12.3: Flatworms, Nematodes, and Arthropods - Biology

12.3: Flatworms, Nematodes, and Arthropods - Biology

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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.


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.


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.


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.


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.


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


Crustaceans are _____.

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


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.


  1. 1 “Number of Living Species in Australia and the World,” A.D. Chapman, Australia Biodiversity Information Services, last modified August 26, 2010,
  2. 2 “Number of Living Species in Australia and the World,” A.D. Chapman, Australia Biodiversity Information Services, last modified August 26, 2010,


a phylum of Ecdysozoa with jointed appendages and segmented bodies
a fused head and thorax
a modified first pair of appendages in subphylum Chelicerata
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
having separate male and female sexes
the internal body cavity seen in arthropods
a phylum of worms in Ecdysozoa commonly called roundworms containing both free-living and parasitic forms
a respiratory openings in insects that allow air into the tracheae

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 .

Types of Arthropods


Trilobites were an ancient family of marine arthropods that went extinct during the Permian-Triassic extinction event. Today, they are known to us mostly through fossils like the one below.

They lived on the ocean floor and occupied ecological niches similar to those occupied by crustaceans today.


Chelicerata are a branch of the arthropod family tree that, at first glance, may not appear related to each other.

This family includes arachnids (such as spiders and scorpions), sea spiders (which look similar to arachnids but have some important differences), and horseshoe crabs (which, despite their name, have important differences from other crustaceans).


The term “myriapod” means “many legs” – so it is not surprising that centipedes, milipedes, and other many-legged creatures are part of this family.

Myriapods can have anywhere from less than ten legs – to over 750! That just seems excessive.

Myriapods are typically found in forests and other ecosystems where there is lots of decaying plant and animal material for them to feed on.


Crustaceans are a family of primarily aquatic arthropods that include lobsters, crabs, shrimp, crayfish, barnacles, and the odd one out – wood lice, also known as pill bugs or “roly polys.”

Unlike their aquatic cousins, wood lice live mostly on dry land and are found in environments such as gardens and forests, where they survive by eating decaying plant and animal material.

It may also surprise you to see barnacles included on this list: adult barnacles develop hard shells that stick them to their surroundings, such as the bottoms of boats or other underwater surfaces.

But earlier in their lives before they freeze in place, barnacles have bodies with legs much like the other crustaceans!


The term “hexapod” literally means “six feet.” It might not surprise you to learn that insects – which all have six legs – are hexapods.

Insects include most “bugs” with six legs, such as flies, ants, termites, beetles, dragonflies, mosquitoes, cockroaches, butterflies, and moths.

There are also three much smaller groups of animals in the “hexapod” category. Collembola, Protura, and Diplura were all once considered to be insects, but later found to have small differences that set them apart from other insects.

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). 

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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.

12.3: Flatworms, Nematodes, and Arthropods - Biology

Unit Five. Evolution of Animal Life

19.3. Six Key Transitions in Body Plan

The evolution of animals is marked by six key transitions: the evolution of tissues, bilateral symmetry, a body cavity, segmentation, molting, and deuterostome development. These six body transitions are indicated at the branchpoints of the animal evolutionary tree in figure 19.3.

Figure 19.3. Evolutionary trends among the animals.

In this chapter, we examine a series of key evolutionary innovations in the animal body plan, shown here along the branches. Some of the major animal phyla are shown on this tree. Lophophorates exhibit a mix of protostome and deuterostome characteristics. The traditional tree shown here assumes segmentation arose only once among the invertebrates, while molting arose independently in nematodes and arthropods. The newly proposed molecular phylogenies assume molting arose only once, while segmentation arose independently in annelids, arthropods,and chordates.

The simplest animals, the Parazoa, lack both defined tissues and organs. Characterized by the sponges, these animals exist as aggregates of cells with minimal intercellular coordination. All other animals, the Eumetazoa, have distinct tissues with highly specialized cells.

2. Evolution of Bilateral Symmetry

Sponges also lack any definite symmetry, growing asymmetrically as irregular masses. Virtually all other animals have a definite shape and symmetry that can be defined along an imaginary axis drawn through the animal’s body.

Radial Symmetry. Symmetrical bodies first evolved in marine animals exhibiting radial symmetry. The parts of their bodies are arranged around a central axis in such a way that any plane passing through the central axis divides the organism into halves that are approximate mirror images.

