113 



HELIOZOA. 



The Heliozoa of the plankton of the Illinois are few both in 

 number of species and of indiyiduals. They apparently play but a 

 small part in the economy of the plankton. The average number 

 for 1898 was but 4,883. Their occurrences are confined -in ^the main 

 to midsummer and early autumn. But four species were identified, 

 though several others remain undetermined for lack of sufficient 

 material, especially of the living forms. Apstein ('96) reports 

 Heliozoa in considerable numbers in German lakes, with maxima 

 in July- August. It is probable that these delicate forms are fre- 

 quently crushed in manipulation or hidden in silt in our collections. 



DISCUSSION OF SPECIES OF HELIOZOA. 



-Actinophrys sol Ehrbg. Average number, 62. This species 

 occurred irregularly from April to the early part of November at 

 temperatures above 46. It was recorded most frequently in the 

 latter part of the summer, the largest number (28,000) appearing 

 Sept. 7, 1897, at 80. 



Actinosph&rium eichhornii (Ehrbg.) Stein. Recorded a few times, 

 from July to October, at maximum temperatures (75- 80), but 

 always in small numbers. 



Endophrys rotatoriorum Przesm. This heliozoan (?) has been 

 recently described by Przesmycki ( '01) as parasitic, during a part of 

 its existence, in Philodina and Hydatina. A parasite resembling 

 this parasitic stage of Endophrys was observed by me in a bdelloid 

 rotifer (Rotifer tardus) on several occasions, but it was never abun- 

 dant, nor was its connection with any free-swimming condition 

 noted. The heliozoan affinities of this organism seem very ques- 

 tionable. 



Nuclearia delicatula Cienk. Average number, 4,760. This 

 species in 1898 appeared first on June 21, attained a pulse of 78,400 

 on August 9 at 82 and another abrupt one of 65,600 on September 

 27 at 73, and made its last appearance October 25 at 48. Occur- 

 rences in previous years are confined to midsummer. Its optimum 

 conditions of temperature obviously lie near the summer maximum, 

 and its lower limits near 50. Its appearance in the plankton is 

 not traceable to flood conditions, and it is apparently eulimnetic 

 in our waters. 



