MANY water-living animals obtain their food from suspended material, micro-organisms or fine dispersed detritus occurring in the surrounding water. Sponges and ascidians, for example, are typical filtering animals. Most lamellibranchs belong to the same group. But filtering forms may also be found among gastropods, crustaceans, insects, polychætes, vertebrates, etc. Apart from lamellibranchs, little information is available regarding the rate and efficiency with which particles are retained from water passing the filtering organs. Bidder1 calculated the water propulsion through Leucandra aspersa (a sponge), whereas investigations on the rate of feeding of ascidians seems to be lacking. During a stay at the Plymouth Laboratory, I was offered an opportunity of determining the feeding-rates of species of sponges and ascidians. The feeding-rates have been calculated from the rate of disappearance of particles from the surrounding water2. Suspensions of colloidal graphite were used, namely, 'Prodag' grade C and 'Aquadag' grade S, manufactured by E. G. Acheson, Ltd., London ; the former has an average particle size of 4–5µ, whereas the latter has an average particle-size of 2µ. The concentrations of the graphite suspensions were determined photometrically by means of a portable Eel photometer.
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