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MANUAL OF ELEMENTARY ZOOLOGY 



of the outer gill, where they develop. This takes place in 

 the summer. In the following spring the young are set free. 

 They are larvae, very unlike the parent, and are known as 

 glochidia. Each has a shell composed of two triangular 

 valves, hinged along the base and with the apex drawn 

 out into a strong hook. There is no posterior adductor 

 muscle, anus, or foot, but in the place of the latter is a 

 gland which secretes a long sticky thread known as the 

 byssus, comparable with the threads by which the adult 

 sea mussel anchors itself. When some small fish, such as 

 a stickleback, passes over the glochidium, it flaps its valves 

 so as to drive out the byssus, and if the latter touches the 



v.h. J9 



Fig. 232. — Diagrams of a mussel (A), a snail (B), and a cuttle- 

 fish (C).— Partly after Lankester. 



/., Foot ; fun., funnel through which water is squirted by the cuttlefish in swimming ; 

 g., gut; h., bead; ml.c, mantle cavity; sh., internal shell found in some 

 cuttlefish (the "cuttle bone"); ten., tentacles of the cuttlefish ; v.h., apex of 

 visceral hump. 



fish it sticks to it. The movements of the fish now bring 

 the glochidium against its body, whereupon the hooks are 

 used to hold on to its skin. The tissues of the fish become 

 inflamed and swell up, enclosing the little parasite, which 

 lives for some months by absorbing the juices of its host, 

 during which time it undergoes a change into the adult 

 form. Eventually the skin enclosing the young mussel 

 withers and it drops off to lead an independent life. By 

 means of this larva, the slow-moving mussel is dispersed 

 into fresh feeding grounds by the fish, without the risk, 

 which would be considerable if so small an organism were 

 free-swimming, of being carried downstream to the sea. 

 We have seen that the young of the crayfish escape the 



