. 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, 



FIG. 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. 



