312 SYNOPTIC COLLECTION. 



The class illustrates great variation in the process of 

 development, not less than fourteen different modes hav- 

 ing been described by von Linstow. Accompanying this 

 widespread variation there is a marked development of 

 adaptive characters, the cause lying in the fact that the 

 Nematodes " display a variation in the conditions under 

 which they live greater than that of any other group of 

 Helminths." l For these reasons they are placed among 

 the more specialized of the subkingdom of Vermes, 

 although, not at the extreme end of the genealogical 

 record, since their structure is not profoundly modified 

 and the adults have not departed so far from their young 

 as those of the Acanthocephala, Trematodes, or Cestodes, 

 to be described farther on. 



It would seem, indeed, as though the parasitic habit 

 had not been long acquired by the members of this class, 

 and that therefore the effects of this habit had not become 

 fixed in the organization to such a degree as to cause its 

 complete modification. 



The hair worm, Gordius (PI. 774; No. 775, 9), lays 

 its eggs in water where a portion of the first larval life is 

 spent. 



The embryo of Gordius subbifurcus while still within 

 the egg (PI. 774, fig. i) has the body divided into two 

 regions, both of which are segmented (clearly seen in fig. 

 3). At the anterior end the proboscis forms; this is 

 retracted in fig. 2 but is thrown out in fig. 3 ; the latter 

 figure shows also the well developed head. In this con- 

 dition the embryo escapes from the egg and swims freely 

 about. 



Some members of the family to which Gordius belongs 

 have two larval stages. The first is segmented and has 

 the body divided into a distinct head, middle, and poste- 

 rior region ; but the second stage, though still segmented, 

 is without a head. These immature stages are mostly 



1 Leuckart, Parasites of Man (translation), 1886, p. 53. 



