93 



part of some of the rhizopods,orto the appearance of limnetic forms, 

 varieties, or species according to the systematic value placed upon 

 these eulimnetic individuals. I am inclined myself to regard them 

 as seasonal forms of species which are predominantly of the bottom 

 or littoral fauna, which have multiplied rapidly under the stimulus 

 of abundant food. Owing to this fact, to the storage in their 

 tissues of the products of metabolism, such as gas and oil vacuoles 

 which tend to lighten their specific gravity, and to the frailer 

 structure of their shells under conditions of rapid multiplication, 

 they abandon their customary benthal or littoral habitat and assume 

 temporarily a limnetic distribution in the plankton where they con- 

 tinue to find abundant food. Their appearance here under these 

 circumstances is a result of their physiological condition, and with 

 its cessation they decline, as shown by their pulse-like occurrences. 

 Whatever the systematic valuation placed upon these limnetic 

 forms may be, there is no doubt of their occurrence. They have 

 appeared in every year of our operations, but were most prevalent 

 in 1897, a year of most stable conditions, and also in the quieter 

 backwaters, and on the declining spring flood or June rise when hydro- 

 graphic conditions are less catastrophic than those of early flood 

 stages. In 1897 there was a pulse of 68,400 (silk-net only) on August 

 8 and another of 1,268,400 on September 7, both in stable conditions 

 and almost exclusively of limnetic types, differing in this respect 

 from the pulse of 141,524 on February 22, 1898, which was pre- 

 dominantly of an adventitious character, resulting from the flood 

 of that period (Pt. I., PI. XII.). The contrast in the numbers of 

 Rhizopoda in the plankton during warm and cold seasons of the 

 year is very striking in 1897. The average per m 3 , per collection 

 from May 1 to October 1, that is, above 60, is 161,045, omitting all 

 filter-paper collections, while in the seven months of lower tempera- 

 tures this average is only 4,771. During the warmer period the 

 June rise was the only hydrographic disturbance (Pt. I., PI. XL) 

 to which any adventitious increase might be attributed. This con- 

 trast is less evident in 1898, when the summer hydrograph was more 

 disturbed. These larger numbers during warmer months may be 

 attributed in part to the greater numbers of the Rhizopoda in their 

 littoral habitat, and in part, doubtless, to the fact that at low water 

 the shore and bottom fauna are brought into more intimate relation 

 with the plankton, and in the river the disturbance of these regions 



