MOLLUSCA : GENERAL CHARACTERS. 227 



MOLLUSCA. 



CHAPTER XLI. 



8 UB-KINGD OM MOLL USCA. 



SUB-KINGDOM MOLLUSCA. The Mollusca may be defined as in- 

 cluding soft-bodied animals, which are usually provided with an 

 exoskeleton. The intestinal canal is bounded by its own proper 

 walls, and is completely shut off from the perivisceral cavity. 

 The alimentary canal is situated between the hasmal system, 

 which lies dorsally, and the neural system, which is situated 

 towards the ventral aspect of the body. The nervous system 

 in its highest development consists of three principal ganglia, 

 which are reduced to one in the lower forms. Usually there 

 is a distinct propulsive organ by which the circulation is carried 

 on, but this is occasionally absent. Distinct respiratory organs 

 may, or may not, be present. Reproduction is sexual, though 

 gemination is, also, occasionally superadded. The higher Mol- 

 lusca are all simple animals, but many of the lower forms are 

 capable of forming colonies by continuous gemmation. 



The digestive system in all the Mollusca consists of a mouth, 

 gullet, stomach, intestine, and anus though in some of the 

 Srachiopoda the intestine ends cascally. In some the mouth 

 is surrounded by ciliated tentacles (Polyzoa, fig. 73) ; in others 

 it is furnished with two ciliated arms (Brachiopoda, fig. 75) ; 

 in the bivalves (Lamellibranchiata) it is mostly furnished with 

 four membranous processes or palpi ; in others it is provided 

 with a complicated apparatus of teeth (Gasteropoda, fig. 77, and 

 Pteropoda) ; and, lastly, the Cephalopoda have in addition 

 horny or calcareous mandibles, forming a kind of beak. Well 

 developed salivary glands are usually present ; the liver in the 

 higher forms is of large size, and pours its secretion either 

 into the stomach or into the commencement of the intestine ; 

 and a renal organ has been detected in most of the Mollusca 

 proper. There is no distinct absorbent system, but the pro- 

 ducts of digestion pass by exosmose into the general abdominal 

 cavity, and thence into the larger veins, which are pierced by 

 numerous round holes for this purpose. 



The blood is colourless, or nearly so. In the Polyzoa the 

 circulation is carried on by ciliary action, and there is no dis- 



Q2 



