262 



THE BIOLOGY OF MARINE ANIMALS 



role amoebocytes play in the digestive processes of lamellibranchs by 

 permitting the utilization of diatoms and other food particles which are 



too large to gain access to the tubules 

 of the digestive diverticula. 



Extracellular digestion is well de- 

 veloped in echinoderms, but there 

 is also some evidence for the co- 

 existence of intracellular digestive 

 mechanisms. Amoebocytes in the gut 

 epithelium and lumen of echinoids 

 and holothurians participate in di- 

 gestion to greater or lesser degree 

 by ingesting particles and absorbing 

 dissolved nutriment. Phagocytes in 

 the pyloric and intestinal caeca of 

 asteroids have a similar role (62). 



Fig. 6.5. Section through a Liver 

 Tubule of a Shtpworm, Showing 

 Phagocytic Cells Containing Par- 

 ticles of Wood (after Potts, 1923.) 



Extracellular Digestion 



Intracellular digestion is the more 

 primitive mechanism from which ex- 

 tracellular digestion has developed as a specialization, notably in the 

 annelids, Crustacea, cephalopods and chordates, all groups containing 

 active forms. Extracellular digestion has certain apparent advantages in 

 that it breaks down food substances and eliminates indigestible material 

 rapidly and also permits a compact and reduced alimentary canal. Various 

 specializations of structure and function have appeared in conjunction 

 with the secretion of digestive enzymes into the gut lumen. There is regional 

 differentiation for secretion of enzymes, absorption of digestive products 

 and elimination of faeces. Special digestive enzymes are developed to meet 

 particular needs, secretion is regulated and the enzymatic environment is 

 controlled with apparent nicety. 



Digestive enzymes are secreted by unicellular glands lining the digestive 

 tract or localized in special diverticula and by compound glands of many 

 sorts. In coelenterates secretory cells are dispersed through the endoderm 

 in hydrozoa, localized in gastric filaments in scyphozoa or in mesenterial 

 filaments in anthozoa. A protease is secreted which initiates the breakdown 

 of proteins, the final conversion of polypeptides to amino-acids taking 

 place intracellularly. Digestive enzymes (proteases) are secreted into the 

 stomach and intestine of nemertines and in the stomach of bryozoa. In 

 most polychaetes enzymes are secreted by unicellular glands in the stomach 

 and intestine (terebellids, sabellids and serpulids). Enzymes identified in 

 the gut fluids of sabellids and terebellids include a protease, lipase and 

 amylase (18). A protobranch Nucula differs from most bivalves in that 

 digestion is extracellular, taking place in the stomach lumen into which 

 enzymes (protease, lipase and amylase) are secreted (56a). The stomach 

 of the starfish (Asterias) produces a strong proteolytic secretion, and pro- 



