SURVEY OF VERTEBRATES 



113 



Lung-fishes to be mentioned later, the Fishes fall into two main 

 groups: the Sharks and Rays with an internal skeleton of gristle, 

 or cartilage, and the Bony Fishes in which the cartilaginous 



-Gill slits 



Fig. 68. — A Cyclostome. Lamprey, Petromyzon marinus. (Redrawn, 



after Dean.) 



skeleton is largely replaced by one of bone. The latter group com- 

 prises the dominant Fish population of the Earth to-day, repre- 

 sented, for example, by the Mackerel and Perch, Goldfish and 

 Guppy, and a Goby less than one-third of an inch long — the 

 smallest known Vertebrate. (Figs. 68, 75.) 



1. Sharks and Rays 



The most primitive of the true Fishes are the Sharks and Rays, 

 or Elasmobranchs : a small remnant of a once dominant group 



Fig. 69. — Sharks. A, Frilled Shark, Chlameidoselachus anguineus; B, Thresher 

 Shark, Alopecias vulpes; C, Angel Shark, Rhina squatina. (From Newman.) 



of Vertebrates. They differ from higher Fishes chiefly by a car- 

 tilaginous skeleton, by gills communicating directly with the 



