122 VERTEBRATE ZOOLOGY 



use to their possessors. It would be very difficult therefore to ex- 

 plain their origin on a natural selection basis. Those who have worked 

 experimentally with fish embryos have often observed in abnormal 

 embryos tendencies for the eyes to project on stalks, and it may well 



Fig. 67. — Group of Sharks (Selachii). A, Frilled Shark {CMameidoselachus 

 anguineus) after Gunther. B, Female Dog-Fish {Scyllium canescens), after 

 Gvinther. C, Thresher Shark {Alopecias vulpes), after Jordan and Everraann. 

 D, Hammer-head shark {Sphyrna zygce'na), male, after Bridge. E, Angel Shark 

 {Rhina squatina), after Bridge. 



be that some similar explanation would account for this specific char- 

 acter in the Hammer-Heads. 



The Whale-Sharks {Rhinodontidm) are of interest because they 

 are much the largest true fishes that have ever lived. They are said 

 sometimes to exceed fifty feet in length and to be of proportionate 

 bulk. Such a shark would be able easily to swallow a man, but it 



