278 NBMATHELMINTHES. 



and receptacula of the generative products. The usually paired 

 ovarian tubes are distinguishable into the following regions : the 

 ovary where the eggs are developed; the oviduct along which they 

 pass to reach the uterus, a more dilated region where they are 

 fertilised and often pass through a part or the whole (viviparous 

 forms) of their development; the two uteruses lead into the single 

 vagina, which opens to the exterior on the ventral surface somewhat 

 in the anterior region of the body or near the middle, rarely near 

 the hind end. The male generative, apparatus, which contains 

 amoeboid spermatozoa without flagellum or vibratile appendage, 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 pro- 

 truded and retracted by a special muscular apparatus, and serve to 

 fasten the male body to the female during copulation. In many 

 cases (Strong ylidae) an umbrella-like bursa is added, or the terminal 

 portion of the cloaca can be protruded like a penis ( Trichina) ; in 

 such cases the cloacal aperture lies almost at the extreme end but is 

 still ventral (Acroplialli). 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. The 

 upper ends of the generative organs in both sexes (ovary and testis) 

 consist of a multinucleated cord in which cell limits are not dis- 

 cernible ; lower down cell limits become discernible, but all the 

 cells are attached by a stalk to a central protoplasmic cord the 

 rachis. In the oviduct and vas deferens the germ cells (progametes) 

 become entirely free and lie loose in the cavity of the duct, and 

 divide into the definite genital cells or gametes (ova or spermatozoa). 

 Development. The Nematoda for the most part lay 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 

 Nematodes 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 

 membrane. The segmentation is equal, and leads to the formation 

 of a kind of invaginate gastrula, from the two cell-layers of which 

 are developed the body-wall and the alimentary canal. The embryo 

 gradually assumes an elongated cylindrical form, and comes to lie 



