NUTRITION AND FEEDING MECHANISMS 



229 



brushed off the basket by the maxillipeds and transferred to the mouth. 

 Finally, we may mention here the parasitic pea'crabs (Pinnotheres), some 



Fig. 5.22. Anterior View of Porcelain-crab Porcellana longicomis while 



Feeding, to Show the Direction of the Water Currents Drawing Food in 



Suspension towards the Animal (from E. A. T. Nicol (81).) 



of which inhabit the mantle cavity of lamellibranchs. These animals collect 

 mucous food-strings from the gills of their hosts with the aid of setous 

 fringes on their claws (62, 81, 84). 



Efficiency of Filter-feeding among Invertebrates 



The food available to filter feeders consists of minute zooplankton, 

 phytoplankton, protozoa, bacteria and organic detritus. This organic 

 matter varies greatly in particle size, from several hundred micra in certain 

 algal cells to sub-microscopic colloidal dimensions in the case of detrital 

 matter. Animals probably differ in the efficiency of their straining appara- 

 tuses: those with structural sieves- — namely sponges, bivalves, copepods 

 and ascidians — retain particles down to 1 // in size; mucous net feeders, 

 on the other hand, filter out material down to 40 A in size. The food intake 

 is a function of the efficiency of the filter, amount of food present in the 

 surrounding sea water and pumping rate. Absolute filtering rates, of course, 

 vary greatly — from less than 100 ml/day in Calanus finmarchicus to 

 10 l./hour in Ostrea virginica. Jorgensen, however, finds that filtration rates 

 of different animals are of about the same order when expressed as litres 

 of sea water per millilitre 2 consumed. Calculated values are as follows — 



Sponges 



Echiuroid 

 Lamellibranchs 



Copepods 



Ascidians 



Grant ia compressa 

 Halichondria panicea 

 Urechis caupo 

 Ostrea virginica 

 Mytilus edulis 

 Calanus finmarchicus 

 Centropages hamatus 

 Ciona intestinal is \ 

 Molgula manhattensis J 



13 1./ml0 2 

 14, 31 l./ml Oo 

 20 l./ml 2 

 16 1./ml0 2 

 14-15 l./ml0 2 

 8 l./ml 2 

 8 l./ml 2 



10-20 l./ml O., 



