72 PHYSIOLOGY OF THE INVERTEBRATA. 



The ducts from the large many-lobed " liver " (pancreas) 

 open into the stomach and anterior portion of the intestine. 

 The secretion of this organ is pancreatic in function. 



The buccal cavity (Fig. 19, b and c) is furnished with hard 

 masticatory structures. The upper portion is provided with a 

 horny jaw, and on the floor is the odontophore or radula. 

 This odontophore (lingual ribbon) lies over a cartilaginous 

 support (Fig. 19, c). Powerful muscles are attached to this 

 support, and by the alternate expansion and contraction of 

 these muscles the odontophore is worked backwards and 

 forwards. By this mechanism the food taken into the mouth 

 is ground down against the horny palate. The odontophore is 

 a chitinous" product of the radular membrane (Fig. 19, c), and 

 is armed with tooth-like projections. The projections are 

 constantly being replaced as they are worn away by the 

 friction which ensues during mastication. 



In the Branchiogastcropoda (which includes JBucmium, 

 Patella, Cypra'a, &e.) the alimentary canal does not, as a rule, 

 vary very much from that of the Pulmo(jasteropoda. 



(6) The Ptcropoda are marine Molluscs. The foot, which 

 is small, is provided with two large, muscular, wing-like fins 

 (epipodia).* In Hyahva the oesophagus dilates into a kind 

 of crop or proventriculus, which is followed by a cylindrical 

 stomach. The intestine is tubular, describes two convolutions 

 in the substance of the liver, and then terminates in an anus 

 situated beneath the right fin. 



(7) The Ccphcdopoda. — This order is divided into the 

 BihrcmcJiiata and the Tctrahranchiata ; and the former order 

 is subdivided into the Dccapoda and the Octoijoda. 



The Dibranchiata include Sepia, Octopus, Argo7iauta, &c. 

 As an example of the digestive system (Fig. 20) we describe 

 that of Sepia ojficinalis. The mouth, armed with two 

 chitinous jaws which overlap each other, is provided wath an 



* Bee Cuvier's Mimoires pour servir a VJIistoire et V Anatomic des 

 Mollusques. 



