293 



MOLL USC A. 



CHAPTER XLI. 

 SUB-KINGDOM MOLLUSCA. 



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 haemal system, 

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

 towards the ventral aspect of the body. The nervous system 

 (fig. 121) in its highest de- 

 velopment 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 oc- 

 casionally absent. Distinct 

 respiratory organs may or 

 may not be present. Re- 

 production is sexual, though 

 gemmation is also occa- 

 sionally superadded. The higher Mollusca are all simple ani- 

 mals, 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. 124); in others it is furnished with two 

 ciliated arms (Brachiopoda, fig. 128) ; in the bivalves (Lamelli- 

 branchiata) it is mostly furnished with four membranous pro- 

 cesses or palpi (fig. 130) ; in others it is provided with a com- 

 plicated apparatus of teeth (Gasteropoda, fig. 132, and Petro- 



Fig. 121. Diagram of a Mollusc, a Alimen- 

 tary canal ; k Heart ; f Foot ; n Cerebral 

 ganglion ; ' Pedal ganglion ; n" Parieto- 

 splanchnic ganglion. 



