Fused segments may form a head, thorax, and abdomen, or a cephalothorax and abdomen, or a head and trunk. The coelom takes the form of a hemocoel (or blood cavity). The open circulatory system, in which blood bathes the internal organs rather than circulating in vessels, is regulated by a two-chambered heart. Respiratory systems vary, depending on the group of arthropod: Insects and myriapods use a series of tubes (tracheae) that branch throughout the body, open to the outside through openings called spiracles, and perform gas exchange directly between the cells and air in the tracheae. Aquatic crustaceans use gills, arachnids employ “book lungs,” and aquatic chelicerates use “book gills.” The book lungs of arachnids are internal stacks of alternating air pockets and hemocoel tissue shaped like the pages of a book. The book gills of crustaceans are external structures similar to book lungs with stacks of leaf-like structures that exchange gases with the surrounding water (Figure 15.3.5).

Arthropod Diversity

Phylum Arthropoda includes animals that have been successful in colonizing terrestrial, aquatic, and aerial habitats. The phylum is further classified into five subphyla: Trilobitomorpha (trilobites), Hexapoda (insects and relatives), Myriapoda (millipedes, centipedes, and relatives), Crustacea (crabs, lobsters, crayfish, isopods, barnacles, and some zooplankton), and Chelicerata (horseshoe crabs, arachnids, scorpions, and daddy longlegs). Trilobites are an extinct group of arthropods found from the Cambrian period (540–490 million years ago) until they became extinct in the Permian (300–251 million years ago) that are probably most closely related to the Chelicerata. The 17,000 described species have been identified from fossils (Figure 15.3.4).

The Hexapoda have six legs (three pairs) as their name suggests. Hexapod segments are fused into a head, thorax, and abdomen (Figure 15.3.6). The thorax bears the wings and three pairs of legs. The insects we encounter on a daily basis—such as ants, cockroaches, butterflies, and bees—are examples of Hexapoda.

Subphylum Myriapoda includes arthropods with legs that may vary in number from 10 to 750. This subphylum includes 13,000 species; the most commonly found examples are millipedes and centipedes. All myriapods are terrestrial animals and prefer a humid environment (Figure 15.3.7).

Crustaceans, such as shrimp, lobsters, crabs, and crayfish, are the dominant aquatic arthropods. A few crustaceans are terrestrial species like the pill bugs or sow bugs. The number of described crustacean species stands at about 47,000.1

Although the basic body plan in crustaceans is similar to the Hexapoda—head, thorax, and abdomen—the head and thorax may be fused in some species to form a cephalothorax, which is covered by a plate called the carapace (Figure 15.3.8). The exoskeleton of many species is also infused with calcium carbonate, which makes it even stronger than in other arthropods. Crustaceans have an open circulatory system in which blood is pumped into the hemocoel by the dorsal heart. Most crustaceans typically have separate sexes, but some, like barnacles, may be hermaphroditic. Serial hermaphroditism, in which the gonad can switch from producing sperm to ova, is also found in some crustacean species. Larval stages are seen in the early development of many crustaceans. Most crustaceans are carnivorous, but detritivores and filter feeders are also common.

Subphylum Chelicerata includes animals such as spiders, scorpions, horseshoe crabs, and sea spiders. This subphylum is predominantly terrestrial, although some marine species also exist. An estimated 103,0002 described species are included in subphylum Chelicerata.

The body of chelicerates may be divided into two parts and a distinct “head” is not always discernible. The phylum derives its name from the first pair of appendages: the chelicerae(Figure 15.3.9a), which are specialized mouthparts. The chelicerae are mostly used for feeding, but in spiders, they are typically modified to inject venom into their prey (Figure 15.3.9b). As in other members of Arthropoda, chelicerates also utilize an open circulatory system, with a tube-like heart that pumps blood into the large hemocoel that bathes the internal organs. Aquatic chelicerates utilize gill respiration, whereas terrestrial species use either tracheae or book lungs for gaseous exchange.

CONCEPT IN ACTION

Click through this lesson on arthropods to explore interactive habitat maps and more.

Section Summary

Flatworms are acoelomate, triploblastic animals. There are four traditional classes of flatworms, the largely free-living turbellarians, the ectoparasitic monogeneans, and the endoparasitic trematodes and cestodes. Trematodes have complex life cycles involving a secondary mollusk host and a primary host in which sexual reproduction takes place. Cestodes, or tapeworms, infect the digestive systems of primary vertebrate hosts.

Nematodes are pseudocoelomate members of the clade Ecdysozoa. They have a complete digestive system and a pseudocoelomic body cavity. This phylum includes free-living as well as parasitic organisms. They include dioecious and hermaphroditic species. Nematodes have a poorly developed excretory system. Embryonic development is external and proceeds through larval stages separated by molts.

Arthropods represent the most successful phylum of animals on Earth, in terms of number of species as well as the number of individuals. They are characterized by a segmented body and jointed appendages. In the basic body plan, a pair of appendages is present per body segment. Within the phylum, classification is based on mouthparts, number of appendages, and modifications of appendages. Arthropods bear a chitinous exoskeleton. Gills, tracheae, and book lungs facilitate respiration. Embryonic development may include multiple larval stages.

Review Questions

Which group of flatworms are primarily external parasites of fish?

  1. monogeneans
  2. trematodes
  3. cestodes
  4. turbellarians

A

Crustaceans are _____.

  1. ecdysozoans
  2. nematodes
  3. arachnids
  4. parazoans

A

Free Response

Speculate as to what advantage(s) a complete digestive system has over an incomplete digestive system?

In a complete digestive system, food material is not mixed with waste material, so the digestion and uptake of nutrients can be more efficient. In addition, the complete digestive system allows for an orderly progression of digestion of food matter and the specialization of different zones of the digestive tract.

Describe a potential advantage and disadvantage of the cuticle of ecdysozoans.

An advantage is that it is a tough covering that is protective against adverse environments, and predators and parasites. A disadvantage is that it must be shed and regrown for the animal to grow, which requires energy and makes the animal vulnerable during this process.

