MOLL U S C A. 



CHAPTER XL. 



SUB-KINGDOM MOLLUSCA. 



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

 cluding soft-bodied, unsegmented 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 haemal system, which lies dorsally, and the neural system, 

 which is situated towards the ventral aspect of the body. The 

 nervous system (fig. 196) 

 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 ab- / ' 



sent. Distinct respiratory Fig I96 ,_ Diagram of a Mollusc . a Alimen . 



Organs may Or may not be tary canal; h. Heart; / Foot; n Cerebral 



present. Reproduction is gi $ ganglion ; *" Pari "- 

 sexual, though gemmation 



is also occasionally superadded. The higher Mollusca 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 

 Brachiopoda, and in a few other forms, the intestine ends 

 caecally. In some the mouth is surrounded by ciliated ten- 

 tacles (Polyzoa, fig. 199); in others it is furnished with two 

 ciliated arms (Brachiopoda, fig. 204) ; in the bivalves (Lamelli- 

 branchiata] it is mostly furnished with four membranous pro- 

 cesses or palpi (fig. 208) ; in others it is provided with a com- 



