440 
plankton also. The specimens that fall clearly into the group of plankton- 
feeders represented a rather wide rangé of families, including Sphaeriidae 
(as represented by Sphaerium striatinwm) ; young Unionidae, about one 
year old; Bryozoa (Urnatella gracilis) ; Trichoptera (larvae of Hydrop- 
syche species) ; Chironomidae (unidentified red larvae); and Planaria. 
The stomachs of the Sphaeriidae and young Unionidae, though con- 
taining principally settled limnetic plankton, held also small amounts of 
fine dead detritus, as well as many living bacteria, apparently taken in 
with the latter or with dead planktonts. The insect larvae (caddis and 
Chironomidae) had enjoyed a clean feed of settled plankton, some of it 
still alive when eaten. Some living bacteria were seen in the stomachs 
of the caddis larvae. Species whose stomachs contained nothing but 
dead detritus included a small Asellus and several tubificid worms. The 
larger snails of the family Viviparidae (Campeloma subsolidum and 
Vivipara contectoides) had eaten large quantities of loose detritus and 
what appeared to be slime-clotted silt and organic detritus particles such 
as is commonly found as a thin coating on the shells of the snails them- 
selves and on other hard objects in the mud. Living bacteria, presumably 
putrefactive or fermentative types, were exceedingly abundant in the ma- 
terial in their stomachs. In small specimens of Vivipara and Campeloma, 
on the other hand, diatoms and Chlorophyceae from the settling limnetic 
plankton were not much if any less abundant than old dead detritus, At- 
tached incrusting algae (Pleurococcus and Palmella types) were present 
in the stomachs of all Viviparidae examined. 
In going through samples of the loose bottom-ooze taken with the 
mud-sucker (see Figure 6, page 372), I was struck with the fact that 
limnetic plankton, principally diatoms and Chlorophyceae, was, next after 
the flaky particles of decayed vegetable or animal matter that makes up 
the dead organic detritus, the most abundant edible element in the ooze, 
as far as could be determined, being decidedly more important in bulk 
than normal bottom Protozoa and Rotifera. While bottom Ostracoda 
were noted in the ooze they were relatively very rare, and limnetic 
Copepoda, Cladocera, and Rotifera were represented only by fragments 
or nearly whole carapaces or other chitinous parts. 
The enormous numbers of bacteria seen swarming in and among 
the flaky honeycombed particles of dead organic matter, and inside the 
bodies of recently dead planktonts, suggest that these minute organisms 
are themselves not an unimportant part of the food supply of both the 
plankton- and detritus-eating bottom-animals. Both bacteria and minute 
pale flagellates and ciliates were also very abundant in the interstices of 
the slime-bound silt and detritus scum that envelops the upper surface of 
the shells of a large portion of the living and dead snails. That this ma- 
terial on their own backs is used as food by their fellows is apparently 
proven by its presence in the stomachs as well as by the numerous 
tracks of radulae identified in the mantle of scum on the backs of liv- 
ing Vivipara and Campeloma examined. 
