CH. Vl] THE FRESH-WATER MUSSEL 193 



the glochidium disappear, and the two permanent ad- 

 ductors of the adult and the foot are developed. The 

 mantle secretes a new bivalve and transparent shell re- 

 sembling in miniature that of the adult and differing from 

 that of the glochidium, which however is not as yet cast 

 off but sits as a saddle upon the permanent shell beneath 

 it. Eventually the cyst in which the parasite has been 

 contained withers and the young mussel drops off and at 

 length begins an independent life. The formation of the 

 cyst and its subsequent degeneration are to be regarded 

 as pathological phenomena occurring in the skin of the 

 fish whereby it eventually succeeds in liberating itself 

 from its parasite. Sticklebacks, loach, minnows, and pro- 

 bably many other species of fish are successfully affected 

 by glochidia. Normal development also is said to take 

 place when they are attached to axolotls or to newts. 

 In my own experience however glochidia fail to secure a 

 hold upon the skin of a newt. Tadpoles on the other 

 hand succeed in freeing themselves from the glochidia 

 before the latter have advanced far towards their adult 

 structure. 



When first set free the young mussel, as yet no 

 larger than the original glochidium, is still covered 

 by the glochidial shell, which persists outside the small 

 permanent shell for three or four weeks and is the cause 

 of the slight irregularity already mentioned (p. 163) in the 

 lines of growth of the adult shell. The apical teeth of the 

 glochidial shell project ventrally towards the middle line 

 and as a consequence impinge upon the ventral border of 

 the, at present, soft permanent shell at a point about half- 

 L. 13 