Footnotes

  1. 1 “Number of Living Species in Australia and the World,” A.D. Chapman, Australia Biodiversity Information Services, last modified August 26, 2010, http://www.environment.gov.au/biodiv...c-summary.html.
  2. 2 “Number of Living Species in Australia and the World,” A.D. Chapman, Australia Biodiversity Information Services, last modified August 26, 2010, http://www.environment.gov.au/biodiv...c-summary.html.

Glossary

Arthropoda
a phylum of Ecdysozoa with jointed appendages and segmented bodies
cephalothorax
a fused head and thorax
chelicerae
a modified first pair of appendages in subphylum Chelicerata
chitin
a tough nitrogen-containing polysaccharide found in the cuticles of arthropods and the cell walls of fungi
complete digestive system
a digestive system that opens at one end, the mouth, and exits at the other end, the anus, and through which food normally moves in one direction
dioecious
having separate male and female sexes
hemocoel
the internal body cavity seen in arthropods
Nematoda
a phylum of worms in Ecdysozoa commonly called roundworms containing both free-living and parasitic forms
spiracle
a respiratory openings in insects that allow air into the tracheae

Flatworms, Nematodes, and Arthropods

The animal phyla of this and subsequent modules are triploblastic and have an embryonic mesoderm sandwiched between the ectoderm and endoderm. These phyla are also bilaterally symmetrical, meaning that a longitudinal section will divide them into right and left sides that are mirror images of each other. Associated with bilateralism is the beginning of cephalization, the evolution of a concentration of nervous tissues and sensory organs in the head of the organism, which is where the organism first encounters its environment.

The flatworms are acoelomate organisms that include free-living and parasitic forms. The nematodes, or roundworms, possess a pseudocoelom and consist of both free-living and parasitic forms. Finally, the arthropods, one of the most successful taxonomic groups on the planet, are coelomate organisms with a hard exoskeleton and jointed appendages. The nematodes and the arthropods belong to a clade with a common ancestor, called Ecdysozoa. The name comes from the word ecdysis, which refers to the periodic shedding, or molting, of the exoskeleton. The ecdysozoan phyla have a hard cuticle covering their bodies that must be periodically shed and replaced for them to increase in size.


Arthropods: Body Cavity, Digestive System and Life History

In this article we will discuss about Arthropods:- 1. Integumentary System of Arthropods 2. Muscular System of Arthropods 3. Body Cavity 4. Digestive System 5. Circulatory System 6. Nervous System 7. Reproductive System 8. Life Cycle.

Integumentary System of Arthropods:

In all arthropods, the integument consists of:

(i) An innermost extremely thin stellate cell layer, called basement membrane,

(ii) A monolayer of closely packed hexagonal cells, hypodermis (epidermis) and

(iii) Outer non- cellular layer, cuticle.

The cuticle is secreted by the hypodermis and excepting the re­gions of joints it is many-layered. The cuticle also lines the inner wall of foregut, hindgut, trachea and genital atrium.

The cuticle con­sists of two layers—outer epicuticle and inner procuticle. The cuticle is extremely thin and usually does not contain chitin (exceptions are a few Centipeds and Pycnogonids).

Specially in insects, it has been seen to contain wax, lipids, proteins and steroids. The wax and lipids make it impermeable to water. The procuticle con­tains chitin, a special kind of polysaccharide and is divisible into two layers—exocuticle and endocuticle.

The outer exocuticle is a tough layer and the inner endocuticle is many-layered and flexible. In Crustacea and Diplopods, various calcareous substances are seen to be deposited in the exocuticle. The different colouration of the pigments is due to the presence of pigment cells in the hypo­dermis.

The outer part of the integument may have various striations and markings. Its out-pushings may form spiny structures and in-pushings give rise to apodemes for the attachment of muscles.

Muscular System of Arthropods:

In arthropods, the muscles are striated. In the thorax are longitudinal muscles are present as a pair of dorsal and a pair of ventral bundles. Each joint is provided with two sets of muscles, one of which is antago­nistic to the others.

Some of these somatic muscles can work at astounding speed, e.g., wing muscles of insects, muscles operating the stridulating organs in various arthro­pods. The splanchnic muscles are present around the gut, heart, aorta, diaphragm, etc. These muscles are arranged either as layers of longitudinal and circular muscles or as myofibrilar network.

Body Cavity of Arthropods:

In Arthropoda, true coelom appears as pouches in the embryonic stage In course of development its walls are used up in the formation of organs and the space becomes continuous with the blastocoel. It is called mixocoel and as blood flows through it, this is also referred to as haemocoel.

In Crustacea true coelom is restricted to the space of ophthalmic artery, within excretory and re­productive parts. Almost similar condition is found in Onychophores, where true coelom is restricted only around the excretory and reproductive parts. But in Myriapods and Insects, the coelomic spaces, are retained only in reproductive parts.

Digestive System of Arthropods:

The digestive system is concerned with nutrition. The process primarily involves three phases—ingestion, digestion and egestion. As arthropods live in varied habitats, they carry out these phases in different ways. Each group has developed the structures perfectly suited to its particular way of life.

The digestive system includes:

The digestive system is absent in certain adult insects, e.g., Mayflies and much modified in a few parasitic crustaceans like Sacculina.

(1) Alimentary Canal of Arthropods:

In general the alimentary canal is divisible into three parts:

The structure varies in different arthropods (Fig. 18.126), but in all the fore and hind guts are lined internally by cuticle.

In parasitic Crustaceans, spe­cially in endoparasites the alimentary canal shows marked degeneration. But in free-liv­ing forms it extends along the entire length of the body. In most Crustaceans, the foregut includes mouth, gullet and oesophagus. But in Malacostraca, the next part, stomach is also included within foregut.

The mouth in gen­eral is ventrally placed and some distance away from the anterior end. The gullet is vertical and the stomach is more or less sac-like. In most Crustaceans, the midgut is straight and dorsally placed.

