Unit Five. Evolution of Animal Life


19. Evolution of the Animal Phyla


19.7. Roundworms: The Evolution of a Body Cavity


A key transition in the evolution of the animal body plan was the evolution of the body cavity. All bilaterally symmetrical animals other than solid worms have a cavity within their body. The evolution of an internal body cavity was an important improvement in animal body design for three reasons:

1. Circulation. Fluids that move within the body cavity can serve as a circulatory system, permitting the rapid passage of materials from one part of the body to another and opening the way to larger bodies.

2. Movement. Fluid in the cavity makes the animal’s body rigid, permitting resistance to muscle contraction and thus opening the way to muscle- driven body movement.

3. Organ function. In a fluid-filled enclosure, body organs can function without being deformed by surrounding muscles. For example, food can pass freely through a gut suspended within a cavity, at a rate not controlled by when the animal moves.


Kinds of Body Cavities

There are three basic kinds of body plans found in bilaterally symmetrical animals. Acoelomates, such as solid worms that we discussed in the previous section and that are shown at the top of figure 19.13, have no body cavity. Pseudocoelomates, shown in the middle of the figure, have a body cavity called the pseudocoel located between the mesoderm (red layer) and endoderm (yellow layer). A third way of organizing the body is one in which the fluid-filled body cavity develops not between endoderm and mesoderm but rather entirely within the mesoderm. Such a body cavity is called a coelom (the two arch-shaped cavities in the worm at the bottom of the figure), and animals that possess such a cavity are called coelomates. In coelomates, the gut is suspended, along with other organ systems of the animal, within the coelom; the coelom, in turn, is surrounded by a layer of epithelial cells entirely derived from the mesoderm.



Figure 19.13. Three body plans for bilaterally symmetrical animals.

Acoelomates, such as flatworms, have no body cavity between the digestive tract (endoderm) and the outer body layer (ectoderm). Pseudocoelomates have a body cavity, the pseudocoel, between the endoderm and the mesoderm. Coelomates have a body cavity, the coelom, that develops entirely within the mesoderm, and so is lined on both sides by mesoderm tissue.


The development of a body cavity poses a problem—circulation—solved in pseudocoelomates by churning the fluid within the body cavity. In coelomates, the gut is again surrounded by tissue that presents a barrier to diffusion, just as it was in solid worms. This problem is solved among coelomates by the development of a circulatory system. The circulating fluid, or blood, carries nutrients and oxygen to the tissues and removes wastes and carbon dioxide. Blood is usually pushed through the circulatory system by contraction of one or more muscular hearts. In an open circulatory system, the blood passes from vessels into sinuses, mixes with body fluid, and then reenters the vessels later in another location. In a closed circulatory system, the blood remains separated from the body fluid in a network of vessels that can be separately controlled. Also, blood moves through a closed circulatory system faster and more efficiently than it does through an open system.

The evolutionary relationship among coelomates, pseudocoelomates, and acoelomates is not clear. Acoelomates, for example, could have given rise to coelomates, but scientists also cannot rule out the possibility that acoe- lomates were derived from coelomates. The two main phyla of pseudocoelomates do not appear to be closely related.


Roundworms: Pseudocoelomates

As we have noted, all bilaterally symmetrical animals except solid worms possess an internal body cavity. Among them, seven phyla are characterized by their possession of a pseudocoel. In all pseudocoelomates, the pseudocoel serves as a hydrostatic skeleton—one that gains its rigidity from being filled with fluid under pressure. The animal’s muscles can work against this “skeleton,” thus making the movements of pseudocoelomates far more efficient than those of the acoelomates.

Only one of the seven pseudocoelomate phyla includes a large number of species. This phylum, Nematoda, includes some 20,000 recognized species of nematodes, eelworms, and other roundworms. Scientists estimate that the actual number might approach 100 times that many. Members of this phylum are found everywhere. Nematodes are abundant and diverse in marine and freshwater habitats, and many members of this phylum are parasites of animals and plants, like the intestinal roundworm in figure 19.14a. Many nematodes are microscopic and live in soil. It has been estimated that a spadeful of fertile soil may contain, on the average, a million nematodes.

