316 SYSTEMATIC ZOOLOGY. 



that of other vertebrates, since movable jaws are lacking. 

 Inside of the mouth are horny teeth (few in the hagfishes, 

 many in the lampreys), but these are chiefly used for 

 holding, not for biting or crushing. The tongue is very 

 large. 



There is but a single nostril on top of the head. The 

 gills are placed not in simple slits, but in large pouches on 

 the sides of the neck (hence the name, Marsipobranchs, 

 often given the group), and these pouches may either 

 open separately to the exterior or by means of a tube 

 which leads to a single opening. The number of gill- 

 pouches ranges between six and fourteen on either side. 



The Cyclostomes are subdivided into two groups, accord- 



FIG. 119. Lamprey (Petromyzon marinus). After Goode. 



ingly as the nostril communicates with the throat or not. 

 As examples of the first, the hagfishes may be cited. 

 These are all marine, and are capable of secreting a large 

 amount of mucus from their bodies, so that a few hagfish 

 in a pail will convert the water into a jelly-like mass. 

 These fishes are parasites, and work their way into vari- 

 ous fishes, like the cod, and when once inside they eat up 

 all the flesh and viscera, leaving nothing except the skin 

 and bones. 



The second group is represented by the lampreys. Some 

 of these are marine, others live in fresh water, while many 

 of the marine forms ascend streams in spring to lay their 