The intestine is coiled in Cladocera. Near the posterior end of the mid­gut in Amphipoda, single or paired caeca are present. The hindgut contains a bulb-like rec­tum which opens to the exterior through a posterior terminal aperture, called anus.

Length of the foregut varies in different Chilopods. In Diplopods, a pre- oral cavity is present in front of the mouth and it leads to a pharynx of varied length. The lining of midgut in the same group contains both secretory and absorptive cells. The hindgut is considerably long and in some (Julidae) is subdivided into three parts.

The last part is usually sac-like and eversible. In the Pauropoda, the foregut is contractile and the midgut begins from third segment of the trunk. The hindgut is divisible into a tubular part and a sac-like part. In Symphyla the hindgut is divisible into four parts.

Mouth is placed at the ventral and terminal end of the head. Mouth is bounded by mouth parts which differ ac­cording to the food habit of the insect. The other parts of the foregut include a well- developed pharynx, bag-like crop and a mus­cular gizzard or proventriculus. The gizzard is prominent in Coleoptera and ants. In honey-bee, it acts as a honey stomach.

The midgut varies widely in insects. The anterior end of the midgut in most insects gives rise to diverticulum or caecum. The caecum is absent in Lepidoptera and Collembola. In some Diptera the midgut is long, coiled and divisible into an anterior digestive part and a posterior absorptive part. In Heteroptera, the anterior part is sac-like and known as stom­ach, while the much coiled posterior part is called intestine.

In Homoptera, the foregut and hindgut join with each other and the midgut is set aside as a loop. In Coccidae the last part of the foregut and the first part of the midgut remain inside the hindgut.

In most insects, a constriction separates the midgut from hindgut and the latter is divis­ible into a slender anterior part and broad posterior part. In some Coleoptera and ter­mites the hindgut is sac-like and contains cellulose-splitting bacteria.

The position of mouth var­ies. In Xiphosurids, the slit-like mouth lies in between the second and fifth gnathobases. A pre-oral cavity is present in front of the mouth of Scorpionids and Uropygi. In Palpigradi, mouth is present on the segment which bears pedipalp. In Ricinulei, the pre- oral cavity is covered anteriorly by a flap-­like projection of carapace, called cucullus.

A projected rostrum in Solifugae bears the mouth at its tip. Usually the pharynx and in some cases (spider) the stomach is suctorial. The midgut sends paired and much branched diverticula in all chelicerates, where both digestion and absorption take place. Only unbranched diverticula are seen in Opiliones, Ricinulei and Acari.

In Xiphosurids the di­verticula fill up the prosoma but in Arach­nids it is restricted only to the abdomen (excepting scorpion). In Scorpionids, the hindgut is the straight continuation of the midgut and is called rectum. In Xiphosura, the short tubular rectum has folded walls. In spider and Pseudoscorpionids, dorsal diver­ticula are given out from the rectum.

(2) Digestive Glands of Arthropods:

In Crustacea, the most important diges­tive gland is hepatopancreas. It contains two kinds of cells—hepatic and pancreatic. The gland is formed by numerous finger-like tubules.

In prawns and crabs, the hepatopancreas is placed within the cephalothoracic cavity. But in Amphipoda and Isopoda it extends within the abdomen. In Stomatopoda, these digestive glands are ar­ranged in ten metameric pairs. Salivary glands are known in certain forms.

Among the Myriapods, four pairs of sali­vary glands are seen in Chilopoda and Diplopoda. But in Pauropoda the number are reduced to two pairs and in Symphyla there are only one pair of large salivery glands. In addition to the salivary glands, the cells present in the lining of midgut are also responsible for producing digestive juices.

In Insecta, three sets of glands—labial glands, maxillary glands and mandibular glands are often referred together as salivary glands, which secrete digestive juices. Labial glands are slender, elongated tubes in muscoid flies and paired sac-like structures with lining of secretory cells in mosquito. Labial glands of larval Lepidoptera work as silk glands.

The maxillary glands are func­tional in the adult Protura and Collembola. Mandibular glands are present in most Apterygota and in Dictyoptera, Isoptera, Trichoptera and Hymenoptera. Of these three kinds of salivary glands, usually only one kind is functional and other two are degen­erated. But it may be that two or all the three sets continue to be functional.

In honey-bee the labial glands work as wax glands, and the mandibular glands produce secretion to soften the pupal case. True salivary glands originating from the pharyngeal system open within the pharynx. The saliva, in addition to its enzymes, often contains anticoagulant. The digestive juices are also produced from the lining of the midgut.

In Chelicerates, the salivary function is carried by a pair of rostral glands and a pair of maxillary glands. The diverticula of the midgut produce digestive juices.

Mechanism of Digestive System:

The Crustaceans exhibit an evolutionary trend in the food-getting devices. Primitive crustaceans (Cephalocarida) use their indentical appendages for locomotion and also for filtering food particles from the sur­rounding water. But in advanced groups the appendages are differentiated to capture food.

The best example is prawn, where maxillae and maxillipeds, while producing water current for respiration, assist the food to enter into the mouth. Mandibles cut the food into pieces. Some of the walking legs being chelated can grab the food. The stomach is also modified for crushing the food and also to digest it.

The Myriapods have powerful mandibles for capturing and cutting the food. In this group, the hindgut exhibits special structural changes for preventing any loss of water. The organisation of mouth parts in Insects speaks about the advancement of this group over others regarding food procure­ment.

The intestine and diverticula perform the breakdown and absorption of food effi­ciently to meet the excess demand of energy. Among Chelicerates only the Xiphosurids are capable of ingesting solid food. The Arach­nids have devices by which the prey is pre-digested either by injecting enzymes or by taking it in a special pre-oral cavity. The partly digested liquid food is sucked inside the alimentary canal.

Circulatory System of Arthropods:

The circulatory system is primarily con­cerned with the distribution of metabolic substances and respiratory gases. In all Ar­thropods, excepting Insects, the circulatory system performs this dual role.

