410 



CHAPTEE XXI. 



SUB-KINGDOM MOLL USCA. 



POLYZOA. 



SUB -KINGDOM MOLLUSCA. The Mollusca comprise the ani- 

 mals ordinarily known as Shell-fish, from their commonly 

 possessing an exoskeleton or shell. The Molluscs are soft- 

 bodied and destitute of any evident segmentation. Commonly 

 the integument secretes a hard calcareous or horny envelope, ~but 

 this may be absent. The alimentary canal is always present, 

 and never communicates with the body -cavity. The nervous 

 system consists typically of three pairs of ganglia, disposed in 

 a characteristically scattered manner ; but in the lower forms 

 a single ganglion alone is present. A heart may or may not 

 be present, and there may or may not be distinct respiratory 

 organs. 



As a matter of course, it is only with the shell of the 

 Mollusca that the palaeontologist has to deal, and those 

 forms which are destitute of this structure are wholly un- 

 known in the fossil condition. The special characters of 

 the shell will be treated of in speaking of the separate 

 classes. In the meanwhile it is sufficient to draw atten- 

 tion to some general considerations. In the Sea-mosses arid 

 Sea-mats (Polyzoa), the animal is compound, and the hard 

 structures secreted by the colony would not come under the 

 common designation of a " shell." In these cases the invest- 

 ment of the colony would rather be termed a " polypidom," 

 and when of a horny nature, it does indeed show a very 



