Body Cavities

The Acoelomate Phyla

The most primitive animal phylum is that of the sponges (phylum Porifera). Sponges have a single body cavity known as the spongocoel. The spongocoel is critical to the food gathering strategy of sponges. Water enters the organism through numerous small pores known as ostia. Small food particles are filtered from the water by cells in the sponge walls. The water then flows through the spongocoel and leaves through a large opening known as the osculum. The one-directional flow of water through the sponge is controlled by special flagellated cells which line the spongocoel.

The second most primitive animal phylum is generally considered to be the cnidarians (phylum Cnidaria), which includes the jellyfish, sea anemones, and hydras. Cnidarians are diploblastic, meaning that they have two distinct tissue layers, an ectoderm and endoderm, separated by a third layer called the mesoglea. Cnidarians are characterized by a single gastrovascular cavity, an internal body cavity that functions in digestion. The gastrovascular cavity has a single opening that serves as both mouth and anus, and is typically surrounded by tentacles that are responsible for food gathering.

The other acoelomate phyla are more advanced than the sponges and cnidarians since their species are characterized by bilateral (left-right) symmetry, as well as the presence of three distinct tissue layers, the ectoderm, mesoderm, and endoderm. In these acoelomate bilatarian phyla, there is no body cavity other than the gastrovascular cavity. The mesoderm is solid. Bilaterally symmetric acoelomates include such taxa as the flatworms (phylum Platyhelminthes) and the ribbon worms (phylum Nemertina).

The "Tube-within-a-Tube" Body Plan

Animal species with a secondary body cavity, either a pseudocoelom or a true coelom, have what is called a "tube-within-a-tube" body plan. The secondary body cavity lies between the two tubes. The outer tube (also called the body wall or the somatic tube) typically contains the sense organs and muscles. Structures of the body wall are generally under voluntary control and are most often involved in mediating between an organism and its external environment. The inner tube (also called the gut tube or visceral tube) typically includes structures that control an organism's internal environment. These tubes perform functions such as digestion, blood circulation, the maintenance of internal homeostasis, and reproduction. The functioning of elements of the visceral tube is generally under involuntary control.

Both the pseudocoelom and the coelom are fluid-filled body cavities that lie between the outer body wall and the inner tube of the digestive tract. The distinction between the two lies in the tissue layer origin of the walls of the cavity.

The Pseudocoelomates

The pseudocoelom is an internal body cavity that develops between the mesoderm and the endoderm. Developmentally, the pseudocoelom is the persistent blastocoel, or fluid-filled cavity, of the developmental stage known as the blastula stage.

Several phyla of invertebrate animals are characterized by pseudo-coeloms. These include the roundworms (phylum Nematoda) and the rotifers (phylum Rotifera), as well as a number of lesser-known phyla of wormlike animals.

At one point, all the pseudocoelomate taxa were grouped together in the phylum Aschelminthes. However, recent phylogenetic studies have shown that in all likelihood they do not form a monophyletic taxon (one that shares a common ancestor), and the group has since been split into numerous separate phyla. It is even possible that the pseudocoelom has evolved more than once.

The pseudocoelom serves many of the same functions as the true coelom, which is discussed below. However, certain complex physiological systems are found only in the eucoelomates. Unlike the eucoelomates, for example, the pseudocoelomates lack circulatory systems. Nevertheless, certain innovations made possible by a true coelom can also be found among the pseudocoelomates. In roundworms, for example, the pseudocoelom is fluid-filled and pressurized and functions as a hydrostatic (fluid dependent) skeleton.

The True Coelom and the Eucoelomates

The true coelom differs from the pseudocoelom in that it is lined on either side with cells originating from the mesoderm. It is filled with a fluid known as the coelomic fluid. The surfaces of the coelom are covered with a slick epithelial layer known as the peritoneum.

The cells of the peritoneum are responsible for regulating the transport of substances into and out of the coelom. The visceral peritoneum covers the viscera (internal organs). The parietal peritoneum lines the outer body wall. The lubricated surfaces of the peritoneum allow for the smooth, sliding motion of the organs within the coelom. The internal organs are suspended from the walls of the coelom by membranous sheets called mesenteries.

The coelom may be a single body cavity or may be divided into separate compartments during the course of development. In humans, the coelom is divided into three separate coelomic cavities. The pericardial cavity contains the heart, the pleural cavity contains the lungs, and the peritoneal cavity contains the rest of the viscera, including the digestive organs, liver, kidneys and excretory organs, and reproductive organs. The pleural and peritoneal cavities are separated by the pleuroperitoneal membrane and the diaphragm (which has a critical role in respiration), while the heart and lungs are separated by the pleuropericardial fold.

Species with a true coelom are called eucoelomates. Within the eucoelomates, two different groups may be distinguished based on the way in which the coelom forms during development. The protostomes are characterized by one method, called schizocoely, while the deuterostomes are characterized by another, called enterocoely. The method of coelom formation is in fact one of the key characteristics that separate these two important phylogenetic groups.

