62 Elementary Zoology. 



byssus serves to moor the young to the gill-plates ; it has the 

 interesting feature of being the equivalent of the byssus of 

 the sea-wa,ter mussel, by means of which that mollusc can 

 hang on to piles of wood, etc., a fact which seems to show 

 that the anodon is descended from a byssus-bearing bivalve. 

 The young glochidia are ultilnately expelled from the sheltering 

 gill cavity, and then fall to the bottom of the pond or lake, 

 where the parent form happens to be living. They cannot 

 swim at all, but lie at the bottom on the valves of the shell, 

 with the byssus filaments streaming upwards. With these, and 

 with the nipping valves of the shell, they fasten on to some 

 small fish, such as a stickleback. The glochidia appear to 

 have a very keen appreciation of the presence of small fish. 

 On one being introduced into water swarming with glochidia, 

 the latter were observed violently to close and open the valves 

 of their shell ; had the fish swum near enough they might have 

 succeeded in laying hold of it. The advantage of this power 

 of adhering to a fish is that the young glochidium is carried 

 about, and the currents due to the movement bring particles 

 of food within its reach. 



The adult anodon, like the glochidium, is enclosed in 

 a two-valved shell. But the shell of the adult arises underneath 

 that of the larva, and is marked by a series of lines running 

 parallel with the long axis of the shell, which indicate its 

 growth. These lines about the middle are slightly curved 

 upwards, an effect which is due to die sharp teeth of the 

 glochidium shell impinging upon the soft and growing shell 

 beneath, and checking its growth at that point. The bivalve 

 shell is composed of two separate halves, which are joined 

 near the summit, the umbo, by an elastic hinge, which is 

 stretched during life by the action of the great adductor muscles, 

 but which after death is relaxed, and thus suffers the shell 

 to gape. On the inside of the shell are to be seen the strong 

 impressions of the two adductors, and of three other muscles, 

 two retractors and a protractor of the foot, which muscles serve, 

 as their names indicate, for the protrusion and retraction of 

 the foot. 



Conrhologists often distinguish between the shell of a 



