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COMPARATIVE PHYSIOLOGY 



few words may be inserted here with reference to the methods 

 adopted by animals of sluggish and sedentary habits for 

 maintaining a supply of food. Most widespread of such feed- 

 ing mechanisms are those which involve the entanglement 

 of food particles, such as organic debris and micro-organisms 

 of the plankton, in mucous slime through the production 

 of ciliary currents to maintain a constant flow of water over 

 the slime-glands and to propel the entrapped food-particles 

 towards the mouth. In Amphioxus, for example (Orton), 

 water flows from the pharynx into the atrium by the lashing 



of cilia which line the sides of 

 the pores in two lateral rows ; 

 these cilia do not play a direct 

 part in the collection of food- 

 particles, v/hich are caught in a 

 fine sheet of mucus secreted by 

 the endostylar gland cells and 

 thrown on to the sides of the 

 pharynx by the cilia of the 

 ventral groove. This sheet of 

 slime with its entrapped food- 

 particles is worked up into 

 cylindrical masses driven to- 

 wards the dorsal groove by 

 cilia which line the inner wall 

 of the pharynx. The cilia of the 

 hyperpharyngeal groove maintain 

 a current of this slimy suspension in the direction of the in- 

 testine where digestion and absorption take place. Essentially 

 similar arrangements exist in Tunicates, Amphioxus, and in 

 at least one Vertebrate, the Ammocoete larva of Petromyzon. 

 In the bivalve molluscs it is again the structures which 

 descriptive anatomists have labelled gills which constitute 

 the ciliary net. Water laden with organic debris and micro- 

 organisms filters between the filaments of the gills through 

 the action of currents produced by the lateral cilia. A 

 ventrally directed current due to the frontal cilia washes 

 the food-particles entangled in slime downwards towards a 



Fig. 23. — Ciliary currents on the 

 Lamellibranch gill. 