Two rival theories attempt to explain the evolutionary origin of the coelom. The acoelomate theory argues that the coelom evolved in an acoelomate ancestor. The enterocoel theory suggests that the coelom evolved from the gastric pouches of cnidarians. Current evidence appears to favor the enterocoel theory. It is unclear whether the pseudocoelomates represent an intermediate evolutionary stage between acoelomates and taxa with a true coelom, or whether pseudocoelomates evolved from an eucoelomate ancestor.

Development of the True Coelom

Among the eucoelomates, the coelom develops differently in the protostomes, which include the Annelida, the molluska, and the Arthropoda, and the deuterostomes, consisting of the Echinodermata, Hemichordata, and Chordata. This is one of a few key characteristics distinguishing these two subkingdoms.

In both protostomes and deuterostomes, coelom formation occurs directly after gastrulation, the developmental stage in which the three tissue layers—the ectoderm, mesoderm, and endoderm—become distinguished. In both groups, coelom development is closely linked to the origin of the mesoderm.

The protostomes are schizocoelous. In protostomes, the mesoderm develops during gastrulation as cells migrate from the already existing ectodermal and endodermal layers to form a solid mesodermal layer that lies between the ectoderm and endoderm. This solid mesodermal mass subsequently splits ("schizo-" "split") to form a hollow cavity that becomes the coelom.

The deuterostomes are enterocoelous. In deuterostomes, the mesoderm originates as outpocketings of the endodermal archenteron, or embryonic gut. The mesodermal pouches extend toward one another as they grow, ultimately joining to form a single cavity that becomes the coelom.

The Importance of the Coelom

The evolution of the coelom was a crucial step in the history of the Animalia. The presence of a secondary body cavity, and the acquisition of the tube-within-a-tube body plan, was critical for the evolution of increasing size and complexity within the animal kingdom.

One major disadvantage of having only a single body cavity is that muscle contractions, such as those necessary for locomotion, cause the gastrovascular cavity to become squeezed and distorted. This not only restricts the flow of nutrients and other materials but also makes the evolution of an effective circulatory system impossible. As a result, acoelomate taxa must rely either on diffusion or on muscle contractions for the transport of nutrients, respiratory gases, and waste products around the body. Both these transport strategies are considerably less efficient than those permitted by the evolution of a coelom.

Consequently, the body sizes, shapes, and complexities that can be supported are severely constrained. Many acoelomate groups are small in size and characterized by a flattened, elongated morphology, or shape, one that is suited to diffusion as a transport mechanism, and that makes a complex circulatory system unnecessary. Not surprisingly, the result is that acoelomate phyla all have comparatively simple body plans. Even the pseudo-coelomates are generally fairly small in size and lack circulatory systems. They have only simple locomotory behaviors, which in addition to motion, help to circulate nutrients in the pseudocoelom.

With the evolution of the coelom, the digestive and circulatory functions are separated, which allows for the possibility of separate specialization and improvement in efficiency. The evolution of the coelom permits the internal organs to grow, change shape, and shift in position. The coelom provides not only space but also protection for complex organ systems, because the fluid-filled environment helps to shield the internal organs from injury. The gastrovascular cavity becomes specialized for digestion alone, and the different portions of the digestive system can expand and contract during the processes of feeding and digestion. This increase in efficiency in both digestion and circulation allows for the support of larger body sizes and increased metabolic rates, both of which are prominent features in the evolution of the Animalia.

In some taxa, the coelom can serve other functions as well. In many species, the fluid-filled, pressurized coelom is important in providing hydrostatic support. This is important in the Annelida, for example, which have evolved a compartmentalized coelom, with one coelomic compartment present in each segment of the organism. Slugs are another example of a group that uses the coelom as a hydrostatic skeleton involved in support and locomotion. Echinoderms such as starfish are characterized by a well-developed water vascular system derived from the coelom, which functions in the locomotion of the tube feet.

Modification and Reduction of the Coelom

The coelom has been modified in different ways in different phyla. The compartmentalization of the coelom in some vertebrates, including humans, has already been discussed. The creation of the pleural cavities, which contain the lungs, through the development of the muscular diaphragm is essential to the respiratory strategy of mammals, providing yet another example of a physiological function for which the evolution of the coelom has been necessary.

The unusual segmented coelomic compartments of annelids have also been mentioned. In this case, segmentation affects numerous components of annelid anatomy, and the coelom is only one more instance of this.

In addition, the coelom has been reduced in several other phyla that have evolved other structures to perform its usual functions. Arthropods, for example, have a hard chitinous exoskeleton (external skeleton) that provides support. The coelom is significantly reduced in the group, persisting only as a cavity around the gonads and excretory organs. (The hemocoel, a component of the arthropod's open circulatory system that bathes the internal organs in blood, is unrelated.)

Mollusks also have a body plan in which the coelom is reduced, in this case to a small body cavity that encloses only the heart. It is likely that the sedentary lifestyle of mollusks such as bivalves made a fully developed coelom unnecessary. Like arthropods, mollusks also possess a large hemocoel that bathes the organs in nutrients.

SEE ALSO BODY PLAN.

Jennifer Yeh

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