82 THE LIFE OF THE MOLLUSCA 
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 
chivagra) 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 siphonel 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 