In Insects, the circulatory system is free from the bur­den of carrying respiratory gases. This sys­tem includes, blood, blood vessels and a pumping organ, the heart. The circulatory system in Arthropods shows a trend of tran­sition from primitiveness to specialisation.

In Crustaceans, the blood contains a fluid part, plasma, and a few colourless amoeboid cells. A copper-containing pigment, haemocyanin, is present in the plasma of Decapods to render a blue colour to the blood. In Crustaceans like Triops and Cypris, the colouring pigment is haemoglobin which makes the colour of their blood red.

Among the Myriapods, the Chilopods have colour­less blood and in Lithobius the blood is violet. In Insects, the blood is usually colourless. In leaf-eating Insects, the blood is green and in some other forms it may be brownish or yellowish. In Chironomous larvae, the blood is red. In Arachnids, the presence of haemo­globin in blood plasma has given red colour to the blood.

In general, the blood flows through coelomic spaces, called haemocoel, but in addition there are vessels with definite walls in Crustacea and others. Such vessels are called the arteries. In Arachnida, there are both arteries and veins with definite walls.

Certain Arthropods, like Cirripeds, Ostracods, Copepods and Pauropods do not possess any heart. When present, the heart is always dorsal. The primi­tive heart is tubular and extends along the entire length of the body. In other forms various types of shortening, thickening and compartmentalisation are noted. Among the Crustaceans, the primitive type of elongated tubular heart is noted in Branchiopoda and Anostraca.

In each segment it communicates through a pair of ostia. More or less similar condition is seen in Leptostraca, Stomato­poda, Isopoda and Amphipoda. The heart is short and sac-like in Cladocera and Decapoda and it has only a few pairs of ostia. In Myriapoda, the elongated tubular heart is internally divided into several chambers.

In Scolopendra, it is enclosed within a cardiac diaphragm. The wall of the diaphragm is attached with the body wall and special sets of muscle, called alary muscle, remain asso­ciated with it. In Insecta, the tubular heart is generally confined to the abdomen but in many instances it extends up to the thorax. The heart of cockroach is composed of thir­teen chambers.

The inner chambers of the heart are interconnected by openings which are guarded by valves. The heart opens into the pericardial cavity by several ostia and the wall of the heart remains attached with the body wall by alary muscles.

In Arach­nida, the length of heart and the number of chambers vary. For example, the heart of scorpion has seven chambers and the spider possesses only three. In Xiphosurids, the heart has eight chambers.

Nervous System of Arthropods:

The nervous system includes:

(b) Peripheral nervous system and

Some higher Crustaceans and Insects possess a sympa­thetic nerve cord which begins from the central nervous system and extends along the wall of the alimentary canal. In spite of tremendous diversity, the nervous system in Arthropoda is built up on a basic plan which includes a pair of ganglia per metamere.

These ganglia are interconnected by nerve cords and peripheral nerves are given out to the particular segment. This basic plan has been modified in various Arthropods, ac­cording to the modification of their body and in many instances fusion of the ganglia have taken place (Fig. 18.130).

(a) Central and Peripheral Nervous Systems:

Within Crustacea, the Branchiopods exhibit primitive type of nervous system which re­sembles that of nervous system which re­sembles that of Annelids. Here brain is formed by the fusion of two pairs of ganglia representing the segments bearing eyes and antennules respectively.

These two pairs are known as protocerebrum and deuterocerebrum. From brain arises paired circum oesophageal connectives which come in contact with paired ventral nerve cords.

The nerve cords run posteriorly and carry a pair of ganglia in each segment. The anterior-most pair of ganglia is known as sub-oesophageal ganglia. But in other Crustaceans the ganglia of the antennal segment also fuse with the brain and form its third lobe, called tritocerebrum.

In some Crustacea, paired ganglia of each segment are fused in others the ventral nerve cord is reduced. In the case of prawn, the ganglia of the thoracic seg­ments are fused to form a single ganglionic mass and in crab (Carcinus) entire ventral nerve cord is fused.

Its segmental ganglia are united to form a large ventral ganglionated mass. Similar reduction is also seen in Cirripeds. Such reduction in Cirripeds is possibly due to their parasitic existence.

In Myriapods, the ventral nerve cord exhibits its double nature and this is distinct specially among the Chilopods. Three gan­glia on the ventral nerve cord corresponding to the first three trunk segments unite to form a sub- or infra- oesophageal mass.

In Insects, the usual pattern is that, after suboesophageal mass, each segment of the thorax and abdomen has a pair of ganglia on the ventral nerve cord. But in Nepa and Acanthia, the first thoracic ganglia are fused with the suboesophageal.

In Gyrinus, all the abdomi­nal ganglia are fused and in Lachnosterna the abdominal ganglia are fused with the gan­glia of meso- and metathorax. The best ex­amples of condensation are seen in Diptera. In Sarcophaga, all the thoracic and abdominal ganglia are fused together. Such condensa­tion includes even the suboesophageal gan­glia in the parasitic Diptera, Pupipara.

Among the Chelicerates the nervous sys­tem shows different grades of fusion, in Xiphosurids, suboesophageal ganglia remain fused with the ganglia belonging to the segments second to eighth. The ventral nerve cords bear four ganglia and the last ganglia are formed by the fusion of ganglia belong­ing to the last three segments.

In Scorpionids, the thoracic ganglia are fused with the suboesophageal ganglionic mass but most of the abdominal ganglia are distinct. In Araneida, all the ganglia are fused into a mass, which is piereced by the oesophagus.

The possession of well- developed sense organs has played impor­tant role in the success of arthropods. Start­ing from setae and bristles there are ex­tremely specialised sense organs like com­pound eyes (Fig. 18.131) to act as ports of entry of different stimuli.

Crustacean Sense Organs:

The important sense organs are eyes, statocysts and olfactory setae. In addition, there are various sensory hairs and bristles.

