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Here is a compilation of term papers on ‘Lower Animals’ for class 8, 9, 10, 11 and 12. Find paragraphs, long and short term papers on ‘Lower Animals’ especially written for school and college students.
Term Paper on Lower Animals
Term Paper Contents:
- Term Paper on the Introduction to Lower Animals
- Term Paper on the Sources of Diversity of Lower Animals
- Term Paper on the Sources of Continuity of Lower Animals
- Term Paper on the Question of Size of Lower Animals
- Term Paper on Phylum Porifera: Sponges
- Term Paper on Phylum Coelenterata: Polyps and Jellyfish
- Term Paper on Phylum Platyhelminthes: Flatworms
- Term Paper on Phylum Rhynchocoela: Ribbon Worms
- Term Paper on Phylum Nematoda: Roundworms
- Term Paper on Phylum Annelida: Segmented Worms
- Term Paper on Phylum Mollusca: Mollusks
- Term Paper on Phylum Echinodermata: Starfish
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Term Paper # 1. Introduction to Lower Animals:
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Animals are many-celled heterotrophs. They depend directly or indirectly for their nourishment on photosynthetic autotrophs-algae or plants. Most digest their food in an internal cavity, and most store food as glycogen or fat. Their cells do not have walls. Most move by means of contractile cells (muscle cells) containing characteristic proteins. Reproduction is usually sexual.
As adults, most are fixed in size and shape, in contrast to plants, in which growth often continues for the lifetime of the organism. The higher animals-the arthropods and the vertebrates—are the most complex of all organisms, with many kinds of specialised tissues, including elaborate sensory and neuromotor mechanisms not found in any of the other kingdoms.
For most of us, animal means mammal, and mammals are, in fact, the chief focus of attention in other section. However, the mammals, or even the vertebrates as a whole, represent only a small fraction of the animal kingdom. More than 90 percent of the different species of animals are invertebrates-that is, animals without backbones and most of these are insects. Indeed, the enormous variety displayed by the invertebrates is partly why they are so endlessly fascinating to study. They are, in addition, of great ecological importance; the insects, for example, have long challenged human dominance of the earth.
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Finally, and perhaps most important, the invertebrates, confronted with the same biological problems that we face, demonstrate a spectrum of ingenious solutions. In this way, they illuminate the essential nature of these problems and so help us to understand and evaluate the solutions arrived at by mammals.
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2. The Sources of Diversity of Lower Animals:
The tremendous versatility of the prokaryotes, as exemplified by the wide range of environments they inhabit and the many ways in which they satisfy their energy requirements.
Among the invertebrates, we see, on a slightly different scale, this same pattern of adaptation to many different ways of life. Thus, for instance, on a single coral head, only a meter or two in diameter, one finds a dazzling array of different forms-sponges, jellyfish, starfish, sea urchins, anemones, and the coral animals themselves.
Similarly, to take terrestrial examples, a single spadeful of soil turns up earthworms, pillbugs, spiders, nematodes, and various other tiny animals; and the branch of a single tree may harbor a dozen different kinds of insects. Given the relentless force of natural selection, why do not the larger ones crowd out the smaller? Why are not the “lower” animals replaced by “higher” animals with superior strength or intelligence?
Darwin, again, offered the answer: The different organisms, he noted, “occupy different positions in the economy of nature.” Each has been shaped by the long process of evolution to occupy a different niche in the environment. Natural selection has worked not to make one “superior” to the other but to continuously adapt the different forms to different ways of life.
This process of adaptation, of course, continues. Every species, including our own, is a traveler through time, caught for only an instant in the present.
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3. The Sources of Continuity of Lower Animals:
Through the patterns of diversity, there is a strong theme of continuity. One reason is simply that of “descent.” We are all related; not only are we made of the same atoms and molecules and even macromolecules, but from E. coli to elephant, we even share many of the same enzymes.
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Although the evolutionary relationships between us and the invertebrates are obscure, we can read into them traces of our own biological beginnings. In the twitch of a tiny segment of an earthworm’s artery, we sense the echo of our own heartbeat.
Second, all organisms face the same set of problems. These problems can best be defined by recalling that an organism is a cell or group of cells. A primary need is to supply the cell or cells with, first of all, a source of energy. Also, most cells-on this planet, at least-require water, oxygen, a source of nitrogen, fixed carbon (in the case of heterotrophs), and a few ions.
Another requirement is to eliminate wastes, including excess carbon dioxide, nitrogenous wastes from the breakdown of amino acids, and, in some cases, excess water. Cells that live individual or colonial lives in a watery environment can solve these problems in relatively simple ways, but as organisms get larger, thicker, and more complex, the problem of servicing each individual cell becomes correspondingly more complicated.
Another set of problems that a multicellular organism must solve in order to exist arises from the fact that it is more than just a group of cells. It is, in fact, a complex society of cells, in which the needs of each individual cell are subordinated to the needs of the society. In a population of Paramecium, the organisms have common requirements, but each is in competition with the others.
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In a society of even a few thousand cells—a small crustacean, for instance-the individual cells are dependent on the existence of the group and are organised in a system of mutual cooperation. The second group of problems faced by organisms, therefore, relates to the organisation or integration of activities.
Hormones are one of the chief means of integration in both plants and animals. In the animals, another, more rapid integrating mechanism has evolved: the nervous system, by which the organism keeps in touch with its environment and coordinates its own activities.
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4. A Question of Size of Lower Animals:
At this point, one might well ask – Considering the problems faced by larger organisms, why did larger animals evolve? What selective advantages do the multicellular, more complex animals have as compared with the smaller ones? Some answers to these questions are obvious and simple.
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Larger animals are, in general, more likely to eat than be eaten. Larger organisms, especially those that live underwater or on land, are generally able to travel faster and farther than small ones and this is an advantage.
A small ciliate, for instance, might starve only a few centimeters from a food supply. On the other hand, its requirements are very modest.
Perhaps even more important, however, than mobility and edibility is what the French physiologist Claude Bernard called the milieu interior, the internal environment of the animal, as distinct from the external environment that surrounds it.
A single-celled organism is as cold or as hot and as wet or as dry as its surroundings, whereas a larger animal is more independent and, to some extent, controls the environment in which its cellular society lives. Control of the internal environment is more readily achieved by the many-celled animal because of the simple surface-to-volume geometry.
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Exchanges between a cell and its surroundings take place across a cell’s available surface area. This is a principal reason why a cell, which depends for its existence on the exchange of substances with its environment, cannot be very large.
