424 



ANIMAL LIFE-HISTORIES 



Sharks and Rays (Elasmobranchii). — Some few members of 

 the order are viviparous, but, as a rule, the eggs are laid in horny 

 cases (fig. 950), within the shelter of which development takes 



Fig. 950. — Egg-capsules of Skate (a), Spotted Dog-Fish (b), and Port Jackson Shark (c). c reduced. 



place, and from which the young fishes ultimately emerge in a 

 form closely resembling that of the adult. There is here no necessity 

 for the early hatching of a larval form, since the egg is large, and 

 contains abundant food-yolk. The egg-cases vary in shape in 

 different species, and are popularly known as 

 "mermaids' purses". Those of the Common 

 Skate {^Raia batis) are flat and squarish, with 

 the corners drawn out into pointed processes, 

 while in the Spotted Dogfish iyScyllium cani- 

 cnld) they are somewhat narrower, and pro- 

 vided with four tendril-like threads, by means 

 of which they are attached to sea-weeds. An 

 external spiral fold and two tendrils are 

 present in the case of the Port Jackson Shark 

 {Cestracion Philippi). 



The abundant yolk makes up the greater 

 part of the egg, and cleavage is consequently 

 partial (see p. 346), the body of the embryo 

 being developed from a small patch of living substance placed 

 at one end. If the egg-case is cut open after development has 

 gone on for some time the immature fish will be found within. 



Fig. 951. — Advanced Embryo 

 of Dog-Fish removed from the 

 Egg -capsule. Note the large 

 yolk-sac (with branching blood- 

 vessels} to left. The embryo is 

 provided with long external gills. 



