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 flgor 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 expan'sion 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 Branchiogasteropoda (which includes Buccinum^ 

 Patella/, Cyprma, &c.) the alimentary canal does not, as a rule, 

 vary very much from that of the Pulmogasteropoda. 



(6) The Pteropoda are marine Molluscs. The foot, whicli 

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

 (epipodia).* In Syalcea 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 Cephalopoda.-^l^h.is order is divided into the 

 Dibranchiata and the Tetrahranchiata ; and the former order 

 is subdivided into the Decapqda and the Octopoda. 



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

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

 that, of Sepia officiTialis. The mouth, armed with two 

 chitinous jaws which overlap each other, is provided with an 



* See Cuvier's Mimoires pour servir a VHistoire et V Anatomic del 

 Molliisques. 



