258 



THE BIOLOGY OF MARINE ANIMALS 



chitinous plates and rods (Fig. 6.2). Hepatopancreatic secretion passes 

 into the cardiac foregut by way of the gland filter and ventral channels. 

 Food is ground up in the gastric mill and attacked by enzymes. Finely- 

 divided material passes back into the pyloric chamber by the midgut 

 filter and lateral channels; dissolved material passes back to the hepato- 

 pancreas through the gland filter and ventral channels. The filters prevent 

 all but the finest matter from reaching the hepatopancreas ; larger particles 

 are passed on to the midgut (69). 



Among filter-feeding lamellibranchs the stomach is concerned with the 

 final sorting of finely divided food. The wall of the stomach is ciliated and 



Chi tin 

 pla 



Lower ventral valve 



Fig. 6.2. Gland-filter in the Pyloric Foregut of Nephrops ( x 13) 



(From Yonge (69).) 



bears a caecum, a cuticular gastric shield and several sorting areas (Fig. 

 6.3). The ducts of the digestive gland open into the stomach, as does a 

 style sac containing a rod-like structure termed the crystalline style, which 

 bears against the gastric shield. Food material on entering the stomach is 

 carried by ciliary action into the caecum where sorting occurs: heavy 

 particles fall into a groove which conducts them across the floor of the 

 stomach to the intestine ; lighter particles are carried outwards by a ciliary 

 tract in the caecal wall. The ciliary fields on the stomach wall transport 

 fine particles, sorted out in the caecum and elsewhere, towards the ducts of 

 the digestive gland, into which they are directed by inwardly beating cilia; 

 while cilia on the opposite sides of the ducts beat outwards, thus maintain- 

 ing a circulation within the diverticula. The style itself is kept revolving 

 by the action of cilia in the style sac, and its motion assists in mixing 

 particles in the stomach cavity. Coarse particles and mucous masses are 

 carried across ridges in the stomach wall to the midgut. The whole ciliary 

 mechanism is one which allows only fine particles and liquid matter to 

 enter the digestive diverticula. Carbohydrases released from the crystalline 



