82 THE LIFE OF THE MOLLUSC A 



concha, Entocolax, Entosiphon and Enteroxenos, etc., 

 whose exact relationships are unknown, and whose 

 appearance, so transformed are they, suggests almost 

 any vermiform animal rather than a Snail. One 

 instance has also lately been placed on record of a 

 mollusc (Epistethe gonodactyli) parasitic on the ven- 

 tral surface of a Stomatopod crustacean (Gonodactylus 

 chiragra) from shallow water in the Persian Gulf. 



A more interesting form of parasitism is afforded 

 in the early life-history of the freshwater Mussels, 

 and it is said in that of the marine Bivalve Philobrya 

 also. The young Mussels are hatched between the 

 gill-folds of the parent, and, having passed there 

 through the veliger stage, escape through the ex- 

 current siphonal opening in the form of a peculiar 

 larva, which when first observed, being thought to 

 be a distinct creature, received the name of 

 Glochidium (Plate XXIX., Fig. 9). This larva has a 

 hook in the middle of the margin of each of its two 

 valves, which, moreover, are perforate, and also a long 

 byssal thread. It is incapable of supporting an 

 independent existence, and cannot, as has been 

 stated, swim by rapidly opening and closing its 

 valves. This valve-clapping takes place vigorously 

 if a fish comes near, and has the effect of forcing the 

 byssus straight out. Should the latter touch the 

 fish, it adheres to it. Any chance movement then 

 bringing the Glochidium itself into contact with the 

 fish it immediately snaps to. The hooks, if the part 



