Characteristics of the Vertebrata. Ixxix 



the fifth and eighth pairs^ and from the spinal series, are distri- 

 buted. The nerves fuse with one surface of these plates, which is 

 electro-negative, whilst the other is electro-positive. 



In several species of the genus Serranus, a testis has been ob- 

 served overlying the ovary, and a similar hermaphroditism has 

 been observed occasionally in Cyprinoids, and in some other Fishes. 

 On the other hand, asymmetry is often, as in the Perch, seen to be 

 produced by the stunting of one or other of the (typically sym- 

 metrical) ovaries. The reproductive glands in the two sexes are 

 often so much alike externally, that an examination of their sub- 

 stance is necessary for deciding the sex to which they belong. 

 Sometimes, however, and especially at the breeding season, the 

 sexes may be distinguished by external differences. Fishes are 

 mostly oviparous, but are sometimes viviparous. Rudimentary in- 

 tromittent organs exist in the male Elasmohranchi'i , as the so-called 

 ' claspers.' Sexual congress or contact takes place in many ovi- 

 parous as well as in the viviparous Teleostei. 



The products of the generative glands sometimes find their way 

 into the water by extrusion, by an abdominal pore or pores, as in 

 Marsipobranchii, Anguilla, and the females of Salmonidae, from 

 the abdominal cavity into which they have been set free by dehis- 

 cence. But in most Teleostei, the walls of both the generative 

 glands are directly continuous with their ducts, so that neither ova 

 nor spermatozoa are ever set free into the peritoneal cavity. This 

 is the case also in Lepidosteus amongst the Ganoidei ; but in the 

 other members of that order, the sexual products of both kinds are 

 taken up after dehiscence by the open mouths of ' Fallopian tubes.' 

 In the Elasmohranchii, where the ovary is single, and the mouths 

 of the oviducts approximated, the lower parts of the oviduct are 

 specially modified to secrete the shell in the oviparous Sci/lliuni 

 and in the Rays, and to serve as a uterus in the viviparous species 

 of Squalidae. In the males of Elasmohranchii, as of all higher 

 Vertebrata, the testis is continuous on each side, with an epididymis 

 and a vas deferens. The lower segment of the vas deferens is 

 expanded into a vesicula seminalis. The Elasmohranchii have 

 large yolked ova with partial segmentation, and in Mustelus laevis, 

 the vessels on the yolk sac come into such relation with the ma- 

 ternal vessels on the walls of the uterus, as to form a sort of 

 placenta. In osseous Fishes, where the ova are very much 



