474 CYCLOSTOMATA. 



Second Type. Petromyzon — The Lamprey. 



There are three British species — the sea lamprey 

 {Petromyzon marinus), over 3 ft. in length ; the river 

 lampern (P. fluviatilis), nearly 2 ft. long ; and the 

 small lampern or " stone-grig " (P. planeri). They eat 

 worms, small crustaceans, insect larvae, dead animals, etc. ; 

 but they also attach themselves to living fishes, and scrape 

 holes in their skin. As their names suggest, they also fix 

 their mouths to stones, and some draw these together into 

 nests. 



The spawning takes place in spring, usually far up rivers. 

 Before laying the eggs, the lamprey seems to fast (cf. 

 salmon, Protopterus, frog), and its muscles undergo a 

 granular degeneration (cf. Protopterus, tadpole, etc.). Soon 

 after spawning the adults of both sexes die. For reproduc- 

 tion is often the beginning of death as well as of life, though 

 in higher animals the nemesis may be slow. The young 

 are in many ways unlike the parents, and after two or 

 three years pass through a metamorphosis. To the larvae 

 before metamorphosis the old name Ammocates is often 

 applied. 



Form, skin, and muscles. — The body is eel-like, with two 

 unpaired dorsal fins, and another round the tail. Two 

 ridges, one on each side of the anus, Dohrn compares to 

 rudimentary pelvic fins. Otherwise there is no trace of 

 limbs. 



The skin is scaleless, slimy, and pigmented. Its structure, 

 like that of Myxine, is complex. Sensory structures occur 

 on the head and along the sides, and form a lateral line 

 system. 



The muscle segments or myomeres are well marked. 

 The suctorial mouth and the rasping "tongue" are very 

 muscular. 



The skeleton. — The skeleton is wholly cartilaginous. 

 The notochord persists unsegmented, but its firm sheath 

 forms rudimentary neural arches. The skull is imperfectly 

 roofed. There are no distinct jaws, but a cartilaginous ring 

 supports the lips of the mouth. There is a complex basket- 

 work around the gill-pouches, and it is likely that its 

 elements correspond to visceral arches. Fin-rays support 



