LAMELLIBRANCHIATA 



503 



material may, according to its nature, either be swept straight into 

 the mouth or come under the influence of ciHa working along re- 

 jection paths which direct it away from the mouth and toward the 

 outgoing circulation on the mantle. 



This complicated but well co-ordinated ciliary mechanism is 

 nearly always working when the lamellibranch is covered with water, 

 and the amount of water which passes through the mantle cavity of 

 a single mussel is surprisingly large. But it must be remembered that 

 this current also serves the purpose of respiration though the ex- 

 change of gases takes place through the medium of the mantle rather 

 than the ctenidia. At low tide the animal must close its shells and COg 

 accumulates within the mantle cavity. This chemical change depresses 

 ciliary activity and finally brings the cilia to rest, so that the store of 

 oxygen in the tissues is conserved. When the tide rises, however, 

 the cilia immediately resume activity. 



Though the majority of the lamellibranchs have the power of 

 movement it is thus seen that they feed in the manner of a sedentary 

 organism, and it is not surprising that there are many fixed and 

 burrowing forms. 



A short oesophagus leads directly into the stomachy which is a wide 

 sac receiving on each side the ducts of the digestive gland. The intestine 

 runs into the foot and makes one or more loops, eventually return- 

 ing to near the hind end of the stomach. It then passes through the 

 pericardium when it is usually surrounded by the ventricle and the 

 posterior aorta, ending as the rectum. The peculiarity of the digestive 

 system is the presence of a diverti- 

 culum of the intestine, the cells of 

 which secrete a crystalline style (Fig. 

 369); cilia in the diverticulum rotate 

 this always in the same direction so 

 that at its free end, projecting into 

 the stomach against a structure called 

 the gastric shield^ it is constantly 

 worn away and so mixed with the 

 contents of the stomach. It is com- 

 posed of protein to which is adsorbed 

 an amylolytic ferment and it may be 



Fig. 369. A, Section of part of the 

 alimentary canal of Donax. cc.m. 

 caecum of the intestine containing 

 C.St, crystalline style; g.s. gastric 



broken down and re-formed periodi- shield; int. intestine; M. mouth; 



cally. There is no doubt that this «^- oesophagus; st. stomach. B, 



for 



Transverse section across the cae- 



represents a special provision ... cum showing a7. ciliated epithelium 

 the digestion of carbohydrates and it and est. crystalline style composed 

 is also found in some gasteropods. of concentric layers of material. 

 For the rest, digestion of proteins After Barrois. 

 and resorption take place in the digestive gland, the cells of which 