The eye may be single or paired. In Ostracoda, single median eye is placed on the anterior end. Similar median eyes on the dorsal side are seen in free-swimming Copepods. The eyes are ab­sent in parasitic Copepods and degenerated in Cirripedia and Rhizocephala. The paired eyes are generally mounted on stalks but may be sessile as in Cumacia, Tanaidacea and Amphipoda.

In many Branchiopods, the eyes remain within a fold of epidermis. The eye stalk is usually jointed but in many Branchiopods the stalk is unjointed. In Cladocera, the paired eyes are fused. The unpaired median eye is known as nauplius eye and is simple in design. But the com­pound eye contains numerous visual ele­ments, called ommatidia (sing, ommatidium). The structure of an ommatidium is same as in prawn.

The balancing or­gans or statocysts are present in the antennule. In Mysidacea, the balancing or­gans are present in the uropods. In the land- living Crustaceans (including true crabs and hermit crabs), the statocysts are also respon­sible for receiving vibrations.

These setae work for receiving smell and are distributed over the antennae.

Myriapod Sense Organs:

The sense or­gans are generally seen as sensory hairs, eyes and other specialised organs.

These hairs either remain scattered all over the body or are found, in specialised groups, in certain regions of the body. In Pauropoda, five pairs of tactile hairs are arranged along the sides of the tergal plates of second to sixth segments.

In between eyes and antennae of Diplopods, a small pit contains projectile hairs. Its gnathochilarium (formed by the fusion of antennae, maxillae and mandibles) carries tufts of sensory hairs. In Diplopoda, sensory hairs and sensory spines are seen in different parts of the body.

Eyes are absent in Symphyla (excepting a few cases, where only one pair is present). An eye-like surface is visible in Pauropoda, but true eye is not present. Several simple eyes are clumped together in Diplopoda. In others the eyes, when present, are usually simple. In Scutigera, a kind of pseudocom-pound eye is present.

(c) Specialised sensory organs:

It is present on the ventral branch of Pauropoda.

(ii) Organs of Tomosvary:

In Diplopoda, the head bears a pair of small projections. It is dressed externally with fine hairs and inside the projection, the hypodermal cells are provided with nerves to receive sounds.

The inner side of the base of the first maxilla possesses a pit with profuse lining of setae.

In Symphyla, last ten pairs of legs have wart-like stumpy processes, called parapodia. Each parapodium contains a sac having sensory function.

Following sense organs are usually seen in insects:

(iii) Organs for touch and taste,

The eyes of insects may be com­pound or simple. The compound eyes are without stalks and are spread on the mar­ginal part of the head. Many subterranean insects are completely blind. The number of simple eyes varies widely. These are absent in Dermaptera and in some Hemiptera. When present, the simple eyes are grouped along the sides of the head.

The tip of each antenna bears hairs which are embedded in pits and act as the sense organ for smell.

(iii) Organs for touch and taste:

Numer­ous -hairs with nerve connections protrude throughout the surface of the body for get­ting stimuli in the form of touch. Specially the hairs around palp serve dual functions of touch and taste. The hairs present around mouth parts are probably for the function of taste.

(iv) Organs for receiving sound:

Some insects (male Culicids) have specialised hairs over the surface of the antennae for detecting sound. A special organ, called tympanum, is seen in several insects (in the anterior seg­ments of Acridiidae, in anterior legs of Locustidae). This organ has a fluid-filled vesicle inside to act as membranous laby­rinth. The organ has connections with nerves supplied from third thoracic ganglion.

These specia­lised sense organs are located in different parts of the body specially in legs and abdo­men. Each organ consists of a pack of sen­sory cells and accessory structures, called scolopales.

These may remain tightly stretched and attached on both the ends of hypodermis or only one end remains at­tached. They perform various functions which include regulation of leg movement (Proprioceptive organ) and also reception of sound waves.

(v) Chelicerate sense organs:

Following sense organs are seen in different Chelicerates— eyes, sensory setae, trichobothria, slit sense organs, frontal organs and pectines.

In Eurypteridae, the cephalothorax bears a pair of large eyes and a pair of small eyes. In Limulus, similar disposition of eyes is marked but the median eyes are com­pound in nature. Scorpions have a pair of large centrally placed eyes on the cephalo­thorax and several pairs of small eyes along the anterolateral margin.

The median eyes are intermediate between simple and com­pound eyes. Each eye has a single cuticular lens like that of a simple eye but retinal cells are disposed like compound eyes.

In the Pedipalpida, two median eyes are large and six small eyes are placed along the margin. The arrangements of 6-8 simple eyes in spiders vary widely and serve as the criteria in classification. The Solifugae and Phalangida possess a pair of eyes on the head and the Acarids are without eyes.

All over the cuticle, specially on the masticating processes of the thoracic limbs, numerous hair-like structures are present. These setae are provided with the branches of nerves and are sensory in function.

These flask-shaped sense organs with a mobile seta are arranged on each chela in different planes. These are responsible for determining air current.

These slit-like sense organs are distributed all over the body, specially over the appendages. Each minute slit is covered by a membrane and leads to a crevice. Inner end of the crevice leads to a membrane-lined tube which is internally supplied by numerous nerve fibres. These sense organs co-ordinate joint movement and work as vibration receptors.

These specialised sense or­gans are found in Scorpion. Males with the help of these sense organs detect suitable surface for depositing spermatophores and the females use these for collecting spermatophores.

These specialised sense organs in the form of hairy areas are seen in Xiphosurids. These organs act as photoreceptors in larva but their function is not known in adult.

Reproductive System of Arthropods:

The secret of success of the Arthropods as a phylum lies in its prolific rate of multipli­cation. This is done by efficient reproductive organs and effective reproductive behaviour. Majority of the arthropods are unisexual. A good number of hermaphrodites are seen in all the classes excepting’ Arachnida.

Each reproductive duct is a modified coelomoduct and reproductive gland or gonad opens into it. The position of the reproductive organ varies as also the sites of reproductive open­ings. Majority of the arthropods are ovipa­rous and it is curious enough that in most of them there are certain devices for internal fertilization. Some forms of viviparity are also seen, but only in Onychophores, true viviparous condition is found.

