II.] INTRODUCTION. XXxiii 



it is enabled to keep always in a floating position on the 

 surface of the water, with the point or spire of its shell 

 downwards, and which apparatus also serves as a nidus 

 for its spawn. A description of this curious appendage 

 or "float," as well as of the equally remarkable habits of 

 the lanthina, will be found in its proper place. The 

 Pteropods have feet like wings, and flutter through the 

 water like butterflies. They have conical, and some- 

 times spiral, shells as fragile and transparent as the 

 finest blown glass. 



Care of young. Many of the bivalve Mollusca, inha- 

 biting the sea as well as fresh water (e. g. Teredo and 

 Anodonta), retain for some time their fry, after being 

 excluded from the egg, in the folds of their mantle, this 

 being in some degree an analogous provision to that 

 which is possessed by the marsupial quadrupeds. The 

 common Limpet and some kinds of Pupa (small land- 

 snails) have also been observed to carry about their 

 young, the former within the folds of its foot, and the 

 latter attached to the shells of their mother. These 

 cases of Molluscan cn-opyr) are nearly as wonderful as 

 any which have been adduced in accounts of much more 

 highly organized animals. 



Sociability. Although many of the Mollusca are gre- 

 garious and assemble together on the same feeding- 

 grounds, it does not appear that they ever associate for 

 a common object, like bees or ants. Each Pholas and 

 Teredo makes a hole for itself; and although the com- 

 mon garden-snail often fastens itself to the shell of its 

 companion, when they hibernate and form clusters, a 

 smooth stone or any other object is used for the same 

 purpose. 



^Estivation and Hibernation. Many animals in a state 

 of nature have their periodical seasons of repose, espe- 



