578 THE INVERTEBRATA 



similar to that of Helix ^ but contains only one kind of cell. This cell 

 takes up the finely divided food which reaches the gland and digests it 

 by intracellular ferments. The intestine runs into the foot and makes 

 one or more loops, eventually returning to near the hind end of the 

 stomach. It then passes through the pericardium where it is usually 

 surrounded by the ventricle, ending as the rectum. The peculiarity 

 of the digestive system is the presence of a diverticulum of the 

 intestine, the cells of which secrete a crystalline style (Fig. 394) ; some 

 cilia in the diverticulum rotate this and others move it forward so that 

 at its free end, projecting into the stomach against a structure called 

 the^fl5inc^/zi>/^, it is constantly worn away and the style material mixed 

 with the contents of the stomach. It is composed of protein to which 

 is adsorbed an amylolytic ferment and it may be broken down and 

 re-formed periodically. There is no doubt that this represents a special 



Fig. 394. A, Section of part of the alimentary canal of Donax. cent, caecum 

 of the intestine containing est. crystalline style ; g.s. gastric shield ; int. in- 

 testine ; M. mouth ; oe. oesophagus ; st. stomach. B, Transverse section across 

 the caecum showing cil. ciliated epithelium and est. crystalline style composed 

 of concentric layers of material. After Barrois. 



provision for the digestion of carbohydrates and it is also found in 

 somegasteropods. For the rest, digestion of proteins and absorption 

 take place in the digestive gland, the cells of which have a surprising 

 power of taking up solid particles. In the oyster, it may be mentioned, 

 there is an extraordinary abundance of leucocytes which wander here, 

 there and everywhere, through the body. It has been shown that 

 they enter the stomach and ingest diatoms and other food particles 

 there, speedily digesting them and wandering over the body afterwards, 

 so that they play a unique part in the transport of food. 



The lamellibranchs are most conveniently classified by the structure 

 of their ctenidia. We have firstly three groups which can be arranged 

 in an evolutionary series, showing the ctenidia to become larger, 

 more complex and solid organs. Lastly there is an isolated group, the 

 Septibranchiata, in which the habit of life has completely changed 

 and the ctenidia have practically disappeared : 



