62 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, Ammocetes (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, z.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 
