40 SALT-WATER FISHES 



This has not perhaps been quite satisfactorily proved, the 

 obvious difficulty being that these egg-cases are generally 

 recovered only when they have been washed up after a storm, 

 and have consequently been exposed for some time to the sun 

 or atmosphere, which doubtless destroys the natural glue. 



The larger rays mostly bring forth living young, like the 

 sharks, and it is now generally recognised that the large egg- 

 case attributed by Couch to the eagle-ray {Myliobatis) belonged 

 to one of the genus Raia, known as the " typical " genus. 

 Among the vipiparous kinds are the sting-rays (^Trygon). Their 

 embryo does not appear to have been an object of special study 

 by anatomists in this country, but with regard to that of an 

 Indian species {T. walga), taken in the Godaveri delta, there 

 is an interesting communication from Messrs. Alcock and 

 Wood-Mason in the archives of the Royal Society (November, 

 1901). According to this memoir, the new-born fish had the 

 upper surface quite smooth and devoid of the usual tubercles 

 of the adult, and whereas the latter has, in that species, two 

 spines in the tail, the young fish showed only one. In 

 addition, however, it had a fold of skin on the tail, which 

 apparently disappears with age. The authors of the memoir 

 regarded this fold of skin as a survival of the vertical system 

 of fins. 



The very voung forms of fishes hatched from spawn differ 

 from their elders in a considerably greater measure than those 

 which are born alive. The tiny fish, as it emerges from the 

 egg and for the first few days of its existence, is known as 

 the " larval form," or larva, and at that stage it has a yolk-sac 

 containing nourishment for the first few days of its existence, 

 for it is born without an open mouth, and cannot at once seek 

 its natural food. The yolk-sac may last it for a few days only, 

 or for as long as a couple of weeks, according to the species, 

 and perhaps also according to conditions of temperature, 

 though our knowledge on this is still rather circumscribed. 

 At last, however, the yolk-sac is used up, and the little mouth 



