II.] INTRODUCTION. XXXV 



and winter, except at its surface, the circumstance that 

 most sea- weeds are annual would lead us to conclude 

 that the marine phytophagous MoUusca also retire into 

 winter quarters, and that, as these are preyed upon by the 

 zoophagous kinds, the habit of hibernating is common to 

 all shell- fish. In shallow seas near the land, the number 

 of marine animals is perceptibly diminished during cold 

 and inclement seasons ; and this was noticed by several 

 naturalists to have been the case in the wet year of 1860. 

 The period of hibernation dififers among the MoUusca. 

 Some retire earlier or emerge later than others. Ac- 

 cording to M. Drouet the Anodonta or freshwater mus- 

 sels hibernate before the close of autumn, and bury 

 themselves deep in the mud until the middle of spring, 

 when the water begins to get warmer. 



Nests. — A few marine bivalves, which do not spin a 

 byssus or bundle of tlireads with which they can moor 

 themselves to rocks, or which have not the power of exca- 

 vating for themselves a place of residence in stone, wood, 

 the tunic of Ascidians, or other substances, form a kind of 

 rude nest out of broken shells and zoophytes, which they 

 cement and line internally with a slimy exudation from 

 their bodies ; and thus they remain snugly ensconced 

 and protected from their natural enemies. Among these 

 nest-builders are the Modiola radiata, Lima Loscombii, 

 and the northern form of L. hians. So little is known 

 of the habits of the marine MoUusca, that I am not pre- 

 pared to say whether these are proper nests and used 

 for the protection of the young, as in the case of stickle- 

 backs and other small fishes, or are constructed solely 

 for the use of the adult shell-fish. 



Modes of attack and defence. — Some aquatic moUusks 

 have the faculty of emitting from their bodies a purple 

 or dark fluid, apparently for the purpose of securing 



