XIII 



PHYLUM CHORDATA 



vertebrae : these, the muscle-buds, give rise to the fin-muscles ; 

 at first, from their mode of origin, they present a metameric 



Fie. $45 Side view of head of embryo 

 of Scyllium canicula, with the 

 rudiments of the gills on the first and 

 second branchial arches, eye, eye ; 

 ni. >>rn. mid-brain; mnd. mandible; 

 WM. nasal sac. (After Sedgwick.) 



spir 



Fio. $46. Side view of the head of Scyllium 

 canicula at a somewhat later stage. The 

 gill- filaments have increased in number and are 

 present on the mandibular arch. nnci. angle 

 of the jaw ; hy. hyoid ; in. brn. mid-brain ; ntix. 

 nasal sac ; sjrir. spiracle. (After Sedgwick.) 



arrangement, but this is in great measure lost during develop- 

 ment. 



Ethology and Distribution. The habits of the active, fierce, 

 and voracious Sharks, which live in the sui face- waters of the sea, 

 waging war on all and sundry, contrast strongly with those of 

 the more sluggish Rays, which live habitually on the bottom, 

 usually in shallow water, and feed chiefly on Crustaceans and 

 Molluscs, with the addition of such small Fishes as they can 

 capture. As a group, the Elasmobranchs, more particularly the 

 Sharks, are distinguished by their muscular strength, the activity 

 of their movements, and also by the acuteness of their senses of 

 sight and smell. 



The only deep water Elasmobranch known is a species of Ray, 

 which extends to a depth of over GOO fathoms. 



None of the Elasmobranchs are of very small size, and com- 

 prised among them are the largest of living Fishes : the harm- 

 less Basking Sharks (Selache) sometimes attain a length of 35 feet 

 or more, the formidable Great Blue Shark (Carcharodon) some- 

 times reaches 40 feet, and some of the Rays also attain colossal 

 dimensions. In this respect, however, recent Sharks and Rays are 

 far behind some of the fossil forms, some of which, if their general 

 dimensions were in proportion to the size of their teeth, must 

 have reached a length of as much as sixty feet. 



The earliest fossil remains of Elasmobranch Fishes that have 

 been found occur in rocks belonging to the Upper Silurian period. 

 Throughout the Palaeozoic epoch the Elasmobranchs constituted a 