Bilateral Symmetry. The bodies of all other animals are marked by a fundamental bilateral symmetry, a body design in which the body has a right and a left half that are mirror images of each other. This unique form of organization allows parts of the body to evolve in different ways, permitting different organs to be located in different parts of the body. Also, bilaterally symmetrical animals move from place to place more efficiently than radially symmetrical ones, which, in general, lead a sessile or passively floating existence. Due to their increased mobility, bilaterally symmetrical animals are efficient in seeking food, locating mates, and avoiding predators.

3. Evolution of a Body Cavity

A third key transition in the evolution of the animal body plan was the evolution of the body cavity. The evolution of efficient organ systems within the animal body was not possible until a body cavity evolved for supporting organs, distributing materials, and fostering complex developmental interactions.

The presence of a body cavity allows the digestive tract to be larger and longer. This longer passage allows for storage of undigested food and longer exposure to enzymes for more complete digestion. Such an arrangement allows an animal to eat a great deal when it is safe to do so and then to hide during the digestive process, thus limiting the animal’s exposure to predators.

An internal body cavity also provides space within which the gonads (ovaries and testes) can expand, allowing the accumulation of large numbers of eggs and sperm. Such storage capacity allows the diverse modifications of breeding strategy that characterize the more advanced phyla of animals. Furthermore, large numbers of gametes can be stored and released when the conditions are as favorable as possible for the survival of the young animals.

4. The Evolution of Segmentation

The fourth key transition in the animal body plan involved the subdivision of the body into segments. Just as it is efficient for workers to construct a tunnel from a series of identical prefabricated parts, so segmented animals are assembled from a succession of identical segments. Segmentation was assumed to have evolved only once among the invertebrates in the traditional taxonomy, as it seemed such a significant alteration of body plan.

5. The Evolution of Molting

Most coelomate animals grow by gradually adding mass to their body. However, this creates a serious problem for animals with a hard exoskeleton, which can hold only so much tissue. To grow further, the individual must shed its hard exoskeleton, a process called molting or, more formally, ecdysis.

Ecdysis occurs among both nematodes and arthropods. In the traditional taxonomy these are treated as two independent evolutionary events. The new phylogenies suggest ecdysis evolved only once. This would imply that arthropods and nematodes, both of which have hard exoskeletons and molt, are sister groups, and that segmentation rather than ecdysis must have evolved several times among the invertebrates, rather than once.

6. The Evolution of Deuterostome Development

Bilateral animals can be divided into two groups based on differences in the basic pattern of development. One group is called the protostomes (from the Greek words protos, first, and stoma, mouth) and includes the flatworms, nematodes, mollusks, annelids, and arthropods. Two outwardly dissimilar groups, the echinoderms and the chordates, together with a few other smaller related phyla, comprise the second group, the deuterostomes (Greek, deuteros, second, and stoma, mouth). Protostomes and deuterostomes differ in several aspects of embryo growth and will be discussed later in the chapter.

Deuterostomes evolved from protostomes more than 630 million years ago, and the consistency of deuterostome development, and its distinctiveness from that of the proto-stomes suggests that it evolved once, in a common ancestor to all of the phyla that exhibit it.

Characteristics of the major animal phyla are described in table 19.2.


Key Learning Outcome 19.3. Six key transitions in body design are responsible for most of the differences we see among the major animal phyla.


In contrast with flatworms, nematodes show a tubular morphology and circular cross-section. These animals are pseudocoelomates and show the presence of a complete digestive system with a distinct mouth and anus. This is in contrast with the cnidarians, where only one opening is present (an incomplete digestive system).

The cuticle of Nematodes is rich in collagen and a carbohydrate-protein polymer called chitin, and forms an external “skeleton” outside the epidermis. The cuticle also lines many of the organs internally, including the pharynx and rectum. The epidermis can be either a single layer of cells or a syncytium, which is a multinucleated cell formed from the fusion of uninucleated cells.

The overall morphology of these worms is cylindrical, as seen in Figure 1. The head is radially symmetrical. A mouth opening is present at the anterior end with three or six lips as well as teeth in some species in the form of cuticle extensions. Some nematodes may present other external modifications like rings, head shields, or warts. Rings, however, do not reflect true internal body segmentation. The mouth leads to a muscular pharynx and intestine, which leads to a rectum and anal opening at the posterior end. The muscles of nematodes differ from those of most animals: They have a longitudinal layer only, which accounts for the whip-like motion of their movement.

