5 2 THE LAMPREYS 



rivers, ascending them in numbers in the springtime 

 (April) on the way to the spawning grounds (v. p. 182). 

 During its adult life it is supposed to be exclusively car- 

 nivorous, to some degree, perhaps, parasitic, although many 

 doubt that it is truly parasitic in the sense of entering the 

 body cavities of healthy fishes. It certainly is often taken 

 attached to other fishes, as shark, sturgeon, or salmon. 



Immature lampreys differ so strikingly from the adults 

 that they were formerly regarded as species of a separate 

 genus, Ammoccetes (v. p. 215). In feeding habits the am- 

 moccete is widely unlike the mature form ; it is toothless 

 (Fig. 72, C), and in part mud-eating, i.e. vegetivorous. 



Petromyzon must be regarded as the most highly organ- 

 ized of Cyclostomes. Its mouth has no longer the fring- 

 ing barbels of Myxinoids, which suggest, according to 

 Pollard, the buccal cirrhi of Amphioxus, it has acquired 

 stout supporting cartilages and a funnel-shaped form, 

 studded with a series of conical teeth, as shown in Fig. 

 72, C. The teeth of the hinder mouth region now appear 

 almost as though they were supported by a mandibular 

 cartilage ; the tongue, as in other Cyclostomes, bears the 

 teeth which are probably of the greatest functional impor- 

 tance. The nasal canal of Petromyzon has its outer opening 

 on the dorsal surface of the head ; its inner end, however, 

 does not perforate the roof of the mouth, although produced 

 backward as a blind sac, closely apposed to the pharynx. 

 Petromyzonts are, accordingly, arranged as the sub-group 

 Hyperoartia, in contrast to the Myxinoids. 



Further structural characters, which the lamprey seems 

 to have derived from simpler conditions, may be noted in 

 its unpaired fin, gill chamber, nervous system, and skele- 

 ton. The unpaired fin has subdivided into dorsal and 

 caudal elements, and is now supported by well-marked 



