73 



Cryptomonas ovata Ehrbg.* Average number, 121,154. This 

 species has been recorded principally in the autumnal or hiemal 

 plankton. It escapes through the silk net readily, and was rarely 

 found in collections of earlier years. In 1895 it occurred from July 

 till the last of October, and in 1898 was common in the December 

 plankton. 



Dinobryon sertularia Ehrbg. Like most typical planktonts, 

 Dinobryon is an exceedingly variable organism, and the varia- 

 tion finds its expression in the form and proportions of the 

 loricas and in their arrangement and continuity in colonies. 

 Divergences from described and figured species are thus at once 

 apparent, and they have been utilized by systematists, notably by 

 Lemmermann ('00) and by Brunnthaler ('01) as the basis for the 

 establishment of a large number of new species. The validity of 

 these species, in my opinion, must rest ultimately upon careful 

 experimental evidence of their present mutual genetic independence 

 under normal conditions of growth. From my own observations 

 upon large numbers of colonies and individuals distributed through- 

 out the range of their seasonal recurrence in six years in our waters, 

 I am inclined to regard all as belonging to a single species, and the 

 different types as mere growth varieties. The rapidity of growth 

 and the age of the individual or of the colony are, I believe, impor- 

 tant factors in the determination of the form of the lorica, and its 

 various forms are therefore not of specific value, but rather of 

 physiological significance. It is a simple matter to find individuals, 

 or even colonies, conforming to the descriptions of the several 

 species, but it is not so easy to refer all individuals and all colonies 

 to the described types. They intergrade nay, more, two, or even 

 more, "species" are not infrequently combined in the same colony. 

 I have never found all the forms in a single colony, but such com- 

 binations as angulatum-divergens, diver gens-angulatum-stipitatum, 

 sertularia-angulatum, and sertularia-undulatum have been observed 

 by me. These combinations are most frequent in large colonies, 

 and, indeed, the number of "species" in a colony is apparently a 

 function of its size. The slender growing tips are wont to assume 

 the stipitatum type of lorica and colony, and the older loricas at the 

 base to conform to that of sertularia, divergens, or angulatum. 

 Small colonies as a rule belong to a single "species." These com- 

 binations are generally most evident during the maximum period 



