512 



FRESH-WATER BIOLOGY 



ent general discussion to refer prominently only to points of con- 

 trast or to features peculiar to parasitic forms. For further 

 structural details the student should consult that discussion which 

 should be read in connection with the following description. Some 

 parasitic nematodes are apparently indistinguishable from free- 

 living species, others are classed in the same genera or families, 

 but there are also large groups that contain no free-living species 

 and are highly modified for a parasitic existence. In general the 

 smaller transparent species show the greatest similarity to the free- 



FiG. 8i2. Ascaris lumhricoides . a, top view of head; dorsal Up with two sensory papillae and ventra 

 lips with one each; the shaded areas indicate the muscle attachment, b, lateral view, showing ventral 

 lips. Magnified. (After Leuckart.) Camallanus ancylodirus. c, ventral view of head, X i35; <^> lateral view 

 of head, X 135. (Original.) Necator americanus. e, head of young male, dorsal view, X 160; /, head of 

 young female, from the right, X 160. (After Looss.) 



living species whereas the large opaque forms depart most widely 

 from that type. In general organology, microscopic structure of 

 cells and their arrangement in layers, as well as in fundamental 

 features of reproduction and development, the parasitic nema- 

 todes agree substantially with the free-living forms and manifest 

 their recent differentiation from them. 



The anterior end or "head" of a nematode is usually slightly 

 truncated or bluntly rounded and shows under a lens the presence 

 of lips, papillae, spines, teeth and other special structures. 



In reality the numerous modifications of the anterior end may 