On the other hand, since it may be advantageous to conserve certain substances, such as water and heat, an organism may be better off, within limits of weight and mobility, if its relative surface area is reduced. One-celled animals can live successfully only in water or as parasites in the bodies of other organisms, which amounts to the same thing.
Many-celled animals can live not only in water but also on land, in the sky, and even, as we are now beginning to discover, in outer space-which is a logical extension of an old evolutionary trend. In this article, we shall discuss the so-called lower invertebrates and some that must clearly be considered higher, such as the clever and highly emotional octopus.
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5. Phylum Porifera: Sponges:
Sponges seem to have had a different origin from other members of the animal kingdom and to have traveled a solitary evolutionary route. For this reason, they are often classified in a sub-kingdom of then-own, the Parazoa (“alongside of animals”).
In fact, until the eighteenth century, the sponges were classified as plant-animals (“zoophytes”) since they are all sessile (attached to a substrate) during their adult life. Sponges are found on ocean floors throughout the world. Most live along the coasts in shallow water, but some, such as the fragile glass sponges, are found at great depths, where the water is almost motionless. A few types are found in fresh water.
A sponge is essentially a water-filtering system, made up of one or more chambers through which water is driven by the action of numerous flagellated cells. Sponges are made up of a relatively few cell types, the most characteristic of which are the choanocytes, or collar cells, the flagellated cells that line the interior cavity of the sponge.
Similar cells, the choanoflagellates, are found among the ciliated protozoans, and it is possible that the sponges arose from such organisms. All of a sponge’s digestive processes are carried out intracellularly; hence, even a giant sponge-and some stand taller than a person-can eat nothing larger than microscopic food particles.
The outer surface of the sponge is covered with epithelial cells. Among these epithelial cells are cells that contract in response to touch or to irritating chemicals, and in so doing, close up pores and channels. Each cell acts as an individual, however; there is little coordination among them.
Between the epithelial cells and the choanocytes is a middle, jellylike layer, and in this layer are amoeba-like cells, amoebocytes, which carry out various functions. Some amoebocytes carry food particles from the choanocytes to the epithelial cells. Amoebocytes also secrete skeletal materials.
Sponges are grouped into three classes, according to their skeletal structure. In some species, the skeleton consists of individual spicules of calcium carbonate. Some, the glass sponges, have spicules of silica fused in a continuous and often very beautiful structure.
The third and largest group has unfused silica spicules, or a tough, fibrous keratin like protein called spongin, or a combination of the two. The skeleton of sponges serves only for protection, stiffening, and support, but not for locomotion, since the adult forms are sessile.
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The sponge shown in Figure 11.1 is a small and simple one. In larger sponges, the body plan, although it is essentially the same, looks far more complex. These sponges have greatly increased feeding and filtering surfaces, owing to their highly folded body walls.
We have already encountered this evolutionary stratagem for increasing biological work surfaces at the cellular level—as in the inner membrane of the mitochondrion-and we shall be encountering it again as we examine the structure of gills and lungs.
Sponges are somewhere between a colony of cells and a true multicellular organism. The cells are not organised into tissues or organs; each leads an independent existence. Yet there is a form of recognition among the cells that holds them together and organises them.
If the sponge Microciona prolifera is squeezed through a fine sieve or a piece of cheesecloth, the body of the sponge is separated into individual cells and small clumps of cells. Within an hour, the isolated sponge cells begin to re-aggregate, and as these aggregations get larger, canals, flagellated chambers, and other characteristics of the body organisation of the sponge begin to appear.
This phenomenon has been used as a model for the analysis of cell adhesion, recognition, and differentiation, all of which are basic biological features of development in higher organisms.
Most kinds of sponges are hermaphroditic; that is, they have male and female reproductive organs in the same individual. Gametes appear to arise from an enlarged amoebocyte, but there are reports that choanocytes can also form gametes. A sperm enters another sponge in a current of water.
It is captured by a choanocyte and transferred to an amoebocyte, which then transfers it to a ripe egg (a method of fertilisation unique to the sponges). The fertilised egg develops into a ciliated, free-swimming larva. After a short life among the plankton, the larva settles and becomes sessile.
Sponges also reproduce asexually, either by fragments that break off from the parent animal, or by gemmules, aggregations of amoeba like cells within a hard, protective outer layer. Production of such resistant forms is found, in general, only among freshwater organisms.
In the ocean, conditions are relatively unchanging, but the freshwater environment is much harsher. Invertebrates that live in fresh water are more likely to have protected embryonic forms than even closely related marine species.
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6. Phylum Coelenterata: Polyps and Jellyfish:
The coelenterates are a large and often strikingly beautiful group of aquatic organisms. Their adult-form is generally radially symmetrical; that is, their body parts are arranged around a central axis, like spokes around a hub. As you can see in figure 11.3, the basic body plan is a simple one: The animal is essentially a hollow container, which may be either, vase-shaped, the polyp, or bowl-shaped, the medusa. The polyp is usually sessile; the medusa, motile.
Both consist of two layers of tissue: ectoderm and endoderm (from the Greek ektos, “outer,” and endon, “inner,” plus derma, “skin”). Between the two layers is a gelatinous filling, the mesoglea (“middle jelly”), which is made of a collagen like material. In the polyp form, the mesoglea is sometimes very thin, but in the medusa, it often accounts for the major portion of the body substance.
Most coelenterates go through both a polyp and a medusa stage in their life cycles. In such species, polyps reproduce asexually and medusas sexually. This sort of life cycle, in which the sexually reproductive form is distinctly different from the asexual form, superficially resembles alternation of generations in plants.
There is, however, no alternation between haploid and diploid forms as there is in plants; the only haploid forms are the gametes.
Another distinctive feature of the animals in this phylum is the coelenteron, a digestive cavity with only one opening. Within this cavity, enzymes are released that break down food, partially digesting it extra-cellularly, as our own food is digested within the stomach and intestinal tract.
The food particles are then taken up by the cells lining the cavity they complete the digestive process and pass the products on to the other cells of the animal. Inedible remains are ejected from the single opening.
A third distinctive feature of coelenterates is the cnidoblast. Coelenterates are carnivores. They capture their prey by means of tentacles that form a circle around the “mouth.” These tentacles are armed with cnidoblasts, special cells that contain nematocysts (thread capsules).
Nematocysts are discharged in response to a chemical stimulus or touch. The nematocyst threads, which are often poisonous and may be sticky or barbed can lasso prey, harpoon it or paralyze it or some useful combination of all three. The toxin apparently produces paralysis by attacking the lipoproteins of the nerve cell membrane of the prey.
