PHYSIOLOGY OF THE INVERTEBRATA. 67 



Is suspended in a spacious perivisceral cavity. "The walls 

 of this cavity are provided with cilia, the working of which 

 keeps up a circulation of the contained fluid." In Lingula 

 the intestine is long, and forms two bends before it terminates 

 in the pallial cavity. 



(2) The Clistcntcrata have a hinge uniting the two valves 

 of the shell. In the TcrchraUda the mouth opens downwards 

 into the pallial chambers, and is situated in the middle line, 

 about one-third of the length of the shell from the hinge. 

 The mouth of the BrachiojJod a has no rudiments of a maxillary 

 or dental apparatus. The oesophagus in Tcrehratula is short, 

 and is situated between the anterior portions of the so-called 

 liver. The stomach is an oblong organ which is dilated at 

 the cardiac end ; the narrower may be spoken of as the 

 pyloric portion, but there is no valvular structure at the 

 pylorus. The cardiac portion of the stomach is surrounded 

 by the so-called liver. The intestine is short, straight, and 

 is continued in a line with the pylorus to the interspace 

 between "the attachments of the adductores longi and cardi- 

 nales to the ventral valve," where it ends blindly. The so- 

 called liver (pancreas) is a large organ consisting of numerous 

 ramified follicles. There are usually two ducts from this 

 organ, communicating with the cardiac portion of the stomach. 

 The alimentary canal is freely suspended in the body cavity 

 by delicate membranes which stretch from the body walls. 



The Mollusca. 



The MoUmca form the second division of the Malacozoic 

 Series ; and this division comprises seven orders. 



d) The Lamcllibranchiata include the Ostrca, Anodonta, 

 Mytdus, Fedcn, Cardium, Mya, Unis, &c. The mouth is 

 bounded by lips, which are usually produced into two labial 

 palps. These palps are ciliated, and by the action of the cilia 

 food particles, which have passed into the branchial chamber, 

 are driven into the mouth. The mouth of a Lamellibrauch 



