PHYLUM NEMATHELMINTIIES 



ni 



this the alimentary 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 earlier 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 (Hstributing the digested 

 food some other means must exist, 

 and this is furnished by the body 

 canity. 



206. Classification. — The 

 phylum Nemathelminthes may be 

 considered as including three classes: 



1. Nematoda (nem a to' da; G., _- 



Process /eao/i'ngi fo nerve 



Mai 



Scrrcop/i 



Wuc/eas 



Coni-rcrcti'/e part 

 of ce/r 



nematos, thread, and eidos, form). — 

 Either parasitic or free-living. 



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 {kka.nthosM'- 

 alk'jG., akantha, thorn, and kephale, 

 head). — All parasitic. 



207. Free-living Nematodes. — Fig. 84.— a muscle cell from an ascaris. 



An inconceivably large number of These cells run longitudinally and are 



•^ " shown in cross section in big. 83. At the 



minute free-living threadw<jrms exist right are shown two sections of the same 



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 hab- 

 itats, are very resistant to drying and freezing, and are disseminated in 

 numerous ways. The number of species is now believed to be very 

 large, but to a great extent they are undoscribed. At the present time 

 these free-living nematodes are often called nemas (Fig. 85). 



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

 either blood or other juices from the host if the worm is a parasite; 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 absorption through the thin wall of the intestine into 

 the body cavity. By means of this cavity it is distributed throughout 

 the body. Within tissues it is passed from cell to cell. Kgestion takes 



cell to show how the appearances seen 

 in Fig. 83 are produced. {The cell from 

 Leuckart, "Parasilen des Menschen.") The 

 contractile part of the cell is indicated by 

 lines, the non-contractile part by stippling. 