Cnidoblasts occur only in this phylum, with some interesting exceptions. Certain other invertebrates, including nudibranchs (a kind of mollusk) and flatworms, can eat coelenterates without triggering the nematocysts. The nematocysts then migrate to the surface of the predator and can be fired in their new host’s defense.
Classes of Coelenterates:
There are three major classes of coelenterates: Hydrozoa, in which the polyp is usually the dominant form; Scyphozoa, predominantly medusoid, exemplified by the common jellyfish; and Anthozoa, which includes the sea anemones and the reef-building corals, and has only the polyp.
Class Hydrozoa: Hydra:
One of the most thoroughly studied of the coelenterates is Hydra, which is a small, common freshwater form, convenient to keep in the laboratory. Figure 11.5 shows a small section of the body wall of Hydra. The ectoderm is composed largely of epitheliomuscular cells, which perform a covering, protective function and also serve as muscle tissue.
Each cell has contractile fibers, myonemes, at its base and so can contract individually, like the contractile epithelial cells of the sponge. The endoderm is mostly made up of cells concerned with digestion; these cells also contain contractile fibers.
In Hydra, as in other polyps, the contractile fibrils of the ectoderm attach lengthwise to the mesoglea and the fibrils of the endoderm cells attach transversely, so the body walls can stretch or bulge, depending on which group is stimulated.
In addition to cnidoblasts and epithelia muscular cells, which are independent effectors-cells that both receive and respond to stimuli- Hydra contain two other types of nerve cells: sensory receptor cells and cells connected into a network, the nerve net.
Sensory receptor cells are more sensitive than other epithelial cells to chemical and mechanical stimuli, and when stimulated they transmit their impulses to an adjacent cell or cells. The adjacent cell may be simply an epithelia muscular cell, an effector, which then responds.
Note that this system is one step more complicated than the epithelia muscular cell or cnidoblast, which acts as both receptor and effector. The nerve net, a loose connection of nerve cells lying at the base of the epithelial layers, is the simplest example of a nervous system that links an entire organism into a functional whole. It coordinates the muscular contractions of Hydra, making possible a wide variety of activities. However, there is no center of operations for the nervous system. This type of conducting system occurs in Hydra and certain other coelenterates.
Although Hydra has only the polyp form, many hydrozoans have both hydroid (polyp) and medusoid forms at different times in their life cycles. Coelenterates of the genus Obelia, for example, spend most of their lives as colonial polyps.
The colony arises from a single polyp, which multiplies by budding. The new polyps do not separate but remain interconnected so that their body cavities form a continuous channel, through which food particles are circulated.
Within the colony are two types of polyps: feeding polyps with tentacles and cnidoblasts, and reproductive polyps from which tiny medusas bud off. These medusas produce eggs or sperm that are released into the water and fuse to form zygotes.
Thus colonial polyps, with their division of labor between feeding and reproductive forms, are very like a single organism. Such a high degree of specialisation of structure and function among social organisms is seen in other phyla only among the social insects.
Class Scyphozoa: Jellyfish:
A second major class of coelenterates is Scyphozoa, or “cup animals,” in which the medusas form is dominant. Scyphozoans, more commonly known as jellyfish, range in size from less than 2 centimeters in diameter up to animals 4 meters across and trailing tentacles 10 meters long.
In the adult animal, the mesoglea is so firm that a large, freshly beached jellyfish can easily support the weight of a human being. The mesoglea of some jellyfish is filled with wandering, amoeba-like cells, which serve to transport food from the nutritive cells of the endoderm. Unlike Hydra, scyphozoans have true muscle cells; these underlie the ectoderm, contracting rhythmically to propel the medusa through the water.
Nervous System of Medusa:
In the medusa, there are concentrations of nerve cells in the margin of the bell. These nerve cells connect with fibers innervating (providing the nerve supply for) the tentacles, the musculature, and the sense organs.
The bell margin is liberally supplied with sensory receptor cells sensitive to mechanical and chemical stimuli. In addition, the jellyfish has two types of true sense organs- Statocysts and light-sensitive ocelli.
Statocysts are specialised receptor organs that provide information by which an animal can orient itself with respect to gravity. The statocyst, which seems to have been one of the first special sensory organs to have appeared in the course of evolution, has persisted apparently unchanged to the present day, appearing in many animal phyla.
Ocelli, which may have evolved even earlier, are groups of pigment cells and photoreceptor cells. They are typically located at the bases of the tentacles.
Class Anthozoa:
Anthozoans (“flower animals “)-the corals and sea anemones-are members of a class of coelenterates that, like Hydra, have lost the medusa stage. They differ from Hydra in having a gullet lined with epidermis and a coelenteron divided by vertical partitions.
In most corals, which are colonies of anthozoans, the epidermal cells secrete protective outer walls, usually of calcium carbonate (limestone), into which each delicate polyp can retreat. The coral- forming polyps are the most ecologically important of the coelenterates.
The 2,000-kilometer-long Great Barrier Reef off the northeast shores of Australia, and the Marshall Islands in the Pacific are examples of coral-created land masses.
A coral reef is composed primarily of the accumulated limestone skeletons of coral coelenterates, covered by a thin crust occupied by the living colonial animals. A reef is both the structural and nutritional basis of the complex coral reef community.
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7. Phylum Platyhelminthes: Flatworms:
The flatworms are the simplest animals, in terms of body plan, to show bilateral symmetry. In bilaterally symmetrical animals, the body plan is organised along a longitudinal axis, with the right half an approximate mirror image of the left half.
It also has a top and a bottom or, in more precise terms (applicable even when it is turned upside down or, as in the case of humans, standing upright), a dorsal and a ventral surface. Like most bilateral organisms, the flatworm also has a distinct “headness” and “tailness” anterior and posterior.
Having, one end that goes first-cephalization-is characteristic of actively moving animals. In such animals, many of the sensory cells are also collected into the anterior end. With the aggregation of sensory cells, there came a concomitant gathering of nerve cells; this gathering is a forerunner of the brain.
Flatworms have three distinct tissue layers-ectoderm, mesoderm, and endoderm-characteristic of all animals above the coelenterate level of organisation.
Moreover, not only are their tissues specialised for various functions, but also two or more types of tissue cells may combine to form organs- for example, the muscular pharynx. Thus, while sponges are made up of aggregations of cells and coelenterates are largely limited to the tissue level of organisation, flatworms can be said to exemplify the organ level of complexity. There are three classes of flatworms the free-living turbellaria and two parasitic forms, flukes and tapeworms.
