PETROMYZON. 523 
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 larve 
before metamorphosis the old name Ammocetes 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 AZyxine, is complex. Sensory structures occur 
on the head and along the sides, and form a lateral line 
system. 
Fic. 277,.—Longitudinal vertical section of anterior end 
-of larval lamprey.—After Balfour. 
m., Mouth; th., thyroid ; ¢.., one of the gill-pouches ; v.a0., ven- 
tral aorta; 4., heart; V., notochord; S.C., spinal cord; £., 
auditory. vesicle ; cb., cerebellum; 4.4, pineal body; c.z., 
cerebral hemispheres ; off, olfactory invo, ution 
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, but it is zof likely that its 
elements correspond to visceral arches. Endoskeletal 
cartilaginous rods, not comparable to the dermal fin-rays 
of fishes, support the dorsal and caudal fins, and other 
skeletal parts occur about the “tongue.” The caudal end 
of the notochord is quite straight. 
