PUZZLES AND PARADOXES 261 



but with qualifications. The young glochidia, as they 

 are now called, thus suddenly sent forth slowly sink to 

 the bottom of the stream. At this time each is possessed 

 of a bivalve shell of curious design, the front edges bearing 

 each a triangular tooth beset with smaller teeth, and with 

 a long sticky thread or byssus, which floats loosely in 

 the water. 



Patiently and anxiously they await the arrival of a 

 shoal of fishes. The disturbance in the water caused by 

 their approach throws them all into a state of intense 

 agitation, the valves are suddenly opened and closed with 

 extreme rapidity, and these movements force the thread 

 upwards. If — and this is a tremendous " if," for it means 

 life or death — if the thread touches any part of the body 

 of one of the passing fish, it sticks, and the youngsters, 

 a dozen or more to each fish, are thereupon for a time 

 trailed through the water. Chance movements of the tail 

 or fins are certain to bring some of the glochidia into 

 actual contact with the skin of the fish. Directly this 

 happens the valves are snapped together, and the teeth 

 grip, dragging the skin well into the shell cavity. Speedily 

 a cyst is formed entirely covering the glochidium, and 

 rendering any danger of falling off impossible. 



For the next three months the snugly ensconced glo- 

 chidium lives a parasite upon its host, drawing nourish- 

 ment from its lymph. Meanwhile a new shell is growing 

 within the glochidium shell, and extensive alterations are 

 made in the structure of the body. But sooner or later 

 the cyst bursts and the youngster falls to the bottom of 

 the stream, this time a mussel differing from its parent 

 only in size. It has not, however, succeeded in casting 

 off the glochidial shell, which for some three or four weeks 

 forms a sort of overcoat to the new, and as yet trans- 



