136 NEMATHELMINTHES 



coides take weeks or even months, and the young of Tricho- 

 cephalus seldom develop within a year. 1 The ova only develop 

 in a damp atmosphere, and they can be arrested at almost any 

 stage, and for considerable periods, by desiccation. 



Our knowledge of the processes by which the fertilised egg- 

 cell develops into the larva is very imperfect. As a rule the 

 segmentation is complete and equal ; it results in the formation 

 of a blastula, which may take the form of a hollow sphere of 

 cells A. megalocephala or the cavity may be reduced, and the 

 blastula may consist of a double-layered plate, as in Cucullanus. 2 

 The distinction into cells which will form the three embryonic 

 layers, the ectoderm, mesoderm, and endoderm, is very early 

 evident, in the eight-cell stage. By the growth of one side of 

 the blastula and the tucking in of the other the blastula becomes 

 converted into a gastrula, which is a two-layered stage with a 

 cavity opening to the exterior by a pore termed the blastopore. 

 In Nematodes the blastopore is elongated and slit-like ; it either 

 forms the mouth (Cucullanus) or closes from behind forwards, the 

 mouth ultimately arising at the point where the blastopore finally 

 closed (Khabdonema nigrovenosuin). The mesodermal cells lie 

 between the ectoderm and the endoderni ; they ultimately develop 

 into the muscles of the body- wall, the lateral excretory canals, and 

 the reproductive organs ; the last-named two systems arise each 3 

 from a single cell. The nervous system arises from the ectoderm, 

 which also forms the sub-cuticle, and is turned in slightly at the 

 mouth and anus ; the remainder of the alimentary canal develops 

 from the endoderm. 



The post-embryonic development, which is very variable, and 

 in many cases very extraordinary, will be dealt with under the 

 several families. 



Classification. The classification of the Nematodes is a 

 matter of very considerable difficulty; their structure is unusually 

 monotonous, and, owing perhaps to their largely parasitic mode 

 of life, they show practically none of those external features 

 which are so useful to the systematist in other groups. Schneider 

 in his Monograph divides the group into three subdivisions (i.) 



1 Leuckart, The Parasites of Man, English Trans, by "\V. E. Hoyle, Edinburgh, 

 1886, p. 56. 



2 0. Biitschli, Zeitschr. wiss. Zool. Bd. xxvi. 1876, ]>. 103. 



3 0. Hamann, Centrlb. Bakter. vol. xi. 1892, p. 501. 



