DISUSE AND ITS EFFECTS. 37 
atrophy or even disappear. The curious suctorial disc 
of the recently hatched embryo of the fish, lepidosteus, 
is a case in point. In the adult fish the upper jaw ends 
in a fleshy globular projection ; this, in the embryo, is 
a large disc, as in fig. 18. Agassiz, to whom we are 
indebted for much of our knowledge of this structure, has 
ascertained that the disc is formed two or three days 
before hatching, and the young fish uses it as a sucker, 
Fic. 18..-A, head of a young lepidosteus ; B, the suctorial 
disc seen from below; sd, suctorial disc; m, mouth. 
(From Balfour.) 
by means of which it can attach itself to the sides of 
the vessel in which it is confined, or to other objects. 
The young lepidosteus can fix itself so firmly that con- 
siderable commotion in the water is required in order 
to make the fish lose hold : it can even remain suspended 
after the water has been lowered beyond the level to 
which it is attached. 
