232 



GENERAL ZOOLOGY 



, 

 stome, the brook lamprey of North America, Lampetra 



wilderi, has interesting breeding habits. Stones are car- 

 ried in the mouth^and a space is thus cleared in which 

 mating take splace. This species does not eat as an adult, 

 and is therefore not harmful to fishes. 



FIG. 91. Cyclostomes and clasmobranchs. A, lamprey-eels, resting and at- 

 tacking a shark; B, shark; C, sting-ray. 



CLASS 2. ELASMOBRANCHII 



Elasmobranchs (Fig. 91, B, C) resemble true fishes in 

 general form and in having two pairs of lateral fins, but 

 differ from them in possessing a notochord when adult, 

 placoid scales (Fig. 93), a spiral valve in the intestine, clasp- 

 ing organs in the male, and in lacking an air-bladder and 

 a gill-cover, or operculum. They are therefore placed in 

 a separate class. 



There are two common types of elasmobranchs, sharks 

 and rays. The sharks and dogfishes are cylindrical or 



