HOW THEY FEED
Normally one does not think of clams and oysters as being very active feeders and certainly, in comparison with the voracious methods of fish and squid, the bivalves are rather peaceful eaters. Yet in their characteristic way they are highly efficient and, in proportion to their size, possess a large and varied menu. Most clams feed on minute plants and, in a relatively short time, can filter from the sea water an extraordinary number of living diatoms and dinoflagellates—microscopic, swimming plants—and protozoa of the ocean. A few genera, such as the small Cuspidaria and Poromya clams, are carnivorous and feed upon small living or dead animals, usually crustaceans and annelid worms.
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Figure 11. Extended animals of some bivalves, showing various types of siphons. a, Mya arenaria Linné; b, Tellina agilis Stimpson; c. Tagelus plebeins Solander; d, Ensis directus Conrad. (from A. E. Verrill 1873.)
The bivalves fall into two general classes of feeders—suspension feeders which merely pump water through their mantle cavity and thus obtain free-swimming or suspended creatures from the water; or deposit feeders which suck up food from the muddy bottom with their long, mobile inhalant siphons. Among the suspension feeders are the oysters, scallops, venus clams, cockles, the shipworms and many others. They may or may not possess siphons, but when present these are generally short. The deposit feeders include such forms as Tellina, Macoma and Abra which all have long siphons.
Whether food is taken in through the inhalant siphon as in the tellins or through a slit in the mantle as in the scallops, it must pass over the gills. These filament-like organs are covered by a thin sheet of mucus. Food passing through the gills becomes ensnarled in the mucus which is transported by water currents and myriads of tiny, hair-like cilia. Mucus is constantly be-
