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 larvae 

 before metamorphosis the old name Ammoc&tes 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. 



FlG. 27 7. 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; k., heart; N., notochord ; S.C., spinal cord; E. t 

 auditory vesicle; cb. , cerebellum ; f.b. pineal body; c.A., 

 cerebral hemispheres ; olf., olfactory involution 



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. t 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 not 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. 