Figure 1. Scanning electron micrograph shows (a) the soybean cyst nematode (Heterodera glycines) and a nematode egg. (b) A schematic representation shows the anatomy of a typical nematode. (credit a: modification of work by USDA ARS scale-bar data from Matt Russell)

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.


I. Inorganic Chemistry

1) Basics of chemical nomenclature
- Inorganic compounds
- Organic compounds

2) Basics of inorganic chemistry
- Atoms: structure, orbitals and quantum numbers
- Chemical bonds – covalent, ionic, coordination

3) Chemical reactions
- Basic concepts of stoichiometry
- Chemical equation writing
- Chemical equilibrium, equilibrium constants
- Sorts of chemical reactions (redox, substitution, decomposition, partner-exchange etc.)
- Reaction rates
- Solutions
- Strong and weak electrolytes
- Ion product constant of water
- pH

4) Periodic system
- Features of elements in context to their position in the periodic system
- Properties, behaviour, important reactions and compounds of
- Non-metals (namely Halogens, H, C, Si, N, P,O,S),
- s-Block metals (namely Alkali Metals and Alkaline Earth Metals),
- p-Bloc metals (namely Group III metals)
- d-Block Elements

II.Organic Chemistry

1) Basics of organic chemistry
- Carbon and its hybridisation states in organic substances
- Important kinds of organic reactions
- Properties of organic compounds, functional groups
- Basics of isomerism in organic chemistry

2) Hydrocarbons
- saturated, unsaturated, aromatic
3) Halogen derivatives of hydrocarbons
4) Alcohols
5) Phenols
6) Ethers
7) Aldehydes and Ketones
8) Carboxylic acids and their derivatives
9) Biologically important groups of organic compounds (basic structures, reactions and properties)
- Carbohydrates
- Lipids
- Proteins
- Nucleic Acids

Tag Archives: nematodes

Non-chordates are animals without a notochord. They are the most abundant and diversified of all animals living or extinct. That makes their study the most fascinating one.

The following chapters appear in this section. Click on the title to open.



Echinoderms can voluntarily and rapidly change the stiffness of their connective tissue, which is called mutable connective tissue. Their bodies can become stone hard or in holothurians it can become so soft that it may flow between fingers.

If attacked by a predator, brittle stars can break their arms at will and grow them again later. This is called Autotomy.

Brittle stars do not have intestine, anus, dermal branchiae, pedicellariae and ambulacral grooves.

Echinoderms have no brain or ganglia, and nerves are made of diffused neurons.

Sea urchins can harden or soften their spines at will.

Starfishes are slowest of predators that take 4-8 hours to kill and consume a mollusc prey.

Aristotle’s Lantern is actually a set of masticatory jaws of sea urchins for feeding on algae from rocks.

Pelagothuriais a pelagic holothurian echinoderm that can swim like a jelly fish with the help of webbed papillae.

Mollusca means soft bodied, although it includes animals having hard shell.

Neopilina galatheae,collected from 3500 m depth is a connecting link between Annelida and Mollusca.

Brachystomiais a tiny shelled snail that sucks blood of clams.

Conusis a predatory snail that lures a fish by a bait (its worm-like modified proboscis) and stings fish’s tongue when it tries to eat the bait. The paralysed fish is then swallowed by the snail.

Foot of sea butterflies (Pteropods) is modified into wing-like parapodia which are used for active swimming.

Giant clam (Tridacna maxima), found in the coral reefs of Indo-pacific region, is 1.5 m long and weighs 225 kg.

Entovalvais the only parasitic bivalve mollusc that lives inside the gut of sea cucumbers. Entochoncha is a worm-like parasitic gastropod that also lives in the body cavity of sea cucumbers.

Glochidium larva of bivalves is parasitic on the gills of fishes.

Scallops (Pecten) can swim by using its valves as wings.

Teredo and Bankiaare wood-boring bivalves that damage boats.

Cephalopods have 3rd or 4th arm hectocotylized (or modified spoon-shaped) which is used to introduce sperms into the mantle cavity of female.

Squids, cuttle fishes and Octopus possess strong beak-like jaws often laced with poisonous saliva that makes them fierce predators of the sea.

Giant squids (Architeuthis Mesonychoteuthes) of the Pacific are the largest invertebrates which are over 50 ft long and weigh more than 500 kg. They live at a depth of 2-3 kilometers in pacific ocean.

Giant Octopus (Octopus dofleini), which inhabits pacific ocean, has a arm-span of ten meters.

Cephalopods swim by a faster jet propulsion method.

