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. 3 ) 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. 



CCELENTERATA. 



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 



