NUTRITION AND FEEDING MECHANISMS 



219 



of that size can be completely retained or let through at will. An 

 estimated diet sheet for bivalves (Tivela, Mytilus) is given in Table 5.1, 

 and data for filtering rates are listed in Tables 4.5 and 4.6 (p. 169). 

 Quantities of water pumped by bivalves are surprisingly large, up to 34 

 l./hour in the oyster, for example. Under suitable conditions the oyster 

 feeds almost continuously, and stomachs are nearly always found to 

 contain food (6, la, 17, 28, 52, 58, 59, 77a, 91a, 110, 114, 116). 



TABLE 5.1 

 Materials Ingested by Ciliary-feeding Lamellibranchs 



Sign + indicates generally; ± commonly, but only small individuals or species; — rarely or not at all. 

 * Quantity of dry organic matter potentially ingested by one adult animal per year. Based on computa- 

 tions for a single mussel (Mytilus californianus) 70 mm in length, filtering 2-5 1. of water per hour. 



(FromCoe(17).) 



Protochordates. Besides the groups we have just examined, ciliary feeding 

 mechanisms are widely utilized by protochordates. Although pharyngeal 

 gills are often involved, considerable diversity exists in the functional 

 patterns concerned with feeding in these forms. 



Pharyngeal gill-bars form a straining apparatus in Amphioxus and other 

 cephalochordates. These animals lie buried in sand, with the oral region 

 at the surface, and draw a feeding current into the mouth, over the gill- 

 bars and out through the atrium. Large particles are strained off by buccal 

 cirri and smaller particles by the gills (Fig. 5.16). The feeding current is 

 created by lateral cilia on the pharyngeal bars, where particles become 

 entangled in mucus secreted by the ventral endostyle, and these mucous 

 masses are driven dorsally by frontal cilia on the pharyngeal surfaces of 

 the gill-bars. In the dorsal wall of the pharynx the food material enters a 

 dorsal ciliated groove and is moved posteriorly to the oesophagus and 

 stomach. There is also some minor collecting of food in the buccal cavity 



