THE FISHES 169 



largely in burrowing. Like the lancelet and other simple 

 chordates, they are of the greatest interest to the zoologist 

 who gains from them some idea of the lowly vertebrate 

 forms that peopled the earth long ago. 



157. True fishes. The third class, Pisces or true fishes, 

 to which the shark as well as the bass and perch belong, has 

 a well-developed skeleton, skull, and brain. The lower jaw 

 is developed, forming a distinct mouth, and there is at least 

 a shoulder-girdle and pelvis ; although the fins these should 

 bear are not always developed, the general traits are those we 

 associate with the fish. Of the true fishes, there are again sev- 

 eral strongly marked groups, usually called sub-classes, two 

 of them wholly extinct. Of these, three chiefly interest us. 



158. The sharks and skates. Very early in the life of 

 the sharks (Fig. 103) and skates (SelachiioY Elasmobrancliii) 



FIG. 103. Soup-fin shark (Galens zyopterus) from Monterey, Cal. 



a notochord appears, similar to that in the lancelet and the 

 lampreys. As growth proceeds its sheath becomes broken 

 up into a series of cartilaginous rings, which thus appear 

 like spools strung on a cord. As the fish grows older these 

 " spools " or vertebrae grow solid, cutting the notochord into 

 little disks, and great flexibility is thus secured. Cartilagi- 

 nous appendages also grow up and cover the spinal nerve- 

 cord lying above, and give strength to the unpaired fins ; 

 the paired fins also have their supports. The shoulder- 

 12 



