274 THE BIOLOGY OF MARINE ANIMALS 



and absorbed. Ciliary beating alone is responsible for the manifold mech- 

 anical processes occurring in the lamellibranch gut — moving food particles, 

 separating fine from coarse particles in the stomach, mixing the food with 

 enzymes, rotating the crystalline style, transporting digestible matter 

 to the diverticula and driving the mucous string through the intestine. 

 Both ciliary and muscular peristaltic movements act co-operatively in 

 gastropods, e.g. Aplysia. Peristaltic waves push food into the crop and 

 gizzard, and fluids are driven backwards and forwards into the anterior 

 gut from the digestive diverticula and stomach by contraction and dilata- 

 tion of the latter structures. Larger particles reaching the intestine are 

 moulded into a faecal rod with mucus, and propelled to the anus by a 

 combination of ciliary and muscular action (35, 47). 



Among polychaetes there is much variation in the way in which the gut 

 contents are transported. In particle-feeding sabellids and serpulids, cilia 

 alone are used ; in Clymenella cilia transport food through the oesophagus 

 and muscular contractions move food through the stomach; regular 

 peristaltic waves occur in the intestine of Tomopteris; terebellids pro- 

 pel food through the gut mainly by muscular action. Such examples could 

 be multiplied (18, 33, 38, 65). 



There are no cilia in Crustacea and movement of food through the gut 

 depends on muscular contraction. In Nephrops, for example, food is 

 pushed through the oesophagus into the cardiac foregut by the action of 

 constrictor and dilator muscles. Anterior and posterior gastric muscles 

 actuate the gastric mill, while rhythmical contraction and expansion of the 

 hepatic tubules (which bear circular and longitudinal muscles) cause 

 secretion to be forced out and dissolved materials to be taken in. 

 Peristaltic and antiperistaltic waves occur in the midgut, and pronounced 

 peristalsis takes place in the hindgut. In smaller Crustacea, such as prawns, 

 there is regular intake of water at the anus, and this seems to stimulate 

 antiperistalsis, followed by defaecation. The muscles of the intestine appear 

 to be under the control of an autonomous internal plexus, but can be 

 regulated by extrinsic nerves originating in the nerve cord (23, 60, 69). 



In fishes the intrinsic musculature of the gut wall likewise propels the 

 food bolus, churns and breaks up the food mass and thoroughly mixes it 

 with digestive secretions. The musculature of the oesophagus is striated 

 and food is pushed into the stomach by peristaltic waves. The walls of the 

 stomach and intestine contain circular and longitudinal smooth muscle, 

 and nervous plexi which are responsible for autonomous activity of the 

 gut wall. Movements take the form of local contractions, peristaltic waves, 

 waving of the pyloric caeca, etc. A strongly-developed sphincter at the 

 junction of the pylorus and duodenum regulates the passage of chyme from 

 the stomach into the intestine. The gut plexi receive extrinsic nerves from 

 the c.n.s. via autonomic pathways, and these regulate and modify activity 

 of the alimentary canal. Stimulation of the visceral vagus causes the oeso- 

 phagus to contract, and evokes local contractions and peristalsis of the 

 stomach in both selachians and teleosts. Stimulation of the sympathetic 



