260 THE BIOLOGY OF MARINE ANIMALS 



Intracellular Digestion 



In the coelenterates and probably the ctenophores extracellular digestion 

 is largely limited to the preliminary digestion of proteins; the digestion of 

 carbohydrates, fats and the final breakdown of polypeptides to amino- 

 acids are accomplished intracellularly. Turbellaria, like coelenterates, are 

 carnivorous animals. In triclads and polyclads there is some preliminary 

 digestion of proteins extracellularly, reducing the foodstuffs to a condition 

 in which they can be ingested by phagocytic cells lining the alimentary 

 canal (36, 36a, 39). 



More advanced forms retaining intracellular digestion include poly- 

 chaetes, brachiopods, molluscs, with the exception of cephalopods, 

 echinoderms and Limulus and pycnogonids among arthropods. 



Intracellular and extracellular modes of digestion coexist in the lugworm 

 Arenicola. Food particles are engulfed by the epithelial cells of the stomach 

 and are passed to amoebocytes in which digestion is completed. Indigest- 

 ible material is deposited in the coelom or gut lumen (38). 



Brachiopods are ciliary feeders and their food consists of finely divided 

 material. It appears that no digestive enzymes are secreted into the gut; 

 the food particles are ingested and broken down by the cells of the digestive 

 diverticula. 



Most lamellibranchs are ciliary feeders, and have sorting mechanisms 

 for selecting fine particles on gills, palps and the stomach wall. In lamelli- 

 branchs such as Ostrea, Ensis and Mytilus, some digestion of carbo- 

 hydrates takes place in the gut lumen through the action of an amylase set 

 free by dissolution of the crystalline style. Fluids and fine materials enter- 

 ing the tubules of the digestive diverticula are taken up by the epithelial 

 cells in which digestion is completed. The cells of the digestive diverticula 

 have been shown to contain protease, lipase and amylase. Small amounts 

 of proteolytic and lipolytic enzymes occurring free in the lumen of the 

 stomach and intestine result from the breakdown of phagocytes and cyto- 

 lysis of the distal ends of cells in the digestive diverticula (8, 17, 50, 59, 74). 

 In septibranch bivalves, which have secondarily become carnivorous, 

 ingested food masses are triturated by a muscular gizzard but, like other 

 members of the class, septibranchs continue to digest protein intracellularly 

 in the digestive diverticula. 



The gastropods show feeding and digestive mechanisms of great diversity 

 which permit exploitation of varied food resources. Primitively digestion 

 is intracellular but there is a tendency for this to be replaced by enzymatic 

 digestion within the gut cavity. In herbivorous prosobranchs conditions 

 are somewhat similar to those in lamellibranchs. An extracellular amylase 

 is secreted — by foregut diverticula in Patella, and from the style in Crepi- 

 dula and Vermetus — but digestion of fats and proteins takes place intracel- 

 lularly within cells of the digestive diverticula. Intracellular digestion occurs 

 in the digestive gland of carnivorous prosobranchs (Natica, Murex), 

 in opisthobranchs {Pleurobranchus, Hermaea) and in gymnosomatous 



