eit Av, 
PNR 
The Relation of Mollusks to Fish in Oneida Lake — 135 
use this dust-fine detritus on the bottom, but take it directly 
from the water in which it is held in suspension, or is brought 
to them by the currents. It has been generally held that the 
plankton furnishes food for the bivalve fauna, but it has not 
been demonstrated that the entire food consists of this 
material. The stomach and digestive tract of the fresh water 
mussels is often filled with an indistinguishable brown mass 
exactly like that of the upper bottom layer. Algv are also 
used to a large degree. 
Fresh water gastropods are normally vegetarians, feeding 
on algze and the soft parts of plants, usually the epidermis. 
An extensive vegetation is, therefore, a prerequisite for an 
abundant and varied molluscan fauna, and it is notable that 
where the water is deep and vegetation absent or only sparsely 
present, molluscan and other invertebrate life is scarce or 
absent. Plants not only afford a place. for support, upon 
which snails may crawl and find a resting place, but they are 
actually eaten as the regular food supply, a fact which any 
one may verify by watching snails upon leaves and stems of 
plants. In view of this fact the statement of Shelford (1913, 
p. 58) that “‘ we could probably remove all the larger rooted 
plants and substitute something else of the same form and 
texture without greatly affecting the conditions of life in the 
water; that is, so far as the life habits of the animals are 
eoncerned ” is misleading. Dissection of snails reveals the 
tissues of plants in their intestines. Green filamentous and 
unicellular alge are largely eaten but do not form all of the 
diet by any means. Dawson (1911, pp. 68-69) observed 
Physa eating tender green shoots of Chara and Elodea, also 
the leaves of grass, maple and elm when partly decayed, the 
snail eating only the soft tissues, leaving the hard skeleton 
nntouched.* 
The fresh water pulmonates, Physa, Lymnea, Planorbis, 
also feed upon animal matter. Dawson records a Physa eat- 
ing amphipods (Gammarus) confined with them in an 
aquarium, although it is believed that the crustaceans died 
*Op. cit., p. 69. See this author on the use of mucus while feeding. 
