38 EVOLUTION 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 — 
