. Animal biology. Zoology; Biology. 180 METAZOAN PHYLA. seems to be effectively distributed by the much-branched gastrovascular cavity of the turbellarian, the increasing development of the mesoderm makes such distribution correspondingly more difficult. In contrast to this the aUmentary canal, open at the two ends and running the length of the body, makes it possible for the animal to feed continuously, taking food at the mouth and passing it gradually through the intestine. Here each successive increment is kept separate, at least to a considerable degree, from the food taken eariier and lat
. Animal biology. Zoology; Biology. 180 METAZOAN PHYLA. seems to be effectively distributed by the much-branched gastrovascular cavity of the turbellarian, the increasing development of the mesoderm makes such distribution correspondingly more difficult. In contrast to this the aUmentary canal, open at the two ends and running the length of the body, makes it possible for the animal to feed continuously, taking food at the mouth and passing it gradually through the intestine. Here each successive increment is kept separate, at least to a considerable degree, from the food taken eariier and later, and the feces from each are egested in due time. An alimentary canal, however, will be most effec- tive if it is a straight unbranched tube, and it seems to be a significant fact that when we come upon such an enteron it is at once divested of all branches. In the absence of this means of distributing the digested food some other means must exist, and this is furnished by the body cavity. 208. Classification.—The phylum Nemathelminthes may be considered as including three classes: 1. Nematoda (nem a to' da; G., nematos, thread, and eidos, form).— Either parasitic or free-Hving. 2. Gordiacea (gor di a'she a; L., gordius, referring to a complicated knot).—Parasitic in the larval stages and free-living and aquatic as adults. 3. Acanthocephala (a kan tho sef'- a la; G., akantha, thorn, and kephale, head).—All parasitic. 209. Free-living Nematodes.— contractile part of the cell is indicated by ^y^ inCOnCcivably large number of lines, the nou-contractile part by stippling. , „ t • j i i mmute free-living threadworms exist in the soil, in sand, mud, and debris from standing and running water, and in the sea. They are thus adapted to a great variety of habitats, are very resistant to drying and freezing, and are disseminated in numer- ous ways. The number of species is now beUeved to be very large, but to a great extent they are undescribed. At the present time these
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