114 



Hempel ( '99) reports Raphidiophrys pallida Ehrbg. and R. elegans 

 Hertwig and Less, in the plankton of Quiver Lake adjoining the 

 river, and I have found an undetermined species of Acanthocystis 

 and a small heliozoan resembling Nuclearia in the river plankton. 



SPOROZOA. 



Triactinomyxon sp. In the plankton collections of each year 

 there have been found free limnetic spores which unquestionably 

 belong to that highly aberrant and peculiar group of organisms 

 described by Stole ('99) as Actinomyxidia and regarded by him as 

 Mesozoa, but later referred by Mrazek ('00) Caullery and Mesnil 

 ('04), and Leger ('04) to the Myxosporidia. The organisms de- 

 scribed by Stole were parasitic in fresh-w r ater oligochaetes, and it is 

 not improbable that the limnetic spores taken in our plankton 

 collections are derived from parasites in some of the numerous 

 aquatic oligochaetes, or other invertebrates, found along the bottom 

 and shores of the stream. 



The species here referred to Triactinomyxon differs in some 

 details from T. ignotum Stole. It was found in the course of the 

 six years at least once in every month of the year, but most regularly 

 in May-September, and rarely and in small numbers in the colder 

 months. Its transparency and long, slender, radiating, tripod-like 

 arms give it a typically limnetic habit. 



Actinomyxidia, gen. et sp. indet. Clusters of eight, or less, 

 cylindrical spores radiating from a common center and bearing a 

 marked resemblance in structural features to those of Triactinomyx- 

 on, but lacking any anchor-like projections, were found sparingly 

 in the plankton in June-September. 



The distinctively limnetic habit of these spore stages in the life- 

 history of these parasites is unique among the Sporozoa, and has 

 not, to my knowledge, been before noted. 



Many of the rotifers of the summer plankton, especially Brachi- 

 onus and an occasional Asplanchna, have been heavily parasitized 

 internally by small sac-like bodies, often pear-shaped, with the 

 smaller end attached to the lorica, or of spherical or flattened form. 

 They occur in such numbers at times as to be a menace to the 

 rotifer population. They are usually most abundant in any given 

 species at the time of, or subsequent to, its maximum occurrence. It 



