DIGESTION 



257 



enzymes secreted in the midgut. The stomach is relatively simple in the 

 Entomostraca. In the Malacostraca, especially in decapod crustaceans, 

 a powerful gastric mill is present, provided with calcareous teeth which 

 grind up the food. 



Sorting Mechanisms 



The stomach is often the site of special mechanisms for sorting out the 

 finely divided food and passing it on to other regions where digestion is 

 completed. In many Crustacea these filtering mechanisms are very complex, 

 and guard the entrance to the hepatopancreas in which absorption of 



Cardiac foregut 



Lateral food stream 



Midgut 

 filter 



Anterior 

 pyloric 

 chamber 



Midgut 



groove 

 Oesophagus 



Hepatopancreatic 

 duct 



Press 

 Gland filter 



Card 10- pyloric 

 valve 



Fig. 6.1. Median View of the Foregut of Nephrops (after Yonge (69).) 



digested foodstuffs takes place. The foregut of Ligia bears a series of chitin- 

 ous cushions and lamellae furnished with spines and bristles. These are 

 arranged so as to form a filter for separating the liquid portion of the 

 food from the solid particles. When the foregut contracts, liquid food is 

 squeezed through the filter, carrying with it very fine particles. Secretions 

 from the hepatopancreas are discharged into the gut cavity and attack the 

 food. Filtered fluids enter the hepatopancreas; more solid portions of the 

 food, mixed with digestive enzymes, pass into the intestine, where further 

 digestion occurs (52). 



In the Norway lobster {Nephrops) the cardiac gizzard is succeeded by a 

 pyloric chamber. The two chambers are separated by a cardio-pyloric 

 valve (Fig. 6.1). Dorsally, there is a channel termed the midgut filter which 

 opens into the midgut. Behind the valve the walls of the pyloric foregut 

 are thickened to form a press, and beneath this is a gland filter made of 



M.A. — 9 