Modification of different classes:

Free-living Crustaceans are generally unisexual. But in Cirripeds, para­sitic Isopods and in a few other species, hermaphroditism is encountered. Such her­maphrodites are called protandrous because in them male reproductive organs appear first and then the female organs. Though males often possess well-developed append­ages, yet generally they are smaller in size than females.

In some forms of parasitic Crustaceans, males are extremely minute and cling to the body of the females. Such males are called complemental males. In many Crustaceans, males may have modified ap­pendages to act as clasping or intromittent organs. The intromittent organs (structures for transferring sperms to the female body) may also be formed by the modification of the protrusible terminal part of the vasa deferentia.

The reproductive organs in both the sexes are hollow and are usually united either completely or incompletely above the ali­mentary canal.

In all the Crustaceans (excepting Cirripedia, Malacostraca and some Cladocera) both the sexes have reproductive openings in the same segment. The genital apertures in most Crustaceans are placed near the posterior end of the thorax. Both in Cirripeds and in some Cladocera the male apertures are terminal and the female open­ing in Cirripeds is at the first thoracic seg­ment.

The sperm cells may be of varied forms—Polyphemus (Cladocera)—amoe­boid Copepods—sausage-shaped Decapods, Euphausids and Stomatopods—spherical and with rigid radial processes Isopods and Amphipods—thread-like Ostracods— sperms are many times larger than the body of the individual.

Round eggs may have varied concentra­tions of yolk. Usually the female carries the eggs after fertilization through some devices but in some cases the eggs may remain uncared.

The reproductive organs are unpaired. In Centipeds, genital aperture is placed near the posterior-most end of the body. But in Millipeds the apertures are not far away from the head. Some Myriapods protect their eggs up to certain period after lying.

All insects are unisexual except­ing Icerya purchasi, which is hermaphrodite and practises self-fertilization.

The testes are generally small, paired and sometimes follicular. Number of follicles varies from one in Diptera to many in Orthoptera. A duct, called the vas efferens, connects the follicles. Thread-like sperms are usually packed as spermatophores.

The ovary is made up of ovarioles. The number of ovarioles is usually six to eight but in female Termite it is 1500, in queen bee several hundreds, but in Tsetse fly only one.

Usually two oviducts unite to form a single duct but in Ephemidae and Lepisma two oviducts open separately.

The genital openings are placed in the ninth and tenth abdominal segments and have in many cases copulatory structures.

The egg is usually ladden with yolk, excepting parasitic Hymenoptera and has a covering of vitelline membrane and rigid chorion. Several openings are present on the chorion for sperm entrance.

Copulation usu­ally takes place long before fertilization and sperms are kept within the spermathecae of the female body. Fertilization occurs at the time of egg laying and thus is under volun­tary control of the female. For the laying of eggs many insects are provided with special structures which are generally used for dig­ging.

The number of eggs laid varies in differ­ent insects. Termites, ants and bees lay a few thousands of egg at a time. But in Tsetse fly, one egg is released every 9-10 days. Here fertilization and major part of the develop­ment take place within a special chamber, called ‘uterus’. Many insects lay their eggs directly within the body of some other insects.

Excepting spiders and cer­tain mites, no sexual dimorphism is noted in Arachnida. In Scorpion both the processes, fertilization and development are internal and some form of courtship is noted. The eggs are small and are without yolk. Similar viviparous developments are also noted in Pedipalpi and mites.

In Limulus, fertilization is external and occurs on land.

In Araneida, in both the sexes the open­ings are present on the middle line of the epigastric furrow. A special structure, called epigynum is associated with the genital open­ing of the female. The epigynum is simplest in Pirata and modified for egg laying in Aranaea angulate.

Male Araneids have pedipalps modified to act as intromittent organs. In females a single oviduct connects both the ovaries. In both the sexes of Chelicerates, single genital opening serves as the outlet, except in Limulus where it is paired.

Life History of Arthropods:

Though sexual reproduction is a pre­requisite for the initiation of development in Arthropods yet instances of parthenogenesis are plenty. Among the Crustaceans, Branchiopoda and Ostracoda usually develop parthenogenetically. In Triops, sexual reproduction is restricted only at a particular time of the year and during the rest of the time development takes place parthenogenetically.

Among the Insects parthenogenesis is com­mon in aphids and certain members of Hy­menoptera. The common black wasp usually develops parthenogenetically, because male varieties are extremely rare in nature. The larvae of a Dipteran insect, Master can pro­duce eggs which develop parthenogenetically.

Usually one embryo develops from one egg but in parasitic Hymenoptera belonging to the family Chlacididae, one egg splits into several hundred bits, each of which forms a complete embryo. This phenomenon of polyembryony is extremely interesting from the point of view of embryology, because it re­sults in the production of identical twins (here, of course, identical hundreds) having similar genetic make-up.

A general survey of life history in the different groups of Arthropods reveals the existence of three categories of development:

From the egg hatches out an individual, which resembles the adult in all respects except the size.

(2) Incomplete metamorphosis:

The young resembles the adult but many adult structures are lacking. Such structures ap­pear later in course of further development.

(3) Complete metamorphosis:

The young which comes out of the egg has no sem­blance with the adult and it lives an inde­pendent and completely different sort of life. From this condition it passes usually into a stationary phase and from it emerges the adult. The details of these three categories of development will be discussed separately in the different classes of Arthropoda.

The metamorphosis is usually complete in Crustacea. The young one which comes out of the egg is called a larva. It usually passes through an independent life and subsequently transforms into an adult. In certain Crustaceans, e.g., Palaemon, Argulus, larva does not come out of the egg.

Thus transformation occurs internally and a young resembling the adult is hatched out. Within the class Crustacea, variety of larval forms (Fig. 18.132 and 18.133) are seen and in many groups one type of larva transforms into another type and finally becomes the adult.


