143 
HELIOZOA. 
The Heliozoa of the plankton of the Illinois are few both in 
number of species and of individuals. 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 
Heltozoa 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°. 
Actinospherium eichhornu (Ehrbg.) Stein.—Recordeda 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 le near the summer maximum, 
and its lower limits near 50°. Its appearance in the plankton 1s 
not traceable to flood conditions, and it is apparently eulimnetic 
in our waters. 