The flatworms are believed by some zoologists to have evolved from the coelenterates (or, perhaps, vice versa), not by way of either adult form, however, but from the ciliated larval form.
Others are persuaded that the flatworms had independent origins among the ciliates. Still another group maintains that they are degenerate annelids. It is unlikely that this matter will soon be resolved.
The Planarian:
The free-living flatworms form a large and varied group, and we shall single out just one for examination, the freshwater planarian. The ectoderm of the planarian is made up of cuboid epithelial cells, many of which are ciliated, particularly those on the ventral surface.
Ventral ectodermal cells secrete mucus, which provides traction for the planarian as it moves by means of its cilia along its own slime trail. Planarians are among the largest animals that can use cilia for locomotion.
The cilia in larger species are usually employed for moving water or other substances along the surface of the animal, as in the human respiratory tract, rather than for propelling the animal.
The planarian has an endoderm composed largely of amoeboid cells, which, although they are phagocytic, are not wandering cells like the amoebocytes of the jellyfish. Between the ectoderm and the phagocytic endoderm is a mesoderm, or middle tissue layer.
In planarians, as in all other groups to follow, the muscle cells and the principal organ systems are of mesodermal origin.
The planarian, like the coelenterates, has a digestive cavity (gut) with only one opening, located on the ventral surface. This digestive cavity has three main branches, which is why planarians are placed in the order of flatworms known as Tricladida.
Like other flatworms, the planarian is carnivorous. It eats either dead meat or slow-moving animals it can fasten itself to or subdue by sitting on, such as smaller planarians. It feeds by means of a muscular tube, the pharynx, which is free at one end. The free end can be stretched out through the mouth opening.
Muscular contractions in the tube causes strong sucking movements, which tear the meat into microscopic bits and draw them into the internal cavity, where they are phagocytized by the endodermal cells.
Unlike the sponges or coelenterates, most flatworms have an excretory system. In the planarian, the system is a network of fine tubules that runs the length of the animal’s body. Side branches of the tubules contain flame cells, each of which has a hollow center in which a tuft of cilia beats, flickering like a tongue of flame, moving water along the tubules to the exit pores between the epidermal cells.
The flame-cell system appears to function largely to regulate water balance most of the metabolic waste products probably diffuse out through the ectoderm or the endoderm.
Planarians have a complicated reproductive system. The eggs are fertilised internally. At mating, each partner deposits sperm in the copulatory sac of the other partner. These sperm then travel along special tubes, the oviducts, to fertilise the eggs as they become ripe.
Organisms like planarians, in which both types of gametes are produced by one individual, are known as hermaphrodites (from Hermes and Aphrodite).
Solitary, slow-moving animals, such as earthworms and snails, are often hermaphrodites; these animals may seldom encounter another adult member of the species, but every such encounter can result in a mating. Some types of hermaphroditic animals can fertilise themselves, although they do not usually do so if another individual is present.
The Planarian Nervous System:
The evolution of bilateral symmetry brought with it marked changes in the organisation of the nervous system as well as of other systems. Even among primitive flatworms, the neurons (nerve cells) are not dispersed in a loose network, as in Hydra, but are instead condensed into longitudinal cords.
In the planarians, this condensation is carried further, and there are only two main conducting channels, one on each side of the flat, ribbon like body. These channels carry impulses to and from the aggregation of nerve cells in the anterior end of the body. Such aggregations of nerve cell bodies are known as ganglia (singular, ganglion).
The ocelli of the planarian are usually inverted pigment cups. They have no lenses, and they cannot form an image. However, they can distinguish light from dark and can tell the direction from which the light is coming.
Planarians are photonegative if you shine a light on a dish of planarians from the side, they will move steadily away from the source of light.
Among the epithelial cells are receptor cells sensitive to certain chemicals and to touch. The head region in particular is rich in chemoreceptors. If you place a small piece of fresh liver in the culture water so that its juices diffuse through the medium, the planarians will raise their heads off the bottom and, if they have not eaten recently, will lope directly and rapidly (on a planarian scale) toward the meat, to which they then attach themselves to feed.
The animal locates the food source by repeatedly turning toward the side on which it receives the stimulus more strongly until the stimulus is equal on both sides of its head. If the chemoreceptor cells are removed from one side of the head, the animal will turn constantly toward the intact side.
Planarians, with their simple nervous systems, their capacity to react to a variety of stimuli, and their powers of regeneration, have been the subject of a number of experiments. In one group, planarians trained to avoid electric shock were fed to untrained planarians.
The result, it was claimed, was that the untrained planarians that had eaten the trained ones behaved as if they had been trained. These experiments, still not verified, on the “transfer of training by cannibalism” led to a great deal of controversy in the 1960s and also, as you might expect, to a number of suggestions for more meaningful student- teacher relationships.
Tapeworms and Flukes:
Phylum Platyhelminthes includes also the tapeworms and the trematodes (flukes), parasitic forms that can cause serious and sometimes fatal diseases among vertebrates. Members of both of these parasitic classes have a tough outer layer of cells that is resistant to digestive fluids and, usually, suckers or hooks on their anterior ends by which they fasten to their victims.
Trematodes feed through a mouth, but the tapeworms, which have no mouths, digestive cavities, or digestive enzymes, merely hang on and absorb predigested food molecules through their skin. Tapeworms are found in the intestines of many vertebrates, including humans, and may grow as long as 5 or 6 meters.
They cause illness not only by encroaching on the food supply but also by producing wastes and by obstructing the intestinal tract. The most common human tapeworm, the beef tapeworm, infects people who eat the undercooked flesh of cattle that have eaten fodder contaminated by human feces containing tapeworm segments.
All parasites, including parasitic flatworms, are believed to have originated as free-living forms and to have lost certain tissues and organs (such as the digestive tract) as a secondary effect of their parasitic existence, while developing adaptations of advantage to the parasitic way of life. Such adaptations also often include a complex life cycle.
Term Paper # 8. Phylum Rhynchocoela: Ribbon Worms:
The ribbon worms (sometimes called nemertines), although a small phylum, are of special interest to biologists attempting to reconstruct the evolution of the invertebrates. They appear to be closely related to the flatworms, but with an important difference: They have a one-way digestive tract beginning with a mouth and ending with an anus.
This is a far more efficient arrangement than the one-opening digestive system of the coelenterates and flatworms. In the one-way tract, food moves assembly-line fashion, with the consequent possibilities- (1) that eating can be continuous and (2) that various segments of the tract can become specialised for different stages of digestion. The ribbon worms also have a circulatory system, usually consisting of one dorsal and two lateral blood vessels that carry the colourless blood.
