91 



animal in their composition, and when they decline the organisms 

 upon which the disappearing animals were feeding have an oppor- 

 tunity to multiply with less decimation in their ranks. 



This species is one of the most abundant of the synthetic organ- 

 isms in the summer plankton, and next to Euglena is the foremost 

 among the synthetic elements of the food cycle of the plankton. 

 The presence of many light-colored or even colorless forms (forma 

 hyalina Kl.) justifies the suspicion that members of this genus, like 

 those of its near relative Euglena, adopt holozoic nutrition in the 

 presence of abundant organic matter suitable for food. 



This species, as well as the others above listed, is exceedingly 

 variable in the proportions of the lorica, in its color, and in the 

 development of the neck. It is very desirable that its life history 

 and the full limits of its variation be determined before many more 

 new species are proposed in the genus. 



In addition to the forms above listed, the following have been 

 noted as present in small numbers in the summer plankton, viz. : T. 

 armata Ehrbg., T. caudata Ehrbg., T. torta Stokes, T. urceolata 

 Stokes, and T. volvocina var. rugulosa Kl. 



Uroglena americana Calkins. This species was found in small 

 numbers in July and September, 1897, and in January, 1899. 



Uroglena radiata Calkins. This species was found in January, 

 1896 ; in April and May, 1897 ; and in March and April, 1898. There 

 was a vernal pulse of 15,279 on April 29, 1896. 



Uroglena volvox Ehrbg. This species was found sparingly in the 

 spring plankton in 1896. Uroglena is one of the few organisms 

 which the usual method of plankton collection and preservation 

 fails to keep in fair condition for subsequent indentification. 

 The gelatinous matrix is easily crushed, and debris adheres to it so 

 as to obscure it beyond recognition. Judging from the frequency 

 of Uroglena in the living plankton it is very probable that the genus 

 is much more abundantly represented in the Illinois River than the 

 data at hand indicate. The genus seems to prefer the cooler waters 

 of autumn and spring to those of midsummer. 



Volvox aureus Ehrbg. This species was found from March to 

 August, but in small numbers and irregularly. 



Volvox globator L. This was somew T hat more abundant than the 

 previous species, and was found more frequently, especially during 



