DIGESTION 275 



(splanchnic) nerves produces gastric and intestinal activity. Both para- 

 sympathetic and sympathetic systems are excitatory, and their effects are 

 additive in the regions where they overlap (11, 54). 



ABSORPTION 



When an extensive complement of digestive enzymes is secreted, foodstuffs 

 are broken down into their component units before absorption takes place. 

 In that event carbohydrates are absorbed as monosaccharides, proteins as 

 amino-acids, and fats as fatty acids and glycerol. Emulsification is aided 

 by the presence of bile salts and monoglycerides, and when small-enough 

 particles are produced they are absorbed directly. Products of digestion 

 have been detected in the body fluids after feeding. In Strongylocentrotus, 

 for example, reducing sugars and non-protein nitrogen increase in the 

 coelomic fluid as digestion proceeds. Digestion has also been followed 

 histologically by looking for uptake of iron saccharate and fat spherules. 

 Physiological studies on absorption in marine animals are rather meagre, 

 however. 



Regions of absorption coincide with digestion in many instances. 

 Absorption takes place in the intestine of polychaetes (Sabella) and sea- 

 urchins, intestine and caeca of Aphrodite, intestine and caecum of squid, 

 pyloric caeca of starfish, and intestine and pyloric caeca of fish. In lower 

 crustaceans (Calanus) absorption takes place in the midgut, but with the 

 development of the hepatopancreas, absorption becomes limited to that 

 organ (decapods). Absorptive cells are usually distinct from glandular 

 secretory cells. In other groups the reader will recall instances in which 

 particles are ingested to be followed by intracellular digestion, e.g. coelen- 

 teron of sea-anemone, digestive diverticula of lamellibranchs and many 

 gastropods, and so forth. 



ELIMINATION OF FAECES 



Faeces contain the undigested and indigestible residues of food, together 

 with water, salts, mucus, bacteria, desquamated cells, etc. In gastropods 

 and lamellibranchs, indigestible particles and chlorophyll break-down 

 products, extruded by the digestive cells of the midgut gland, are eliminated 

 in the faeces. Bile pigments, the breakdown products of haemoglobin 

 metabolism, are discharged into the intestine of vertebrates and pass into 

 the faeces after undergoing modification. In some animals the intestine 

 participates in ionic regulation. The gut of fish is relatively impermeable 

 to magnesium and sulphate, which pass out in the faeces (p. 72), and 

 studies with marine molluscs (Acanthodoris, Mytilus) have shown that 

 radioactive strontium is excreted in part by cells of the digestive gland (25). 

 Special means of dealing with the faeces occur in certain animals. Thin 

 peritrophic membrances of chitin are formed about the faeces in some 

 crustaceans (shrimps, barnacles, etc.). In gastropods and lamellibranchs 

 the intestinal contents are consolidated with mucus and moulded by 

 ciliary and muscular action into a faecal cord. From this cord firm faecal 



