Segmented flatworms

The populations of worms living in intertidal areas fall into two major groups flatworms and segmented worms. Of the... 

Segmented worms are much more advanced and complex than flatworms. The digestive systems of flatworms are one-way tubes sandwiched between two body walls. However, segmented worms have a space between their two body walls called the body cavity, or coelom, that represents an important evolutionary advance, one that provides a place for the body s internal organs. In segmented worms, organs are held in their proper places inside the coelom by a membrane, the peritoneum. 

Segmentation is an advance in animal evolution because segmented animals can increase in size by adding more body portions. In addition, segments can become specialized to carry out certain jobs. Flatworms are therefore limited in size as well as in the degree of specialization they can reach because they lack segments. 

Segmented worms have much more advanced digestive systems than flatworms do. A flatworm has one opening, a mouth, for food and wastes. A segmented worm has two openings, a mouth at one end and an anus at the other. The mouth opens to an esophagus that leads to a muscular pharynx. 

Worms are plentiful in the shallow intertidal waters. Both simple flatworms and more complex segmented worms can be found there. Because of its size, the milky flatworm may be the most obvious, but the shoreline also supports many other types, including the speckled flatworm and oyster flatworm. 

Less obvious, but just as numerous, are flatworms and segmented worms. Some species are free living, but many inhabit tubes just under the surface of the sand or mud. Crawling slowly among theses relatively simple animals are the larger arthropods—mollusks and echinoderms. 

There are two types of flatworms the flukes, whose appearance can best be described as lanceolate or leaflike, and the tapeworms, which are ribbonlike. The latter consist of repeating egg-containing segments, or proglottids (numbering up to several thousand) linked in a chain called a strobilia and attached to a head or scolex. They have no digestive system. Their nutrients are absorbed directly from the host s intestines they are true parasites. 

The flatworms are normally of two kinds, namely segmented (trematodes). 

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