308 ANIMAL SKETCHES. CHAP. 



by a straight hinge. The mouth, which as yet of course 

 has no moustache, is large and opposite the hinge. There 

 are no gills. The shell is closed by a muscle similar in 

 function to that of the mother, but different in position. 

 But the most noticeable point of difference between the 

 little rascal and its mother is the possession of an oval 

 cushion projecting between the edges of the valves, and 

 bearing on its edges the delicate swimming hairs by which 

 the little embryo mollusk propels itself through the water 

 amid its myriad companions, and enjoys for a while a 

 vigorous and active life. By means of special muscles, the 

 cushion with its swimming-hairs may be withdrawn into 

 the shell, whereupon the oyster sinks. 



It is pleasant to think that even the sedate and seden- 

 tary native enjoys, if only for a few days, an active, frisky, 

 mischievous boyhood. In this it resembles the vast 

 majority of bivalve mollusks. Our oyster is indeed 

 peculiar in affording any protection to its young. Most 

 bivalves, and even such near relations as the Portuguese 

 oyster and the American oyster, are cast adrift as soon as 

 they are born, and undergo no period of incubation beneath 

 the mantle-wing of the mother. A curious example of 

 a somewhat similar protection is afforded by the fresh- 

 water mussel. 



This shell-fish in some respects resembles, and in other 

 respects differs from the oyster. The figure shows one 

 lying in its right shell, the left valve having been removed 

 and the mantle cut away along the dark line. There are 

 two strong muscles (mm.) for closing the shell instead of 

 one as in the oyster. The mouth is seen at mo., and close 

 to it the moustache-like pelps. The foot, /., is large. The 

 gills, o.g. and i.g, lie on either side of the foot. 



When the mussel is at home the foot-end is buried in 

 the mud or sand and, though the valves gape a little, the 