This phylum is called Rhynchocoela (“beak” plus “hollow”) because these worms are characterised by a long, retractile, slime-covered tube (proboscis). The proboscis, sometimes armed with a barb, seizes prey and draws it to the mouth where it is engulfed. Some inject a paralyzing poison into their prey.
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9. Phylum Nematoda: Roundworms:
The number of species of roundworms (nematodes) has been variously estimated as low as 10,000 and as high as 400,000 to 500,000. Most are free-living, microscopic forms. It has been estimated that a spadeful of good garden soil usually contains about a million nematodes. Some are parasites; most species of plants and animals are parasitised by at least one species of nematodes.
Humans are hosts to about 50 species, including hookworms, pinworms, and Trichinella. This causes trichinosis, which is transmitted by eating uncooked or undercooked pork, a single gram of which may contain 3,000 cysts (resting forms) of Trichinella. Ingestion of only a few hundred of these cysts can be fatal to human beings.
Nematodes have a three-layered body plan and a tubular gut with a mouth and an anus. They are un-segmented and are covered by a thick, continuous cuticle, which is molted periodically as they grow.
An interesting, and unique, feature of nematode construction is the absence of circular muscles. The contraction of the longitudinal muscles acting against the tough, elastic cuticle gives the worm its characteristic whipping movement in water. The sexes are usually separate.
Nematodes may have evolved from early Platyhelminthes, possessing, as they do, a three-layered body plan without a true coelom. They have, however, what is known as a pseudocoelom, a body cavity that is between the endoderm and the mesoderm and lacks the epithelial lining of a true coelom.
Six other minor (in terms of species and numbers) phyla, mostly small, wormlike animals, have body plans based on the pseudocoelom, but Nematoda is the only major pseudocoelomate phylum.
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10. Phylum Annelida: Segmented Worms:
This phylum includes almost 9,000 different species of marine, freshwater, and soil worms, including the familiar earthworm. The term annelid means “ringed” and refers to the most distinctive feature of this group, which is the division of the body into segments, visible as rings on the outside and with partitions on the inside.
This segmented pattern is found in a modified form in higher animals, too, such as dragonflies, millipedes, and lobsters, which are thought to have evolved from the same ancestors that gave rise to modern annelids.
The annelids have a three-layered body plan, a tubular gut, and a well-developed circulatory system that transports oxygen (diffused through the skin or through fleshy extensions of the skin) and food molecules (from the gut) to all parts of the body.
The excretory system is made up of specialised paired tubules, nephridia, which occur in each segment of the body except the head. Annelids have a nervous system and a number of special sense cells, including touch cells, taste receptors, light-sensitive cells, and cells concerned with the detection of moisture. Some also have well-developed eyes and sensory antennae.
In the flatworms and ribbon worms, the mesoderm is packed solid with muscle and other tissues, but in the annelids there is a fluid-filled cavity, the coelom, (pronounced see-loam) in this middle layer. (Note that the term coelom, although it sounds similar to coelenteron and comes from the same Greek root, meaning “cavity,” refers quite specifically to a cavity within the mesoderm, whereas the coelenteron is a digestive cavity lined by endoderm.)
Within the coelom, the gut-lined with an epithelium-is suspended by double layers of mesoderm known as mesenteries. The fluid in the coelom constitutes a hydrostatic skeleton for the annelid, stiffening the body in somewhat the same way water pressure stiffens and distends a fire hose.
The muscles of the earthworm work against this hydrostatic skeleton much as our muscles work against our bony skeleton. Although the opening of a cavity within the mesoderm may seem less dramatic than other evolutionary innovations, it is extremely important.
Within such a space, organ systems can bend, twist, and fold back on themselves, increasing their functional surface areas and filling, emptying, and sliding past one another, surrounded by lubricating coelomic fluid. Consider the human lung, constantly expanding and contracting in the chest cavity, or the 6 or 7 meters of coiled human intestine; neither of these could have evolved until the coelom made room for them.
Earthworms:
The earthworm is the most familiar of the annelids. Figure 11.8 shows a portion of the body of an earthworm. Note how the body is compartmented into regular segments. Most of these segments, particularly the central and posterior ones, are identical, each exactly like the one before and the one after.
Each identical segment contains four pairs of bristles, or setae; two nephridia, excretory tubules that pick up waste materials from the body fluids and excrete them through pores on the ventral surface of the worm; four sets of nerves branching off from the central nerve cord running along the ventral surface; a portion of digestive tract; and a left and right coelomic cavity.
The chief exceptions to this rule of segmented structure are found in the most forward segments. In these, sensory cells, a cluster of nerve cells (ganglia), and specialised areas of the digestive, circulatory, and reproductive systems are found.
The tube like body is wrapped in two sets of muscles, one set running longitudinally and the other encircling the segments. When the earthworm moves, it anchors some of its segments by its setae, and the circular muscles of the segments anterior to the anchored segments contract, thus extending its body forward. Then its forward setae take hold, and the longitudinal muscles contract while the posterior anchor is released, drawing the posterior segments forward.
Digestion in Earthworms:
The digestive tract of the earthworm is a long, straight tube. The mouth leads into a strong, muscular pharynx, which acts like a suction pump, drawing in decaying leaves and other organic matter, as well as dirt, from which organic materials are extracted.
The earthworm makes burrows in the earth by passing such material through its digestive tract and depositing it outside in the form of castings, a ceaseless activity that serves to break up, enrich, and aerate the soil. The narrow section of tube posterior to the pharynx, the esophagus, leads to the crop, where food is stored.
In the gizzard, which has thick, muscular walls lined with protective cuticle, the food is ground up with the help of the ever-present soil particles. The rest of the digestive tract is made up of a long intestine, which has a large fold along its upper surface that increases its surface area. The intestinal epithelium consists of enzyme-secreting cells and ciliated absorptive cells.
Circulation in Earthworms:
In the protists and in the smaller and simpler animals, food molecules and oxygen are supplied to cells largely by diffusion, aided by the movement of external fluids and, sometimes, as we saw in coelenterates, by wandering, amoeboid cells.
A circulatory system that propels extracellular fluid around the body solves the problem of providing each cell with a more direct and rapid line of supply.
The circulatory system of the earthworm is composed of longitudinal vessels running the entire length of the worm, one dorsal and several ventral. The largest ventral vessel underlies the intestinal tract, collecting nutrients from it and distributing them by means of many small branches to all the tissues of the body and to the three smaller ventral vessels that surround the nerve cord and nourish it.
