Body Cavities in the Bilateria Help
Introduction to Body Cavities in Bilateria
Remember that the bilateria are eumetazoans with true body tissues, as well as mature bodies that have the form of bilateral symmetry. These mature bodies grow from embryos having all three germ layers.
Acoelomates: No Main Body Cavity
In all animals except sponges, there is an archenteron present in the embryo. This archenteron is a hollow tube essentially representing the beginning structure of the digestive tract. All animals must have such a digestive tract, because they are heterotrophs consuming food which provides them with needed energy. Since metabolism is never 100% efficient, some of the ingested food matter is excreted through the anus ( AY -nus) as feces.
A further way of classifying the bilateral invertebrates can now be considered. It is the answer to the following question: “Does the animal’s body contain a coelom ( SEE -loam) – central body “cavity” – around its archentereon (digestive tract), or not?”
The additional way of classifying animals thus becomes one of distinguishing the coelomates ( SEE -luh- mates ) from the acoelomates ( ay - SEE -luh-mates). The coelomates, quite obviously, are those bilateral invertebrates whose bodies contain a coelom (central cavity), whereas the acoelomates have no central body cavity.
The distinction becomes clear when one examines Figure 10.4. The acoelomates are generally considered the more primitive or most ancient organisms (according to the Fossil Record). Representative of the acoelomates are the planaria (plah- NAIR -ee-uh). The planaria are free-swimming flatworms that have a solid body (containing no coelom around their digestive tube). The planaria are carnivores. They catch and eat smaller animals, and feed on dead organisms in the water. The planaria have amazing powers of regeneration . This means that they are able to re-grow large portions of their bodies when they are cut off and removed. This trait has made them a valuable research tool for biologists seeking to learn how to promote regeneration of lost or damaged human body parts.
Study suggestion: Compare the name planaria with similar words like plantar and plaintain. From a look at Figure 10.4 (A), after what specific characteristic is the planaria worm named?
Pseudocoelomates - "False" Coelom Cavity
Midway between the acoelomates and the coelomates is a very large group of invertebrates called the pseudocoelomates ( SOO -doh- see -luh-mates). This group is so large because it contains over 90,000 known species of nematodes ( NEM -ah-toads). Nematodes are slender, “thread” ( nemat )-“shaped” (- ode ) worms. The nematodes (threadworms) are alternately called the roundworms . But their bodies tend to be narrow and cylinder-shaped, and they are often tapered at either end (Figure 10.4, B).
Nematodes - Roundworms and Threadworms
The nematodes (roundworms, threadworms) are classified as pseudocoelomates because their bodies contain a “false” ( pseudo -) “cavity” ( coelom ). These slender worms have a complete digestive tube, which extends from the mouth all the way to the anus. Their muscles run lengthwise through their entire bodies, which are nonsegmented (not divided into small segments). But there is no “true” coelom (cavity with an actual lining of mesoderm) between the muscles and the digestive tube. There is, instead, a pseudocoelom cavity that is filled with fluid, but is not lined with mesoderm.
Coelomates - "True" Coelom Cavity
Finally, we turn to the coelomates. The coelomates are commonly represented by the annelids ( AN -eh- lids ) – the worms with many “little rings” of muscles encircling their bodies. The most familiar annelid is the common earthworm (Figure 10.4, C).
The earthworm is considered a segmented worm, due to the division of its body into numerous ring-like segments. The fluid-filled space surrounding its digestive tract is a true coelom, because it is lined by cells from the mesoderm. Earthworms are probably the most frequently studied of all coelomates in introductory biology classes. But human beings, like earthworms, are also coelomates! In both annelids (such as earthworms) and humans, the fluid within the coelom acts as a valuable shock absorber, cushioning the internal organs from blows hitting the outer body surface. It also moistens and lubricates them, reducing their friction and rubbing during body movements.
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