THE ANIMAL KINGDOM 



15 



however, their heads were covered by heavy bony armor. Like the lam- 

 preys they lacked jaws and paired appendages. As in cyclostomes the 

 nasal aperture was median and dorsal in position. It has been asserted 

 but not demonstrated that the ostracoderms are the ancestors of carti- 

 laginous fishes, which are consec^uently assumed to have lost their heavy 

 body exoskeletons. Most morphologists, however, consider ostracoderms 

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

 laspis and Pterichthys are characteristic genera. 



HEPTANCHUS - AN ELASMOBRANCH 



„.Aii^,e^d,^_^__ 



Fig. 17. — Types of three sub-classes of fishes — Heptanchu,s, an elasmobranch; 

 Polypterus, a crossopterygian ganoid; and Scomberomorus, 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. 

 Dorsal and ventral spinal nerves join to form mixed trunks. Sympathetic 

 ganglia are differentiated. The liver has at least two lobes. 



Of special interest are the orders of fishes which are believed to be near 

 the line of ancestry of land animals. Probably cartilaginous forms like the 

 Elasmobranchs (sharks and skates) were the common stock from which the 

 remaining orders of fishes were evolved. Their gills are not covered by an 

 operculum ; their skull is devoid of covering membrane bones ; their intestine 

 has a spiral valve. The dogfish Squalus (Fig. 16) is a familiar example. 



