NEMATODA. 347 



(Ascaris) niyrovenosum, which produces 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 single or paired and often much coiled 

 tubes, at the upper end of which the generative products are de- 

 veloped, the lower ends representing the efferent ducts and recep- 

 tacula of the generative products. The usually paired ovarian tubes, 

 at the upper ends of which the ova arise, terminate in a short 

 vagina, which opens on the ventral surface, rarely near the posterior 

 end of the body. The male generative apparatus, which contains 

 hat-shaped spermatozoa, is almost invariably represented by an 

 unpaired tube, and usually opens on the ventral surface near the 

 posterior end of the body in a common opening with the intestine. 

 As a rule, the common cloacal portion contains two pointed chitinous 

 rods, the so-called spicula, in a pouch-like invagination. These 

 spicula can be protruded and retracted by a special muscular ap- 

 paratus, and serve to fasten the male body to the female during 

 copulation. In many cases (Strong ylidce) an umbrella-like bursa is 

 added, or the terminal portion of the cloaca can be protruded like 

 a penis (Trichina) ; in this case the cloacal aperture lies almost at 

 the extreme end but is still ventral (Acrophalli). In the male 

 papillae are almost always present in the region of the posterior end 

 of the body, and their number and arrangement afford important 

 specific characters. 



Development. The Nematoda for the most part lays eggs ; it is 

 only in rare cases that they bear living young. The eggs usually 

 possess a hard shell and may be laid at different stages of the 

 embryonic development or before it has begun. In the viviparous 

 ISTematodes the eggs lose their delicate membranes in the uterus of 

 the mother (Trichina, Filaria). Fertilization takes place by the 

 entry of a spermatozoon into the ovum, which is still without a mem- 

 brane. The segmentation is equal, and leads to the formation of 

 a kind of invaginate gastrula. From the two cell layers are de- 

 veloped the body wall and the alimentary canal. The embryo 

 gradually assumes an elongated cylindrical form, and comes to lie 

 rolled up in several coils within the shell. The excretory pore and 

 the rudiments of generative organs, as well as a nerve ring, are 

 present in the embryo, which is also provided with mouth and 

 anus. The free development is a metamorphosis, usually com- 

 plicated by the circumstance that it is not undergone in the habitat 

 of the mother. The young stages or larvae, probably of most IsTerna- 



