styles across the one-drop electrode, was found by 

 Dean (1958) to vary between 5.8 and 6.0. 



Extracts of the digestive diverticula of 0. 

 edulis have pH values from 5.6 to 5.9; the varia- 

 tions are probably associated with the resting and 

 active stages of digestion. The styles of C. 

 rirginica contain a heat-labile substance, probably 

 an enzyme, which has the ability to attack certain 

 algal cells only during the dissolution of the style 

 (ir within a very short period after the dissolution. 

 This has been reported by Dean (1958) who 

 observed rapid disintegration of Cryptornonas cells 

 in buffered sea water (pH 6.0) containing style 

 extract. Monoehrysis sp. were immobilized by 

 the extract while Isochrysis sp. were not affected 

 and were able to swim near or even touch the 

 style. Dean thinks the "enzyme" may be a 

 protease, lipase, or amylase. The observed results 

 may be interpreted as the differences in the resist- 

 ance to digestion by different species of algae used 

 by the oyster as food. 



It has been pointed out in support of the im- 

 portance of extracellular digestion that fragments 

 of partially disintegrated large diatoms (Coscin- 

 odiscm, Melosira, Skeletonema) are frequently 

 found in the stomachs of C. virginica (Nelson, 

 19.34), but the question of the significance of 

 extracellular digestion in bivalves has not been 

 settled. Weak proteolytic action was found in 

 the stomach of the giant clam, Tridacna, the 

 pearl oyster, Pinctada (Mansour-Bek, 1946, 

 1948), and in the crystalline style extract of C. 

 virginica (Chestnut, 1949) and strong amylo- 

 lytic activity in the stomach of the oyster was 

 demonstrated by a number of investigators. 

 Oysters apparentl}^ have a great capacity to 

 utilize materials rich in carbohydrates. 



ABSORPTION OF FOOD BY GILLS AND 

 MANTLE 



The idea that the exposed surfaces of bivalves, 

 particularly the gills, palps, and mantle, absorb 

 the organic matter dissolved in sea water (Ranson, 

 1926, 1927) is not substantiated by experimental 

 evidence. In experiments with 0. edulis Yonge 

 (1928) has shown that the oyster absorbs glucose 

 from the water but that this absorption takes 

 place tlu'ough the alimentary canal and digestive 

 diverticula. No absorption was recorded in the 

 animals in which the access of water to the esopha- 

 gus was prevented by stuffing the mouth with wax 

 plugs. Glucose may be absorbed, however, by 



the phagocytes which accumulate on the surface of 

 the mantle. The results of Yonge's observations 

 were confirmed by Roller (1930) in his experiments 

 with MytUus edulis and Mya arenaria. 



Since phagocytes normally aggregate on the 

 surface of the mantle and gills, it is possible that 

 the oyster may absorb the substances present in 

 the surrounding media by means of these wander- 

 ing cells. Yonge admits this possibility in the case 

 of oysters fed u-on saccharate, and I observed that 

 the particles of iron oxide added to the water in 

 which I kept C. virginica were ingested by the 

 phagocytes of the gills and transported to the 

 deeper parts of the body. 



FOOD AND FEEDING 



The study of food of the oyster has attracted 

 the attention of many investigators who examined 

 the stomach contents and recorded the variety of 

 organisms found in it. One of the earliest obser- 

 vations was made more than a century and a half 

 ago by Reade (1844, 1846), who was "induced" to 

 examine the contents of the stomach of British 

 oysters and the "well known ciliary currents In 

 the fringes of the oyster." His curiosity was well 

 satisfied, for he found "myriads of living nomads, 

 the Vibrio also in great abundance and activity, 

 and swarms of a conglomerate and ciliated living 

 organism, which may be named Volvox ostrearius, 

 somewhat resembling the Volvox globator, but so 

 extremely delicate a structure, that it must 

 be slightly charred to be rendered permanently 

 visible." He listed also a number of common 

 diatoms, silicoflagellates, and desmids which he 

 called "Infusoria." It is impossible to guess the 

 true identities of the "Vibrio" and "Volvox." 



Since the oyster is a filter feeder it is natural 

 to expect that the contents of its alimentary canal 

 would reflect the material suspended in water. 

 Many of the investigators were unduly impressed 

 by the occurrence of one or several species in the 

 stomach and because of their abundance considered 

 them to be of primary importance in the oyster 

 diet. Opinions based on such examinations re- 

 ferred to the following forms found in the European 

 oyster as important food materials: Navicula 

 fusiformis v. ostreari, Griin. (Puysegur, 1884); 

 desmids, minute animals, and dead organic matter 

 (Hoek, 1883); bottom diatoms Nitzschia punctata, 

 N. acuminata, N. sigma, Grammatophora oceanica, 

 and Diploneis bombus var. densestriata, the latter 

 species being considered of special importance for 



ORGANS OF DIGESTION AND FOOD OF THE OYSTER 



231 