A second phylum consisting of animals with a pseudo-coelomate body plan is Rotifera, the rotifers. Rotifers are common, small, basically aquatic animals that have a crown of cilia at their heads, which can just barely be seen in figure 19.14b; they range from 0.04 to 2 millimeters long. About 2,000 species exist throughout the world. Bilaterally symmetrical and covered with chitin, rotifers depend on their cilia for both locomotion and feeding, ingesting bacteria, protists, and small animals.



Figure 19.14. Pseudocoelomates.

(a) These nematodes (phylum Nematoda) are intestinal roundworms that infect humans and some other animals. Their fertilized eggs pass out with feces and can remain viable in soil for years.

(b) Rotifers (phylum Rotifera) are common aquatic animals that depend on their crown of cilia for feeding and locomotion.


All pseudocoelomates lack a defined circulatory system; this role is performed by the fluids that move within the pseu- docoel. Most pseudocoelomates have a complete, one-way digestive tract that acts like an assembly line. Food is broken down, absorbed, and then treated and stored.


Phylum Nematoda: The Roundworms

Nematodes are bilaterally symmetrical, cylindrical, unsegmented worms. Shown in longitudinal and cross sections in the Phylum Facts illustration on the facing page, they are covered by a flexible, thick cuticle, which is molted as they grow. Their muscles constitute a layer beneath the epidermis and extend along the length of the worm, rather than encircling its body. These longitudinal muscles, which can be seen in the cross section attaching to the outer layer of the body, pull both against the cuticle and the pseudocoel, which forms a hydrostatic skeleton. When nematodes move, their bodies whip about from side to side.

Near the mouth of a nematode, at its anterior end (toward the left side of the diagram), are usually 16 raised, hairlike sensory organs. The mouth is often equipped with piercing organs called stylets. Food passes through the mouth as a result of the sucking action of a muscular chamber called the pharynx. After passing through a short corridor into the pharynx, food continues through the other portions of the digestive tract, where it is broken down and then digested. Some of the water with which the food has been mixed is reabsorbed near the end of the digestive tract, and material that has not been digested is eliminated through the anus.

Nematodes completely lack flagella or cilia, even on sperm cells. Reproduction in nematodes is sexual, with sexes usually separate (a female with a uterus, ovary, and oviducts is shown in the Phylum Facts illustration). Their development is simple, and the adults consist of very few cells. For this reason, nematodes have become extremely important subjects for genetic and developmental studies. The 1-millimeter-long Caenorhabditis elegans matures in only three days, its body is transparent, and it has only 959 cells. It is the only animal whose complete developmental cellular anatomy is known, and the first animal whose genome (97 million DNA bases encoding over 21,000 different genes) was fully sequenced.

Some nematodes are parasitic in humans, cats, dogs, and animals of economic importance, such as cows and sheep. Heartworm infections in dogs and cats are caused by a parasitic nematode that infests the heart of the animal. About 50 species of nematodes, including several that are rather common in the United States, regularly parasitize human beings. Trichinosis, a nematode-caused disease in temperate regions, is caused by worms of the genus Trichinella. These worms live in the small intestine of pigs, where fertilized female worms burrow into the intestinal wall. Once it has penetrated these tissues, each female produces about 1,500 live young. The young enter the lymph channels and travel to muscle tissue throughout the body, where they mature and form cysts. Infection in human beings or other animals arises from eating undercooked or raw pork in which the cysts of Trichinella are present. If the worms are abundant, a fatal infection can result, but such infections are rare.



A more prevalent human parasitic nematode is Ascaris lumbricoides. This intestinal worm infects approximately one in six people worldwide but is rare in areas with modern plumbing. These worms live in the intestines and spread their fertilized eggs in feces, which can remain viable in the soil for years. Adult females, which are up to 30 centimeters long, contain up to 30 million eggs, and can lay up to 20,000 of them each day.


Key Learning Outcome 19.7. Some body cavities develop between the endoderm and mesoderm (pseudocoelomates), others within the mesoderm (coelomates). Roundworms have a pseudocoel body cavity. Nematodes, a kind of roundworm, are very common in soil, and several are parasites.