NEMATODA. 



277 



Sn- 



.Rn. 



the excretory pore, and sending off posteriorly six and anteriorly six 

 nerve trunks (Ascaris megalocepTiala). The two largest posterior 

 trunks run in the dorsal and ventral lines (N. dorsalis, ventralis), 

 the four smaller, two in each lateral line, to the extremity of the 

 tail ; while of the six anterior nerves, two run in the "lateral lines 

 (N. laterales), and four in the interspaces between the lateral and 

 median lines (N. submediani) ; they supply 

 the papillae around the mouth. The gan- 

 glion cells lie partly near, in front of and 

 behind the nerve ring, partly on the fibrous 

 cords themselves, and are arranged in groups 

 which can be distinguished as ventral, dorsal, 

 and lateral ganglia. There are in addition 

 groups of ganglion cells in the median lines 

 and in the lateral lines in the caudal region. 



As sense organs we must mention the 

 eyes found in the free-living Nematoda, and 

 the papillae and tactile hairs found princi- 

 pally in the neighbourhood of the mouth. 

 Each papilla is supplied by one nerve fibre, 

 which is swollen to a knob and forms the 

 axis of the papilla. 



The body -cavity is a continuous space 

 between the longitudinal muscles of the 

 body-wall and the outer cuticle of the gut- 

 wall ; it contains a corpusculated and colour- 

 less fluid. The homology of the cavity is 

 doubtful ; it occurs between the muscles and 

 the endoderm, and it has no relation to the 

 gonads or excretory organs. There is no vas- 

 cular system. 



Generative organs. The Nematodes are 

 dioecious (with exception of the hermaphro- 

 dite Pelodytes, and of RJiabdonema (Ascaris) 

 nigrovenosum and Allantonema mirabile, 

 which produce first spermatozoa and later 



ova). The males are characterised by their smaller size, and by 

 the posterior end of the body being generally curved. Both kinds 

 of generative organs consist of a single tube or of paired and 

 often much coiled tubes, at the upper end of which the generative 

 products are developed, the lower ends representing the efferent ducts 



FIG. 228. Nervous system of 

 Nematodes, diagrammatic 

 after Butschli. C lateral 

 ganglion on the nerve ring ; 

 S anterior lateral nerve ; 

 Sm submedian nerve ; SI 

 sublateral nerve ; Bn ven- 

 tral nerve ; Rn dorsal 

 nerve ; Ag anal ganglion ; 

 A anus. 



