MODES OF REPRODUCTION 315 



in length. Comparatively few of the shark-like fishes (Selachii) 

 in addition to Scyllium are known to be oviparous : Pristiurus, 

 the black-mouthed dog-fish of the Mediterranean, is one of these ; 

 Heterodontus or Cestracion, the Port Jackson Shark and allied 

 species of the Australian, Japanese, and Californian coasts, have 

 an egg-shell of very peculiar shape, consisting of an elongated 

 conical capsule with two broad flat flanges winding spirally round 

 it and two long coiled tendrils at the pointed end. 



The common skates and rays of the genus Raia are also 

 oviparous, the shell being broader than in Scyllium and having 

 stiff processes at the corners instead of tendrils. 



Among the viviparous forms the best known are Carcharias 

 to which genus belong most of the large dangerous sharks of 

 tropical seas, Galeus vulgaris, the Tope, in one of which were 

 found thirty-two young, Sphyrna or Zygcena, the curious 

 hammer-headed sharks in which the head is produced on each 

 side into a great outgrowth bearing the eye at its extremity, 

 the Notidanidse distinguished by having more than five bran- 

 chial clefts, Mustelus, the hounds, one of which occurs in British 

 seas, Rhina the monk or angel-fish, the Torpedinidas or elec- 

 tric rays, the Trygonidae or sting-rays, and the Myliobatidae or 

 eagle-rays. Internal development involves the possibility of 

 the nutrition of the embryo by absorption of secretions from 

 the wall of the oviduct, a possibility which renders the provision 

 of yolk of less importance, and which in mammals has led to 

 the disappearance of the yolk and the evolution of a special 

 organ connecting the developing young with the wall of the 

 oviduct, the organ called the placenta. In viviparous Elasmo- 

 branchs we find many examples of the evolution of structures 

 adapted to the nutrition of the embryo or foetus : in many 

 species long filaments called villi or " trophonemata " (nourish- 

 ing threads) are developed from the inner wall of the uterus, 

 and these secrete a nutritive liquid which is absorbed by the 

 embryo either by the blood-vessels of its yolk-sac or by its 

 digestive organs. In Pteroplatea, one of the sting-rays of the 

 Indian Ocean, two bundles of such trophonemata pass through 

 the greatly enlarged spiracles of the embryo, and pour their 

 secretion into its stomach. Some species of Mustelus and Car- 

 charias exhibit a closer similarity to the relation of the embryo 

 to the uterus which is characteristic of mammals: folds or pro- 



