742 NATURAL HISTORY OF AQUATIC AIs^IMALS. 



subdivide until they break up into a great number of blind ovoidal sacs, into which the biliary- 

 secretion is poured from the cells of their walls. A thick stratum of these follicles surrounds the 

 stomach, except at its back or dorsal side. It is not correct to speak of the liver of the Oyster as 

 we speak of the liver of a higher animal. Its function in the Oyster is the same as that of three 

 •different glands in us, viz, the gastric follicles, the pancreas, and the liver, to which we may add the 

 salivary, making a total of four in the higher animals which is represented by a single organ in 

 the Oyster. In fact, experiment has shown that the secretion of the liver of mollusks combines 

 characters of at least two, if not three, of the glandular appendages of the intestine of vertebrated 

 animals. There are absolutely no triturating organs in the Oyster for the comminution of its 

 food ; it is simply macerated in the glandular secretion of the liver and swept along through the 

 intestine by the combined vibratory action of innumerable fine filaments with which the walls of 

 the stomach, hepatic ducts, and intestine are clothed. 



In this way the nutritive matters of the food are acted upon in two ways : first, a peculiar 

 organic ferment derived from the liver reduces them to a condition in which they may be absorbed ; 

 secondly, in order that the latter process may be favored it is propelled through an intestinal 

 ■canal which is peculiarly constructed so as to present as large an amount of absorbent surface as 

 possible. This is accomplished by a double induplicatiou or fold which extends for the whole 

 length of the intestine, the cavity of which in consequence, appears almost crescent-shaped when 

 cut straight across. On the concave side the intestinal wall is thrown into numerous very narrow 

 longitudinal folds, which further serve to increase the absorbing surface. Such minor folds are 

 also noticed in the stomach, and some of these may even have a special glandular function. There 

 are no muscular fibers in the wall of the intestine as in vertebrates, and the sole motive force 

 which propels the indigestible as well as digestible portions of the food through the alimentary 

 canal is exerted by the innumerable vibratory cilia with which its inner surface is clothed. The 

 intestinal wall is wholly made up of columnar cells which are in direct contact externally with 

 the connective tissue which is traversed by numerous large and small bloodvessels devoid of 

 specialized walls. 



This apparatus is admirably suited to render the microscopic life found in the vicinity of the 

 animal available as a food supply. The vortices created by the innumerable vibratory filaments 

 which cover the mantle, gills, and palps of the Oyster enables it to draw its food toward itself, 

 and at the same time the microscopic host is hurled into the capacious throat of the animal to 

 undergo conversion into its substance as described above. The mode in which the tissues may 

 become tinged by the consumption of green spores, diatoms, or desmids it is easy to infer from 

 the foregoing description of the digestive apparatus of the animal ; and the colorless blood-cells, 

 moving in a thin, watery liquor sanguinis, would, judging from their amoebiform character, readily 

 absorb any tinge acquired by the latter from the intestinal juices. 



217. LOCAL VARIATIONS IN THE FORM AND HABITS OF THE OYSTER. 



Mr. Darwin ("Variation of Animals and Plants," vol. ii, 2d ed., p. 270) writes: "With 

 respect to the common Oyster, Mr. F. Buckland informs me that he can generally distinguish the 

 shells from difierent districts; young Oysters brought from Wales and laid down in beds where 

 ^natives' are indigenous, in the short space of two months begin to assume the 'native' character. 

 M. Costa ^ has recorded a much more remarkable case of the same nature, namely, that young 



' Bull, de la Soc. Imp. d'Acclimat.,.viii, p. 351. 



