132 
PORIFERA. 
Spongilla spp.—Average number of spicules, 772. The identifi- 
cation of fresh-water sponges by isolated spicules is practically 
impossible, and, moreover, the sponge fauna of the Illinois River is 
as yet practically unknown. No attempt, therefore, was made to 
identify the species to which the spicules which occur in our plank- 
ton collections belong. They belong to the genus Spongilla in part, 
and were usually the simple sarcode forms, the gemmules or their 
spicules not appearing in the plankton. They occurred in all 
months of the year, and were found in 46 per cent. of the collections. 
They are adventitious, and their occurrence in the plankton is there- 
fore dependent in part upon hydrographic conditions. Records in 
December and January are few (3) and always occur on rising floods. 
In February and March, months of rising floods, they are increased 
(8 and 7), but decline again in April—June (3, 5, and 5), months of 
predominantly declining water and more stable conditions. In 
midsummer and autumn months (July to November) they again 
occur more frequently (8 to 12), probably as a result of proximity 
to the season of greatest growth and frequency of sponges in the 
river and its backwaters. Here also they occur most frequently 
in years of greatest hydrographic disturbance, as, for example, in 
1898. The adventitious relation which they bear to the plankton 
is also seen in their erratic and irregular numbers. The maximum 
record (16,000 per m.?) was made June 28, 1897, on the rising flood ; 
the next in size, on August 10 in stable low water. In both instances 
the plankton was probably taken from water in which as a result of 
some local disturbance the remains of some disintegrating sponge 
had been distributed. Living sponges are found in considerable 
abundance on submerged brush and timbers in the channel and 
backwaters during the summer months, and feed on the smaller 
organisms of the plankton, being one of its depleting agencies. 
C@LENTERATA. 
Hydra fusca L.—Average number, 39. Hydra occurred in about 
16 per cent. of our channel collections—a percentage which would 
be considerably increased if the whole of each collection had been 
examined for it, or if backwater collections should be included. With 
one exception the 28 occurrences recorded, all fall in May-September 
