DIGESTION 263 



teases and amylases have been found in the stomach and intestine of sea- 

 urchins. The stomach is also the region of secretion in some tunicates 

 (Ciona). 



The majority of fishes possess a stomach in which enzymes are secreted 

 and the breakdown of foodstuffs commences. As in higher vertebrates 

 the stomach juices are acid, and a proteolytic enzyme (pepsin) and acid 

 are produced by special cells in gastric glands. These, known as chief 

 glands, are largely limited to the descending limb of the stomach, termed 

 the corpus (11, 14). 



In many marine animals the gut is provided with various kinds of diges- 

 tive diverticula, which increase the surface available for secretion and 

 absorption and in some instances permit a differentiation of function. 

 These structures show much diversity in different groups and need separate 

 consideration. The digestive diverticula of most polyclad turbellaria, bra- 

 chiopods, lamellibranchs and certain gastropods (in which they are also 

 termed liver, hepatopancreas or digestive gland) are concerned exclusively 

 with intracellular digestion of food particles. They are the loci both of 

 secretion and intracellular digestion in many gastropods; of secretion and 

 absorption in Aphrodite (a polychaete) and in crustaceans; and of 

 secretion alone in squid (hepatopancreas) and certain ascidians (liver 

 diverticulum). Adnexa of the gut (liver, pancreas) in fishes and higher 

 vertebrates secrete digestive fluids, among their other functions. 



Digestive caeca are a conspicuous feature of the anatomy of Aphrodite. 

 At the bases of the caeca there are little sieves, so that when fluid is forced 

 from the intestine into the caeca all but the finest particles are filtered off. 

 Food undergoes partial digestion in the intestine, and within the caeca 

 digestion is completed and nutrient matter absorbed (20). The pyloric 

 caeca of starfish (Asterias) function in a somewhat similar manner. Particu- 

 late and partially digested food is transported from the stomach into the 

 caeca by special tracts of flagellated cells; digestion is completed and ab- 

 sorption takes place in the caeca (2). 



In simpler crustaceans, exemplified by Calanus, secretion and absorption 

 take place in the large midgut and digestive diverticula are rudimentary. 

 With the appearance of diverticula in Malacostraca (isopods, decapods) 

 secretion becomes limited to this region. In the latter forms we have already 

 noted how hepatopancreatic secretions are discharged into the foregut 

 where foodstuffs are triturated and filtered. In this process enzymes are 

 mixed with the food and enzymatic degradation of nutrient material takes 

 place. Carbohydrates, fats and proteins are attacked (34). 



Gastropods show much specific variation in the matter of extracellular 

 secretion. We have seen how amylases and proteases are liberated from 

 buccal glands or diverticula of the foregut in certain genera (p. 256). 

 Extracellular enzymes are produced by the digestive diverticula of carni- 

 vorous prosobranchs and some opisthobranchs, and are discharged into 

 the stomach (Fig. 6.6). An amylase and, in some forms at least, a cellulase 

 are found in the crystalline style of particle-feeding lamellibranchs and 