Mollusks (Phylum Mollusca)

  1. Bivalvia. Two shells encase the body. Includes the clams, mussels, oysters, and scallops.
  2. Gastropoda. Snails and slugs. Snails have a single shell ("univalves') while slugs have none.
  3. Cephalopoda. This marine group includes the various species of octopus, squid, and cuttlefish, as well as the chambered nautilus. A record 28-foot (8.5 m) octopus and 60-foot (18 m) squid make these the largest of all the invertebrates.
  4. Scaphopoda. Marine, filter-feeding "tooth shells".
  5. Monoplacophora. Until a live specimen was discovered in 1952, these animals were thought to have been extinct for millions of years. It has a single shell (hence the name) and, unlike the other mollusks, is segmented (as are its relatives the annelids).
  6. Polyplacophora. The animals in this group, called chitons, have their dorsal surface protected by 8 overlapping plates or "valves".

Diversity of Flatworms

Flatworms are traditionally divided into four classes: Turbellaria, Monogenea, Trematoda, and Cestoda (Figure 15.16). The turbellarians include mainly free-living marine species, although some species live in freshwater or moist terrestrial environments. The simple planarians found in freshwater ponds and aquaria are examples. The epidermal layer of the underside of turbellarians is ciliated, and this helps them move. Some turbellarians are capable of remarkable feats of regeneration in which they may regrow the body, even from a small fragment.

The monogeneans are external parasites mostly of fish with life cycles consisting of a free-swimming larva that attaches to a fish to begin transformation to the parasitic adult form. They have only one host during their life, typically of just one species. The worms may produce enzymes that digest the host tissues or graze on surface mucus and skin particles. Most monogeneans are hermaphroditic, but the sperm develop first, and it is typical for them to mate between individuals and not to self-fertilize.

The trematodes, or flukes, are internal parasites of mollusks and many other groups, including humans. Trematodes have complex life cycles that involve a primary host in which sexual reproduction occurs and one or more secondary hosts in which asexual reproduction occurs. The primary host is almost always a mollusk. Trematodes are responsible for serious human diseases including schistosomiasis, caused by a blood fluke (Schistosoma). The disease infects an estimated 200 million people in the tropics and leads to organ damage and chronic symptoms including fatigue. Infection occurs when a human enters the water, and a larva, released from the primary snail host, locates and penetrates the skin. The parasite infects various organs in the body and feeds on red blood cells before reproducing. Many of the eggs are released in feces and find their way into a waterway where they are able to reinfect the primary snail host.

The cestodes, or tapeworms, are also internal parasites, mainly of vertebrates. Tapeworms live in the intestinal tract of the primary host and remain fixed using a sucker on the anterior end, or scolex, of the tapeworm body. The remaining body of the tapeworm is made up of a long series of units called proglottids, each of which may contain an excretory system with flame cells, but will contain reproductive structures, both male and female. Tapeworms do not have a digestive system, they absorb nutrients from the food matter passing them in the host’s intestine. Proglottids are produced at the scolex and are pushed to the end of the tapeworm as new proglottids form, at which point, they are “mature” and all structures except fertilized eggs have degenerated. Most reproduction occurs by cross-fertilization. The proglottid detaches and is released in the feces of the host. The fertilized eggs are eaten by an intermediate host. The juvenile worms emerge and infect the intermediate host, taking up residence, usually in muscle tissue. When the muscle tissue is eaten by the primary host, the cycle is completed. There are several tapeworm parasites of humans that are acquired by eating uncooked or poorly cooked pork, beef, and fish.


What features distinguish annelids from roundworms?

Q: How do the skin and respiratory systems of amniotes differ from those of their early tetrapod ancest.

A: Amniotes belongs to the kingdom Animalia and phylum Chordata. They belong to the clade of tetrapod v.

Q: Fill in the blank based off the graph: Most of the mercury emitted in South and Central America was .

A: The mercury is a highly toxic pollutant that is responsible for causing several diseases including c.

Q: . MAKE CONNECTIONS What type of feedback processis exemplified by the production of ethylene during .

A: Ethylene is the plant hormone. It is mainly responsible for the seed, flower production, and fruit r.

A: Ploidy refers to the number of chromosomes present in the nucleus of a cell. Polyploids are organism.

Q: 1. The height at which a parcel of air becomes saturated when it is lifted dry-adiabatically is call.

A: When a volume of air at a given temperature holds the maximum amount of water vapor, the air is said.

Q: Which cell part helps control the matterials that enter and leave a cell

A: Cell is the basic unit or the fundamental unit of life. Basically cell is of two types:- A)PROKARYOT.

A: Tetrapods include all land-living vertebrates, such as frogs, turtles, hawks, and lions. They all ha.

Q: Recessive maternal effect genes are identified in flies (for example)when a phenotypically normal mo.

A: Drosophila is usually used for genetic studies in the laboratory. The phenotype of the offspring can.

Q: What are two reasons why it is important to characterize and understand the human microbiome?

A: The microbiome is referred to as the collective genomes of the microbes that live inside and on the .


Evidence for multiple independent origins of trans-splicing in Metazoa

In contrast to conventional splicing, which joins exons from a single primary transcript, trans-splicing links stretches of RNA from separate transcripts, derived from distinct regions of the genome. Spliced leader (SL) trans-splicing is particularly well known in trypanosomes, nematodes, and flatworms, where it provides messenger RNAs with a leader sequence and cap that allow them to be translated efficiently. One of the largest puzzles regarding SL trans-splicing is its evolutionary origin. Until now SL trans-splicing has been found in a small and disparate set of organisms (including trypanosomes, dinoflagellates, cnidarians, rotifers, nematodes, flatworms, and urochordates) but not in most other eukaryotic lineages, including well-studied groups such as fungi, plants, arthropods, and vertebrates. This patchy distribution could either suggest that trans-splicing was present in early eukaryotes/metazoans and subsequently lost in multiple lineages or that it evolved several times independently. Starting from the serendipitous discovery of SL trans-splicing in an arthropod, we undertook a comprehensive survey of this process in the animal kingdom. By surveying expressed sequence tag data from more than 70 metazoan species, we show that SL trans-splicing also occurs in at least two groups of arthropods (amphipod and copepod crustaceans), in ctenophores, and in hexactinellid sponges. However, we find no evidence for SL trans-splicing in other groups of arthropods and sponges or in 15 other phyla that we have surveyed. Although the presence of SL trans-splicing in hydrozoan cnidarians, hexactinellid sponges, and ctenophores might suggest that it was present at the base of the Metazoa, the patchy distribution that is evident at higher resolution suggests that SL trans-splicing has evolved repeatedly among metazoan lineages. In agreement with this scenario, we discuss evidence that SL precursor RNAs can readily evolve from ubiquitous small nuclear RNAs that are used for conventional splicing.