Cephalopods’ brain is more advanced than any other invertebrates that makes octopus the most intelligent invertebrate that can carry out complex tasks.

Cephalopod eyes are strikingly similar to vertebrate eyes but have evolved independently, which a strange coincidence.

Argonautais a cousin of octopus but secretes an external shell around the body.

Largest eye in the animal kingdom, having a diameter of 12 inches, belongs to the giant squid (Mesonychoteuthis). Its lens is the size of a tennis ball.

Snails are born bilaterally symmetrical but within 15 minutes they become spirally coiled by torsion.

Pearl oysters secrete pearl around any object that is trapped in their mantle cavity and causes irritation.

Arthropods are the most successful of all animals on earth. They also make more that 80% of animal species.

Japaneses spider crab (Macrocheira kaempferi) is the largest living arthropod. Its legs are 5-6 feet long.

Trilobites were the earliest arthropods which have all become extinct. Over 4000 species of fossil trilobites are known today.

More than 1,000 species of sea spiders (Pycnogonida) live up to 6,000 m depth and feed on cnidarians and worms.

Giant water scorpions (Eurypterida) grew up to 3 meter length and were the top predators of the Paleozoic era.

Giant centipedes (Scolopendra) can attain a length of 30 cm and their bite can be fatal to humans.

Tracheal system of insects is a unique respiratory system that conveys oxygen directly to the muscles where it is needed. Therefore, there is no need of respiratory pigment in the blood of insects.

Compound eyes are unique innovation of arthropods. The eye is made of thousands of ommatidia, each of which functions independently. Compound eye is superior because it gives 360 degrees of visual field, a much sharper vision and more depth of field at higher magnification, and detects movement of predators no matter how fast it is.

Insects are the most abundant and most successful of all creatures on earth they make three-fourth of all animal species and also the most species that survived successive mass extinctions.

Male and female scorpions have a honeymoon dance called “Promenade a deux”, after which female kills and devours the male. Similar phenomenon is seen among spiders, in which male is much smaller than the female.

Termites are the earliest social animals, which developed a well organised social system and communication, with division of labour among castes for specific duties. Other insects having advanced social life are ants, honey bees and wasps.

Lobatocerebrum and Jennaria (Rhynchocoela) are marine worms that are intermdediate between flatworms and annelida.

Washington giant earthworm (Driloleirus americanus) is one meter long the Australian giant earthworm (Megascolides australis) is 3 meter long and the South African Microchaetus rappi is 20 feet long earthworm.

Clamworms feed by everting out their entire pharynx which has jaws at the bottom.

Earthworms burrow in mud by producing hydraulic skeleton by pumping coelomic fluid that makes the anterior region stone hard for burrowing.

Amazon leech (Haementaria) is 30 cm long. Leeches do not have true blood vascular system.

Leech has ten eyes but cannot see properly and finds the host by smell.

Some annelids are parasitic on other animals, such as Ichthyotomus is a parasite of fishes and Histriobdella is a parasite in the gill chamber of crustaceans.

Each gravid segment of tape worm carries about one hundred thousand eggs and hundreds of these proglottids are produced each day.

Hydated cyst of the dog tapeworm (Echinococcus) can grow to the size of a football in human tissues and it carries poisonous fluid.

Round worms (Nematodes) grow by enlargement of cells and not by multiplication of cells. Mitosis stops in nematodes after they hatch from eggs.

A single Ascaris can lay about 200,000 eggs daily and about 30 million in its lifetime.

Roundworm and hookworm larvae take a tour of all body organs before they reach intestine to become adult parasites.


Palytoxin obtained from the Hawaiian cnidarian, Palythoa toxica is more toxic than batrachotoxin of dart frog and is used to smear arrow tips.

Sea wasps (Cubozoa) are like small jelly fishes but swim very fast and their sting can kill a man in 15 minutes.

Hydra cannot digest starch.

Jellyfishes are named so because of the presence of enormous jelly-like mesogloea in their bodies.

The north Atlantic sea blubber, Cyanea capillata, is the largest jelly fish with a diameter of two metres.

Corals cannot grow beyond a depth of 50 metres in the sea.

The great barrier reef of Australia is over 1200 miles long and 70 miles wide.

Most of the coral reefs are in the Indo-Pacific region.

Sponges possess such extraordinary power of regeneration that even if they are crushed, mashed and strained through a cloth, the cells still rearrange themselves to form a complete sponge.