Numerous small capillaries in each segment carry blood from the ventral vessels through the tissues to the dorsal vessel. Also in each segment are larger parietal (“along the wall”) vessels transporting blood from the sub-neural vessels to the dorsal vessel. Fluids collected in this way from all over the animal’s body are fed into the muscular dorsal vessel, which propels the blood forward.
Connecting the dorsal and ventral vessels, and so completing the circuit, are five pairs of hearts, muscular pumping areas in the blood vessels. Their irregular contractions force the blood down to the ventral vessels and also forward to the vessels that supply the more anterior segments.
Both the hearts and the dorsal vessel have valves that prevent backflow. Note that the blood flows entirely through vessels. Such a system is known as a closed circulatory system. Evolution of a closed circulatory system, in effect, added a new compartment to the body plan and, in so doing, made possible a degree of control, not previously feasible, over the content of the circulating body fluid.
Excretory System of Earthworms:
The excretory system consists of pairs of tubules, the nephridia; one pair for each segment. Each nephridium consists of a long, convoluted tubule that terminates in a ciliated funnel opening into the coelomic cavity of the anteriorly adjacent segment.
Coelomic fluid is carried into the funnel by the beating of the cilia and is excreted through an outer pore. As the fluid makes its way through the long tubule, water, sugar, salts, and other needed materials are returned to the coelomic fluid through the walls of the tubule, while other materials are absorbed into the tubule for excretion.
Thus the excretory system is concerned not only with the problem of water balance, as are the contractile vacuoles of Paramecium and the flame cells of planarians, but also with the homeostatic regulation of the chemical composition of the body fluids.
Thus, in effect, one segment monitors and regulates this aspect of the physiology of its neighboring segment-a neat trick for integrating the animal as a whole.
Respiration in Earthworms:
The earthworm has no special respiratory organs; respiration takes place by simple diffusion through the body surface. The gases of the atmosphere dissolve in the liquid film on the surface of the earthworm’s body, which is kept moist by secreted mucus and excreted water.
Oxygen travels inward by diffusion, since the surface film, exposed to the oxygen-rich atmosphere, contains more oxygen than does the blood in the network of capillaries just underlying the body surface. The oxygen is consumed by body cells as the blood circulates.
Carbon dioxide moves out to the surface film and then into the air by the same principle. In fact, all gas exchange in animals, whether the organism is land-dwelling or water-dwelling, takes place across moist membranes.
Nervous System in Earthworms:
The earthworm has a variety of sensory cells. It has touch cells, or mechanoreceptors. These contain tactile hairs, which, when stimulated, trigger a nerve impulse. Patches of these hair cells are found on each segment of the earthworm. The hairs probably also respond to vibrations in the ground, to which the earthworm is very sensitive.
The earthworm does not have ocelli-as one might expect, since it lives most of its life in complete darkness-but it does have light-sensitive cells. Such cells are more abundant in its anterior and posterior segments, the parts of its body most likely to be outside of the burrow.
These cells are not responsive to light in the red portion of the spectrum, a fact exploited by anglers who search for worms in the dark using red-lensed flashlights.
Among the earthworm’s most sensitive cells are those that detect moisture. The cells are located on its first few segments. If an earthworm emerging from its burrow encounters a dry spot, it swings from side to side until it finds dampness; failing that, it retreats.
However, when the anterior segments are anesthetised, the earthworm will crawl over dry ground. The animal also appears to have taste cells. In the laboratory, worms can be shown to select, for example, celery in preference to cabbage leaves and cabbage leaves in preference to carrots.
Each segment of the worm is supplied by nerves that receive impulses from sensory cells and by nerves that cause muscles to contract. The cell bodies for these nerves are grouped together in clusters (ganglia). The movements of each segment are directed by a pair of ganglia.
Movement in each segment is triggered by movement in the adjacent anterior segment; thus a headless earthworm can move in a coordinated manner. However, an earthworm without its cerebral ganglia moves ceaselessly; in other words, cephalic ganglia modulate activity.
There are also, as in planarians, conducting channels made up of nerve fibers bound together in bundles, like cables, which run lengthwise through the body. These nerve fibers are gathered together in a double nerve cord that runs along the ventral surface of the body.
The nerve cords contain fast-conducting fibers that make it possible for the earthworm to contract its entire body very quickly, withdrawing into its burrow when disturbed.
Reproduction in Earthworms:
Earthworms are hermaphrodites. Two earthworms, held together by mucous secretions from the clitellum (a special collection of glandular cells), exchange sperm and separate. Two or three days later, the clitellum forms a second mucous sheath surrounded by an outer, tougher protective layer of chitin.
This sheath is pushed forward along the animal by muscular movements of its body. As it passes over the female gonopores, it picks up a collection of mature eggs, and then, continuing forward, it picks up the sperm deposited in the spermathecas.
Once the mucous band is slipped over the head of the worm, its sides pinch together, enclosing the now fertilised eggs in a small capsule from which the infants hatch.
Other Annelids:
Other annelids resemble the earthworm in that they are cylindrical worms divided into a series of similar segments, and they have a complex circulatory system of blood vessels, a main ventral nerve trunk, a complete digestive tube, and a coelom.
The phylum is usually divided into three principal classes:
1. Oligochaeta,
2. Polychaeta, and
3. Hirudinea.
Oligochaeta is the group that includes the earthworms and related freshwater species. The polychaetes, which are marine animals, differ from the earthworms and other oligochaetes in a number of ways.
The most striking difference is that they typically have a variety of appendages, including tentacles, antennae, and specialised mouthparts. Each segment contains two fleshy extensions, parapodia, which function in locomotion and also, because they contain many blood vessels, are important in gas exchange.
Many polychaetes live in elaborately fashioned tubes constructed in the mud or sand of the ocean bottom. Usually the sexes are separate, fertilization is external, and the larvae are free swimming. Hirudinea are the leeches, which have flattened, often tapered, bodies with a sucker at each end.
Bloodsucking leeches attach themselves to their hosts by their posterior sucker, and then, with their anterior sucker, either slit the host’s skin with their sharp jaws or digest an opening through the skin by means of enzymes. Finally, they secrete a special chemical (hirudin) into the host’s blood to prevent it from coagulating.
Term Paper #
11. Phylum Mollusca: Mollusks:
The mollusks constitute one of the largest phyla of animals, both in numbers of species and in numbers of individuals. They are characterised by soft bodies within a hard, calcium-containing shell, although in some forms the shell has been lost in the course of evolution, as in slugs and octopuses, or greatly reduced in size and internalised, as in squids.
