NUTRITION AND FEEDING MECHANISMS 



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Vermetus gigas. By means of a large pedal gland this animal secretes long 

 mucous strings, which extend up to 30 cm away from the shell, and which 

 entangle fine plankton material. At intervals these threads are drawn back 

 towards the mouth, to be seized by the radula and swallowed. In addition 

 Vermetus gigas is able to capture small organisms which come within 

 range of its radula. The ctenidia are small and the ciliary current slight in 

 this form, and are of minor importance in securing food. Certain other 

 filter-feeding gastropods also possess a supplementary mode of feeding 

 by means of thread-like mucus-traps (5, 76, 118). 



Feeding Mechanisms Involving Setae 



Crustacea, in common with other arthropods, lack cilia, but many 

 species nevertheless feed on minute particles which they strain from the 



Fig. 5.20. Ventral View of Calanus finmarchieus, to Show Water 

 Currents when the Animal is Swimming Slowly 

 Large incoming whirls to right and left; smaller feeding whirls on ventral surface. 

 Other arrows indicate direction of water currents towards and away from the animal. 

 (From Cannon, 1928.) 



surrounding sea water with the aid of fine setae occurring on appendages. 

 Feeding by copepods, the most important members of the zooplankton, 

 will be described first, before considering other filter-feeding crustaceans. 



Copepods. When swimming slowly and steadily, pelagic copepods such 

 as Calanus finmarchieus often feed automatically by straining off phyto- 

 plankton, although they are capable of selecting larger food items. 

 Swimming movements are due to rapid vibrations of the anterior appen- 

 dages, and some of the water drawn towards the animal is caught in a 

 vortex created by the activity of the maxillipeds and maxillules (Fig. 5.20). 

 This current passes through the stationary maxillae which bear long setous 

 filters, and food particles which are filtered off are passed to the mandibles. 



Because of their importance in the economy of the sea, copepods have 

 been studied intensively, particularly Calanus finmarchieus, which is widely 



M.A. 8 



