270 ELEMENTS OF COMPARATIVE ZOOLOGY. 



and the body and foot is a mantle chamber, or, since it 

 frequently contains the gills (branchiae), it is frequently 

 called the branchial chamber. 



The outer surface of the mantle and the dorsal part of 

 the body frequently have the power of secreting a shell 

 composed, chiefly, of carbonate of lime. This shell in 

 some forms becomes split along the median line, so that 

 two halves or valves result. In most other forms the shell 

 becomes coiled into a spiral, and when this occurs the 

 primitive symmetry becomes lost in part. 



Shells increase in size during the life of the animal. 

 The mantle is continually laying down new layers of shell 

 inside of those first formed, hence the older parts are thicker 

 than the newer portions. Then the mantle is larger when 

 the new layers are secreted, so these project beyond the 

 layers outside of them. As a consequence there occur on 

 the outside lines of growth. 



In many species there are colored bands or spots upon 

 the mantle, and these parts secrete carbonate of lime 

 similarly colored, the result being that the shell is corre- 

 spondingly striped or spotted. Again, in some, the edge 

 of the mantle is produced into finger-like lobes, etc., and 

 these produce spines and the like upon the shell. 



Shells are frequently spoken of as the houses or homes 

 in which the animals live. As will be seen from the above, 

 the shells are as much a part of the animal as is the 

 carapax of a lobster or the wings of a butterfly. The 

 oyster or snail can never leave its shell. 



In most molluscs folds of the skin extend from the body- 

 wall into the mantle-chamber. These are the branchiae or 

 gills. Inside of them are blood-vessels, and through their 

 thin walls the blood is brought into close connection with 

 the oxygen dissolved in the water, just as is the case in 



