76 FISH- CULTURAL ASSOCIATION, 
— ee" 
numerable 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 absorb- 
ed; secondly, in order that the latter process may be favored it 
is propelled through an intestinal canal which is peculiarly con- 
structed so as to present as large an amount of absorbent surface 
as possible. This is accomplished by a double induplication 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 mus- 
cular fibres in the wall of the intestine as in vertebrates, and the 
sole motive force which propels the indigestible as well as diges- 
tible portions of the food through the alimentary canal is ex- 
erted 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 blood-vessels devoid of specialized walls. 
This apparatus is admirably suited to render the microscopic 
life found inthe 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 itselt and at the same time the micro- 
scopic 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 consump- 
tion of green spores, diatoms or desmids, it is easy to infer from 
the foregoing description of the digestive apparatus of the ani- 
mal; 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 intesti- 
nal juices. 
