MATERNAL CARE AND MOLLUSCS 145 



to touch a fish's body becomes immediately fastened by 

 it to the fish and then grips the skin with its snapping 

 shells, the edges of which are provided with a few long, 

 sharp teeth. The fish probably is quite unaware of the 

 lodgment of the young mussel on its skin, but there it 

 remains, and gets buried for a time in the soft tissues 

 of the fish, becoming thus actually a parasite for some 

 two or three months during the winter season. It 

 nourishes itself on the juices of the fish, and grows to 



FIG. 35. Young of the pond-mussel after escaping from the maternal 

 gill -pouch : A, as it escapes, swimming by opening and shutting the 

 shells ; sh, shell of one side ; al, shell-muscle ; t, teeth of the shell's 

 edge ; by, adhesive filament. B, after it has fixed to a fish ; mt, 

 mantle ; f, muscular foot ; br, gill processes ; pad, aad, al, muscles ; 

 auv, heart. (From drawings by the late Frank Balfour.) 



the size of a pin's head, whilst it is carried away from its 

 birthplace by the peregrinations of its host, the fish. Its 

 shell now ceases to be triangular, and becomes like that of 

 its parents. Eventually the young mussel drops off the 

 fish and rests on the muddy bottom of pond or river, 

 where it remains for many years, growing vastly in size, 

 and barely moving during its long life from the spot 

 where it fell. 



A beautiful little bivalve common in weedy streams 

 in England is known as Cyclas (it has no English name) ; 

 it has a pair of shells shaped like those of a cockle, but 



