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 free- 

 living nematodes are often called nemas (Fig. 91). 



210. Metabolism. — The food of roundworms is mostly fluid, being 

 either blood or other juices from the animal host; the juices of plants; or 

 water containing microorganisms or organic matter in solution if the 

 animal is free-living. This liquid food is pumped into the alimentary 

 canal by the pharynx, is digested in the intestine, and is freely passed by 



Process /ectcfi'n^ /<? nerve 



Scrrcop/^rsm 



Wuc/ec/^ 



'Con-f-rcrctj'/e pa/^f 





of ce/l 



Fig. 90. — A muscle cell from an ascaris. 

 These cells run longitudinally and are 

 shown in cross section in Fig. 89. At the 

 right are shown two sections of the same 

 cell to show how the appearances seen 

 in Fig. 89 are produced. {The cell from 

 Leuckart, " Parasiten des Menschen.") The 



