24 



COMPARATIVE ANATOMY 



rather highly specialized types and not primitive ancestral forms. Cepha- 

 laspis and Pterichthys are characteristic genera. 



CEPHALASPIS - 

 AN OSTRACODERM 



Fig. 17. — Cephalaspis, an Ostracoderm, appears to have affinities with cyclostomes and 

 has been thought by W. Patten to connect vertebrates with arachnids. 



Fig. 18. — Types of three sub-classes of fishes — Heptanchus, an Elasmobranch; 

 Polypterus, a Crossopterygian Ganoid; and Scotnheromorus, a Teleost. (Redrawn 

 after Dean.) 



Class Pisces 



Fishes are vertebrates with usually scaly skins, permanent gills, and 

 paired fins. The heart is two- or three-chambered. The skeletons may 

 be cartilaginous or bony. Gill-apertures number four to seven pairs. 



