38 E VOL UTION AND DISEASE. 



This form of atrophy may be studied in ascidians. 

 These are marine animals which encrust stones, rocks, 

 and weeds on the sea-bottom. Sometimes they are 

 solitary, but often occur combined in masses. In the 

 adult condition they have an appearance recalling that 

 of a tough leathern bottle with two openings ; water 

 enters at one, leaves by the other. The young of some 

 ascidians have a totally different form, resembling a tad- 

 pole not only in external configuration but in internal 

 organization ; the general details of the anatomy of an 

 ascidian tadpole is shown in fig. 19. 



After existing in a free state for some time the 

 young ascidian fixes itself to a stone by its head ; the 

 tail, with the notochord and nervous axis, atrophies, the 

 body changes its shape, the brain remains small and 

 undeveloped, and the eyes disappear. Finally the 

 animal increases in size, its outer case becomes tough 

 and leather-like. 



Among other examples of this form of atrophy, 

 mention may be made of the tail and gills of frog- 

 tadpoles, the external gills of sharks, and the Alpine 

 salamander, the yolk sac of vertebrata. Remarkable 

 instances of the atrophy and disappearance of larval 

 organs may be studied among invertebrates, especially 

 in the echinoderms and star-fish. Many marvel at such 

 things occurring in other animals, and overlook the fact 

 that similar conditions may be studied in our own 

 bodies, for the fall of the milk teeth is induced by the 

 same process which brings about the disappearance 

 of the tadpole's gills and tail. Puppies are born blind : 

 this blindness is due to the existence of a vascular 



