PETROMYZON. 501 



gill-pouches have separate openings, and are extraordinarily variable in 

 number, from six to fourteen on either side a variability perhaps point- 

 ing to ancestral reduction from a larger number (cf. Amphioxus). Large 

 eggs are laid on a shelly or rocky bottom, become connected by polar 

 hooks in chains or clusters, are fertilised after deposition, and exhibit 

 meroblastic discoidal segmentation and direct development. Ayers' 

 experiments show that the removal of one or both ears in this form 

 does net materially affect equilibration. 



SECOND TYPE. Petromyzon The Lamprey. 



There are three British species --the sea lamprey 

 (Petromyzon marimts), 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 2-4 years 

 pass through a striking metamorphosis. To the larvee 

 before metamorphosis the old name Ammoccetes is applied. 



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

 two unpaired dorsal fins, and another round the tail. 



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 



