394 MANUAL OF ZOOLOGY SECT. 



shell-gland. Enclosed in the shell, the form of which varies 

 in different groups, the egg passes to the exterior and under- 

 goes development until the young fish is fully formed, when 

 it escapes by rupturing the egg-shell. In the viviparous 

 forms, on the other hand, the ovum undergoes its develop- 

 ment in the uterus ; and the young fish, when it escapes to 

 the exterior, has assumed all the features of the adult. 



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, cod, 

 sole, herring, eel, salmon, etc., and the Ganoidei or Ganoids, 

 such as the sturgeon, body pike (Lepidosteus), and bow-fin 

 (Amid} of North America, and the Polypterus of the Nile. 

 They are distinguished from Elasmobranchs by the posses- 



1 Sub-class II, the small group Holocephali, or Chivnaeras and their 

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



