XI MOLLUSC A 147 



may very likely catch on to the fish and stick, so that the 

 fish swims off carrying a mass of glochidia with it. Then if 

 any glochidium comes into actual contact with the fish — as it 

 very likely would do as the fish darts about — it seizes the skin 

 with the hooks on its shell and drags it in between the valves, 

 causing such irritation that the tissues of the fish inflame 

 and form a sort of case or " cyst " round the larva ; in this 

 position the larva remains for three months, living as a para- 

 site, absorbing its food from the juices of the fish. Thus it 

 gains not only a temporary home and food, but also transport 

 from one part of the pond to another. Meanwhile, the 

 internal structure of the larva changes gradually to a structure 

 similar to that of the adult ; a new bivalve shell is secreted 

 by the mantle below the glochidium shell, which, however, 

 remains covering it for some time. The cyst formed by the 

 fish finally withers and drops off, and the little mussel now at 

 last begins its independent life, though for several weeks 

 longer it still retains its larval shell ; the teeth of this shell 

 press against the margin of the new shell that is growing 

 within, and often cause a distinct irregularity in the lines 

 of growth at this point. (This irregularity is, however, more 

 marked in Unio than in Anodon.) 



When first it begins its independent life, the little mussel 

 is not much larger than a pin's head and is very transparent, 

 and, therefore, difficult to study closely, except under the 

 microscope. 



Each pair of mussels may produce hundreds of thousands 

 of young. 



There is a fairly well developed nervous system, 



and^enfes'^ ^^'^ ^ ^^"^ P^'^'' °^ ^^ body which is exposed 



beyond the shell is more or less sensitive to touch 



and to light. The tentacles that surround the inhalent siphon 



seem to be organs of taste as well as of touch, but there are 



no other clearly differentiated sense organs. 



Enemies of Mussels are eaten by several kinds of water- 



the Mussel, fowl, and sometimes also by otters and water-voles. 



Other Fresh-water Bivalves. 



jj^^^ The genus Unio differs from Anodon in having 



strongly marked lateral hinge-teeth ; also in having 

 a depressed area in front of the umbo. 