Nematodes are Beautiful to Study and Learn

Nematodes are also known as roundworms. As the name indicates, they are not flat like platyhelminthes. From an evolutionary point of view, the first complete digestive system, which contains a mouth and anus,ਊppears in nematodes. Another evolutionary novelty brought by nematodes is their pseudocoelom.

4. What are the morphological similarities and differences between nematodes and annelids?

Nematodes, like annelids, have an਎longated cylindrical body. Annelids are different from nematodes in that they have a segmented body (a body divided into metameres). Because of this, they are called segmented worms.

5. Are nematodes diploblastic or triploblastic animals?

Just like platyhelminthes, nematodes are triploblastic organisms, meaning that they have three germ layers (the ectoderm, mesoderm and endoderm). 

Select any question to share it on FB or Twitter

Just select (or double-click) a question to share. Challenge your Facebook and Twitter friends.

Nematode Physiology

6. What is the main evolutionary innovation presented by nematodes? What is the advantage of that innovation?

The main evolutionary innovation of nematodes is their complete digestive system, which contains two openings (a mouth and anus).

Since the ingestion and the defecation processes can occur at different extremities of the digestive tube, organisms with a complete digestive system have the advantage of being able to ingest new food while the residue of already eaten food is still inside the body and has not yet been eliminated.

7. Compared to platyhelminthes, which physiological problem was caused the cylindrical body of nematodes? How was that problem solved?

The cylindrical shape of nematodes made it impossible for them to use respiration exclusively via simple diffusion between cells, since they contain tissues far from their exterior. This problem was solved by the presence of an inner cavity in the body filled with fluid, called the pseudocoelom. The pseudocoelom has the function of distributing gases and nutrients to the body as well as collecting waste. In addition, it serves as a hydrostatic base to maintain the shape of the worm.

(Due to the fact that the pseudocoelom fluid and the pseudocoelom do not constitute a true circulatory system with blood and a heart, the respiration in nematodes is not considered to be cutaneous rather, scientists consider that these animals still carry out respiration via diffusion).

8. How does the excretory system of nematodes work?

The metabolic waste of nematodes is collected by two longitudinal lateral excretory channels that open at one single excretory pore near the mouth.

9. How is the nervous system of nematodes organized? Where in their body are their neural cords located?

Roundworms have a ganglial nervous system with an anterior neural ring, which represents (evolutionarily) primitive cephalization.

Nematodes have two main longitudinal ganglial cords, one of which extends dorsally and the other of which ventrally under the epidermis. There may also be nerves that run laterally to these main cords. The nervous system of a free-living nematode, Caenorhabditis elegans, has been well-studied in neurophysiological research and contains 302 neurons.

The nematode C. Elegans was the organism used in the research on the genetic regulation of organogenesis and apoptosis, the researchers of which won the Nobel Prize in Medicine in 2002 (Brenner, Horvitz and Sulston).

Nematode Reproduction and Human Diseases

10. What type of reproduction occurs in roundworms? What is the typical feature of nematode sperm cells?

Nematodes reproduce sexually. Nematode sperm cells do not have cilia or flagella, and they move through amoeboid movement, forming pseudopods.

11. What are the main human diseases caused by roundworms?

The main human diseases caused by nematodes are ascariasis, ancylostomiasis (hookworm infection) and filariasis (commonly known by its sign, elephantiasis).

Roundworms Summary

12. The main features of nematodes.  How can nematodes be described according to examples of representative species, basic morphology, type of symmetry, germ layers and coelom, digestive system, respiratory system, circulatory system, excretory system, nervous system and types of reproduction?

Examples of representative species: ascaris, hookworms, filaria, pinworms. Basic morphology: cylindrical (round) body, not segmented. Type of symmetry: bilateral. Germ layers and coelom: triploblastic, pseudocoelomates. Digestive system: complete. Respiratory system: respiration via diffusion. Circulatory system: circulating fluid within the pseudocoelom. Excretory system: excretory channels and excretory pore. Nervous system: ventral and dorsal ganglial cords, primitive cephalization. Type of reproduction: sexual.


Section Summary

The phylum Mollusca is a large, mainly marine group of invertebrates. Mollusks show a variety of morphologies. Many mollusks secrete a calcareous shell for protection, but in other species, the shell is reduced or absent. Mollusks are protostomes. The dorsal epidermis in mollusks is modified to form the mantle, which encloses the mantle cavity and visceral organs. This cavity is distinct from the coelomic cavity, which the adult animal retains, surrounding the heart. Respiration is facilitated by gills known as ctenidia. A chitinous scraper called the radula is present in most mollusks. Mollusks are mostly dioecious and are divided into seven classes.

The phylum Annelida includes worm-like, segmented animals. Segmentation is both external and internal, which is called metamerism. Annelids are protostomes. The presence of chitinous hairs called chaetae is characteristic of most members. These animals have well-developed nervous and digestive systems. Polychaete annelids have parapodia that participate in locomotion and respiration. Suckers are seen in the order Hirudinea. Breeding systems include separate sexes and hermaphroditism.


Watch the video: Complex Animals: Annelids u0026 Arthropods - CrashCourse Biology #23 (November 2021).