MYTILUS. 295 



hering were all deposited by the iuhabitant^ as this species^ 

 like the cuckoo^ frequently appropriates a neighbour's shell 

 for the temporary reception of her offspring. 



The migration of testaceous tribes is a subject of peculiar 

 interest. Some of the shell-bearing mollusca lay their eggs 

 in a sponge-like nidus, wherein the young remain enveloped 

 for a fixed period after their birth, and this buoyant sub- 

 stance floats like the weed of ocean. Others are affixed to 

 sea-weeds ; and others, light as the finest grains of sand, 

 ride on the billows, and become deposited wherever the 

 currents find their way. Serpulm are often found adhering 

 to floating cocoa-nuts, and even fragments of pumice. Such 

 species of mollusks, on the contrary, as inhabit lakes 

 and rivers usually attach their eggs to leaves and sticks, 

 which, having fallen into the water, are liable to be swept 

 away from tributary waters to the main streams, and from 

 thence to every part of the same basins. Hence it happens 

 that certain species migrate during one season of the year 

 from the commencement of the Mississippi to countries 

 bordering the sea, at the distance of many thousand miles. 



A considerable number attach themselves to the bottom 

 of ships, or nestle in holes, which the Teredo, or ship-worm, 

 has perforated. By one such method the Mi/tilus joolpmr- 



