Ichthyopsida 



12 



The division Ichthyopsida includes three Classes: the fishlike 

 Cyclostomata ; the typical fishes, Pisces; and the Amphibia. 



I. Class Cyclostomata 



Cyclostomes (Fig. 317) are the round-mouthed lamprey eels 

 (Petromyzon) and hagfishes (Bdellostoma and Myxine). The 

 mouth, devoid of jaws, is at the center of a suctorial disk whose surface 

 bears small teeth composed of a horny substance produced by the 

 epidermis. By means of the oral sucker, the cyclostome attaches itself 

 to the body of its prey, usually a fish, and by rasping movements of 

 the toothed sucking disk bores into the flesh of the prey. They have a 

 persistent notochord and, in Petromyzon, suggestions of vertebrae in 

 the occurrence of cartilaginous rods or plates, two pairs to each pair of 

 myomeres, forming a series of incomplete and discontinuous neural 



JILL APERTURES (7] 



PETROMYZON 



Fig. 317. Three characteristic genera of cyclostomes — Bdellostoma, Myxine, 

 and Petromyzon. (Alter Dean. Courtesy, Neal and Rand: "Chordate Anatomy," 

 Philadelphia, The Blakiston Company.) 



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