xii PHYLUM CHORDATA 391 



when it escapes by rupturing the egg shell. In the 

 viviparous forms, on the other hand, the ovum undergoes 

 its development in the wide uterine portion of the oviduct, 

 and the young fishj when it escapes to the exterior, has 

 assumed all the features of the adult. In most instances 

 while in the oviduct it absorbs nutrient matter by means 

 of the elongated gill-filaments (p. 383) and of vessels in 

 the wall of the yolk-sac, but in a few forms (species of 

 Mustelus) there is an intimate relation brought about be- 

 tween the surface of the yolk-sac and the uterine wall (yolk- 

 sac placenta] subserving nutrition. 



The habits of the active, fierce, and voracious Sharks, 

 which live in the surface waters of the sea waging war on 

 all and sundry, are in strong contrast 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 600 fathoms. 



Sub- class III. Teleostomi. 1 



The great majority of existing Fishes belong to the 

 sub-class Teleostomi. As a matter of convenience we may 

 look upon the Teleostomi as consisting of two main divisions 

 the Teleostei, in which are included all the commonest 

 and most familiar Fishes, such as the Perch, Pike, Mackerel, 



1 Sub-class II. the small group Holocephali> or Chimara and its 

 allies, is one of the groups omitted from this work. See Preface, 