There are three major classes of mollusks:
(1) The gastropods, such as the snails, whose shells are generally in one piece;
(2) The bivalves, including the clams, oysters, and mussels, which have two shells joined by a hinge ligament; and
(3) The cephalopods, the most active and most intelligent of the mollusks, including the cuttlefish, squids, and octopuses.
The basic molluscan body plan is shown in figure 11.9. This hypothetical animal was bilaterally symmetrical and segmented.
Among modern mollusks, only the chitons, a relatively small group, bear any obvious resemblance to the archetypal model, but modern mollusks, although diverse in size and shape, all have the same fundamental body plan.
There are three distinct body zones-a head-foot, which contains both the sensory and the motor organs; a visceral mass, which contains the organs of digestion, excretion, and reproduction; and a mantle, which hangs over and enfolds the visceral mass and which secretes the shell.
The mantle cavity, a space between the mantle and the visceral mass, houses the gills; the digestive, excretory, and reproductive systems discharge into it. Water sweeps into the mantle cavity (propelled in the bivalves by cilia on the gills), passing through the gills and aerating them. It then passes by the nephridia, gonopores, and rectum, which are always downstream from the gills.
Water leaving the mantle cavity carries excreta and, in season, gametes. The digestive tract is far more convoluted and so provides more working surface than that of the annelids.
In all mollusks, the digestive tract is extensively ciliated, with many different working areas. Food is taken up by the cells lining the digestive glands arising from the stomach and the anterior intestine, and then is passed into the blood.
A characteristic organ of the mollusk, found only in this phylum, and in all classes except the bivalves, is the radula, a tooth-bearing strap of movable pieces of chitinous tissue covering the tongue.
The radula apparatus, which operates with a rhythmic back-and- forth movement, serves both to scrape off algae and other food materials and also to convey them backward to the digestive tract. It is also used in combat.
Mollusks, have gills. To understand the basic plan of gill structure and function, it is necessary only to recall the moist epidermis of the earthworm, through which oxygen diffuses, and the blood vessel lying close beneath it, which transports the oxygen to other parts of the body. A gill is a structure with an increased amount of surface area, through which gases can diffuse, and a rich blood supply for transport of these gases.
Oxygen diffuses inward, along the gradient, because the cells of the animal have removed oxygen from the bloodstream by cellular respiration. Carbon dioxide, produced by cellular respiration, diffuses outward.
Mollusks have three-chambered hearts; two of the chambers (atria) collect oxygenated blood from the gills, and the third (the ventricle) pumps it to the oxygen-depleted tissue.
Except for the cephalopods, mollusks have what is known as an open circulation; that is, the blood does not circulate entirely within vessels-as it does in the earthworm, for example-but is oxygenated, pumped through the heart, and released directly into spaces in the tissues from which it returns, deoxygenated, to the gills and then to the heart.
Such a blood-filled space is known as a hemocoel (“blood cavity”). Cephalopods, which are extremely active animals, have accessory hearts that propel blood into the gills, and a closed circulatory system.
Class Bivalvia:
In the bivalves, the two-shelled mollusks, the body has become flattened between the two shells, and “headness” has generally disappeared. The bivalves are sometimes called Pelecypoda-“hatchet foot”—because the muscular foot is often highly developed in this group.
A clam, using its “hatchet foot,” can dig itself into sand or mud with remarkable speed. However, the bivalves are largely sessile forms, and many of them secrete strong strands of protein by which they anchor themselves to rocks.
Most bivalves are filter-feeding herbivores; they live largely on microscopic algae. Their gills, which are large and elaborate, collect food particles.
Water is circulated through the sieve-like gills by the beating of gill cilia. Small organisms and particles of food are trapped in mucus on the gill surface and swept toward the mouth by the cilia; the gills also sort particles by size, rejecting sand and other larger particles.
The shells are held together and opened at the hinge by a strong ligament and are drawn closed by one or two large muscles connecting the two shells.
Throughout the molluscan phylum, there is a wide range of development of the nervous system. The bivalves have three pairs of ganglia of approximately equal size-cerebral, visceral, and pedal (supplying the foot)-and two long pairs of nerve cords interconnecting them.
They have statocysts, usually located near the pedal ganglia, and sensory cells for discrimination of touch, chemical changes, and light. The scallop has quite complex eyes; a single individual may have a hundred or more eyes located among the tentacles on the fringe of the mantle.
The lens of this eye cannot focus on images, however, so it does not appear to serve for more than the detection of light and dark and movement.
Class Gastropoda:
The gastropods, which include the snails, whelks, periwinkles, abalones, and slugs, are the largest group of mollusks. They have either a single shell or, as a secondary evolutionary development, no shell. Another feature common to all members of this group, as compared with the ancestral mollusk, is that all of them have undergone torsion.
In other words, the internal organs, the shell, and the mantle have been twisted 180° so that in the modern animal, the mouth and anus and also the gills share the same comparatively small mantle cavity, now pointing forward instead of toward the rear.
Third, the stomach and digestive gland have become twisted upward into a spirally coiled visceral mass. In response to this displacement and consequent crowding of the internal organs, the gill and nephridium of the right side have been lost in many species.
In some close relatives of the snails, such as the slugs, the digestive tract has become straightened out again by another course of evolutionary events, in which the shell was lost but the missing organs were not regained.
Land-dwelling snails do not have gills but the area in their mantle cavities once occupied by gills is rich in blood vessels, and the snail’s blood is oxygenated there.
Some snails that were once land dwellers have returned to the water, but they have not regained gills. Instead, they must bob up to the surface at regular intervals to entrap a fresh bubble of air in their mantle cavities.
Thus the mantle cavity has, in effect, become a lung. Moreover, as with all lungs, the opening is reduced to retard evaporation. Gastropods, which lead a more mobile, active existence than bivalves, have a ganglionated nervous system with as many as six pairs of ganglia connected by nerve cords.
There is a concentration of nerve cells at the anterior end of the animal, where the tentacles, which have chemoreceptors and touch receptors, and the eyes, are located. In some of the animals, the eyes are quite highly developed in structure; they appear, however, to function largely in the detection of changes in light intensity, like the eyes of the scallop.
Class Cephalopoda:
The cephalopods (the “head-foots”) are the most highly developed mollusks. The large head has conspicuous eyes and a central mouth surrounded by arms, some 70 or 80 in the chambered nautilus, 10 in the squid, and 8 in the octopus.
The nautilus, as the only modem shelled cephalopod, offers an indication of some of the steps by which this class disposed of the shell entirely. The animal occupies only the outermost portion of its elaborate and beautiful shell, the rest of which serves as a flotation chamber.
