PISCES 187 



state occurs. They then descend to the ocean and remain 

 there several years, after which they return to the scene 

 of their youth for breeding purposes, and a few days later 

 die without having regained the ocean. The adult is a 

 parasite living on the blood and mucus rasped from the 

 sides of fish. The lake lamprey of New York is probably 

 a descendant of the ocean lamprey, having been cut off 

 from the salt water by the drying up of the streams many 

 years ago. It is destructive to food fishes, preying on 

 nearly thirty different species. 



The lampreys are not true fish, as they lack paired fins 

 containing spines or bony rays, and have only one nostril, 

 which opens on top of the head in the median line. Jaws 

 are wanting. The skull is made of membrane and carti- 

 lage, and the spinal cord lies in a fibrous tube with bars 

 of cartilage along the sides. Beneath the spinal cord is a 

 stiff gelatinous rod known as the notochord, which is pres- 

 ent in the very young stages of all vertebrates, but usually 

 its place is later occupied by the bodies of the bony verte- 

 bra? (Fig. 227). 



Cartilaginous Fish. The fish whose skeletons are made 

 of cartilage instead of bone live in salt water. They are 

 the sharks and rays. Many of them are predaceous crea- 

 tures, and their presence accounts for the scarcity of some 

 of the species of fish desired for food by man. 



The sharks are represented along our coast by several 

 species, of which the best known is the dogfish (Squalls 

 acanthias), which attains a length of three feet. During 

 the summer it congregates in great numbers along the 

 Maine and Massachusetts coasts, and often prevents the 

 fishermen from catching any codfish by seizing the bait 