In asexually reproducing species, offspring always have more deleterious mutations than the parents. This is called Muller’s Ratchet.

Roundworms (Nematodes)

Figure 86-5 shows the structure of nematodes. In contrast to platyhelminths, nematodes are cylindrical rather than flattened hence the common name roundworm. The body wall is composed of an outer cuticle that has a noncellular, chemically complex structure, a thin hypodermis, and musculature. The cuticle in some species has longitudinal ridges called alae. The bursa, a flaplike extension of the cuticle on the posterior end of some species of male nematodes, is used to grasp the female during copulation.

Figure 86-5

Structure of nematodes. (A) Female. (B) Male. Transverse sections through the midregion of the female worm (C) and through the esophageal region (D). (Modified from Lee DL: The Physiology of Nematodes. Oliver and Boyd, Edinburgh, 1965, with permission.) (more. )

The cellular hypodermis bulges into the body cavity or pseudocoelom to form four longitudinal cords𠅊 dorsal, a ventral, and two lateral cords—which may be seen on the surface as lateral lines. Nuclei of the hypodermis are located in the region of the cords. The somatic musculature lying beneath the hypodermis is a single layer of smooth muscle cells. When viewed in cross-section, this layer can be seen to be separated into four zones by the hypodermal cords. The musculature is innervated by extensions of muscle cells to nerve trunks running anteriorly and posteriorly from ganglion cells that ring the midportion of the esophagus.

The space between the muscle layer and viscera is the pseudocoelom, which lacks a mesothelium lining. This cavity contains fluid and two to six fixed cells (celomocytes) which are usually associated with the longitudinal cords. The function of these cells is unknown.

The alimentary canal of roundworms is complete, with both mouth and anus. The mouth is surrounded by lips bearing sensory papillae (bristles). The esophagus, a conspicuous feature of nematodes, is a muscular structure that pumps food into the intestine it differs in shape in different species.

The intestine is a tubular structure composed of a single layer of columnar cells possessing prominent microvilli on their luminal surface.

The excretory system of some nematodes consists of an excretory gland and a pore located ventrally in the mid-esophageal region. In other nematodes this structure is drawn into extensions that give rise to the more complex tubular excretory system, which is usually H-shaped, with two anterior limbs and two posterior limbs located in the lateral cords. The gland cells and tubes are thought to serve as absorptive bodies, collecting wastes from the pseudocoelom, and to function in osmoregulation.

Nematodes are usually bisexual. Males are usually smaller than females, have a curved posterior end, and possess (in some species) copulatory structures, such as spicules (usually two), a bursa, or both. The males have one or (in a few cases) two testes, which lie at the free end of a convoluted or recurved tube leading into a seminal vesicle and eventually into the cloaca.

The female system is tubular also, and usually is made up of reflexed ovaries. Each ovary is continuous, with an oviduct and tubular uterus. The uteri join to form the vagina, which in turn opens to the exterior through the vulva.

Copulation between a female and a male nematode is necessary for fertilization except in the genus Strongyloides, in which parthenogenetic development occurs (i.e., the development of an unfertilized egg into a new individual). Some evidence indicates that sex attractants (pheromones) play a role in heterosexual mating. During copulation, sperm is transferred into the vulva of the female. The sperm enters the ovum and a fertilization membrane is secreted by the zygote. This membrane gradually thickens to form the chitinous shell. A second membrane, below the shell, makes the egg impervious to essentially all substances except carbon dioxide and oxygen. In some species, a third proteinaceous membrane is secreted as the egg passes down the uterus by the uterine wall and is deposited outside the shell. Most nematodes that are parasitic in humans lay eggs that, when voided, contain either an uncleaved zygote, a group of blastomeres, or a completely formed larva. Some nematodes, such as the filariae and Trichinella spiralis, produce larvae that are deposited in host tissues.

The developmental process in nematodes involves egg, larval, and adult stages. Each of four larval stages is followed by a molt in which the cuticle is shed. The larvae are called second-stage larvae after the first molt, and so on (Fig. 86-6). The nematode formed at the fifth stage is the adult. Figure 86-7 summarizes the life cycles of several intestinal nematodes.

Figure 86-6

Stages in the development of nematodes. (Adapted from Lee DL: The Physiology of Nemotodes. Oliver and Boyd, Edinburgh, 1965, with permission.)

Watch the video: Kingdom Animalia: Phylum Nematoda and Phylum Annelida. Biology. iKen. iKen Edu. iKen App (July 2022).


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