In the squid and its relative, the cuttlefish, the shell has become an internal stiffening support, and in the octopus, it is lacking entirely.
The octopus body seldom reaches more than 30 centimeters in diameter (except on the late show), but giant squids sometimes attain sea-monster proportions. One caught in the Atlantic some hundred years ago was 15 meters long, not counting the tentacles, and was estimated to weigh 2 tons.
Freedom from the external shell has given the mantle more flexibility. The most obvious effect of this is the jet propulsion by which cephalopods dart through the water. Usually, water taken into the mantle cavity bathes the gills and is then expelled slowly through a tube-shaped structure, the siphon; but when the cephalopod is hunting or being hunted, it can contract the mantle cavity forcibly and suddenly, thereby squirting out a sudden jet of water.
Contraction of the mantle cavity muscles usually shoots the animal backward, head last, but the squid and the octopus can turn the siphon in almost any direction they choose. In addition to the siphon, cephalopods have sacs from which they can release a dark fluid that forms a cloud, concealing their retreat and confusing their enemies.
These coloured fluids were at one time a chief source of commercial inks. Sepia is the name of the genus of cuttlefish from which a brown ink used to be obtained.
The cephalopods have well-developed brains, composed of many groups of ganglia, in keeping with their highly developed sensory systems and their lively, predatory behaviour. These large brains are covered with cartilaginous cases.
The rapid responses of the cephalopods are made possible by a bundle of giant nerve fibers that control the muscles of the mantle. Many of the studies on conduction of the nerve impulse are made with the giant axon of the squid, which is large enough to permit the insertion of an electrode.
Evolutionary Affinities of the Mollusks:
Although the annelids and the mollusks are quite different in their basic body plans, there are some similarities between them that seem to suggest evolutionary links. One of these is the trochophore larva. Many of the annelids (the oligochaetes and hirudineans excepted) have this very distinct larval form.
Most marine mollusks (except the cephalopods) also pass through a trochophore stage in their development. Until fairly recently, the lack of unequivocal traces of segmentation in the mollusks seemed to argue against the evidence of close affinity provided by the trochophore.
In the 1950s, however, 10 living specimens of Neopilina, a genus of mollusks previously known only from Cambrian fossils, were dredged from a deep ocean trench off the coast of Costa Rica. Neopilina, which is little more than 2.5 centimeters long, resembles a combination of gastropod and chiton, with a single large shell but five pairs of gills, five pairs of retractor muscles, and six pairs of nephridia, all arranged in what seems to be a distinctly segmental pattern.
A third link between mollusks and annelids is the pattern of embryonic development-protostomes vs. deuterostomes.
Term Paper #
12. Phylum Echinodermata: Starfish:
The starfish and its relatives are known as echinoderms, or “spiny skins.” Adult echinoderms are radially symmetrical, like most coelenterates, although the symmetry is imperfect with some traces of bi-laterality in the adults and with bilaterally symmetrical larvae.
Starfish:
The most familiar of the echinoderms is the starfish, whose body consists of a central disk from which radiate a number of arms. Most starfish have five arms, which was the ancestral number, but some have more.
A starfish has no head, and any arm may lead in its sluggish, creeping movements along the sea bottom. The central disk contains a mouth on the ventral surface, above which is the stomach. Like all echinoderms, the starfish has an interior skeleton that typically bears projecting spines, the characteristic from which the phylum derives its name.
The skeleton is made up of tiny, separate calcium-containing plates held together by the skin tissues and-by muscles. Each arm contains a pair of digestive glands and also a nerve cord, with an eyespot at the end.
These latter are the only sensory organs, strictly speaking, of the starfish, but the epidermis contains thousands of neurosensory cells (as many as 70,000 per square millimeter) concerned with touch and chemoreception. Each arm also has its own pair of gonads, which open directly to the exterior through small pores.
The circulatory system consists of a series of channels within the coelomic cavity. Respiration is accomplished by many small fingerlike projections, the skin gills, which are protected by spines. Amoeboid cells circulate in the coelomic fluid, picking up the wastes and then escaping through the thin walls of the skin gills, where they are pinched off and ejected.
The water vascular, or hydraulic, system is a unique feature of the phylum. Each arm of a starfish contains two or more rows of water- filled tube feet. These tube feet are interconnected by a central ring and radial canals. Water filling the soft, hollow tubes makes them rigid enough to walk on.
Each tube foot connects with a rounded muscular sac, the ampulla. When the ampulla contracts, the water is forced under pressure through a valve into the tube foot; this extends the foot, which attaches to the substrate by its sucker. When, the muscles at the base of the tube feet contract, the animal is pulled forward.
If the tube feet are planted on a hard surface, such as a rock or a clam shell, the collection of tubes will exert enough suction to pull the starfish forward or to pull apart a bivalve mollusk, a feat that will be appreciated by anyone who has ever tried to open an oyster or a clam.
When attacking bivalves, which are its staple diet, the starfish averts its stomach through its mouth opening and then squeezes the stomach tissue through the minute space that the starfish has made between the bivalve shells. The stomach tissues can insinuate themselves through a slit as narrow as 0.1 millimeter and begin to digest the soft tissue of the prey.
Echinoderm Evolution:
The echinoderms are believed to have evolved from an ancestral, bilateral, mobile form that settled down to a sessile life, and then became radially symmetrical. The sea lilies represent this second hypothetical stage.
In the third evolutionary stage, some of the animals, as represented by the starfish and sea urchins, became mobile again. Following this line of reasoning, one might expect an eventual return to bilateral symmetry in this group, and, in fact, this is seen to some extent in the soft, elongated bodies of sea cucumbers.
The ancestral bilateral form, like most other hypothetical ancestors, was wormlike. It had a coelom and a one-way digestive tract. However, it differed from the ancestor of the mollusks and annelids in what zoologists consider a very fundamental way, its early embryonic development.
Among mollusks, annelids, and also arthropods, the early cell divisions of the zygote are spiral, occurring in a plane oblique to the long axis of the egg. In the echinoderms, the cleavage pattern is radial, parallel to and at right angles to the axis of the egg.
The second difference appears when the embryo becomes a hollow sphere of cells. In the embryos of both groups, an opening, the blastopore, appears. Among the mollusks, annelids, and arthropods, the mouth (stoma) of the animal develops at or near the blastopore, and this group is called the protostomes—”first the mouth.”
In the echinoderms, which are called deuterostomes, the anus forms at or near the blastopore and the mouth forms secondarily. The chordates, the phylum to which we vertebrates belong, share these characteristics with the echinoderms.
