12 



recently described species. In these instances the difficulty lies 

 not so much in finding representatives of these closely related 

 species, but, rather, in drawing the lines between them and placing 

 every individual enumerated in the proper pigeon-hole. To avoid 

 this difficulty, the separation was not attempted in every case. 

 With the hope that the results would throw some light on the ques- 

 tion of seasonal variation, this separation was attempted in the 

 genus Brachionus, where the species characters are confined to 

 prominent structural features. 



So far as it was feasible, specific distinctions w r ere accepted as 

 found, and utilized whenever possible. In the lists and discussions 

 which follow, the inclusion of a species does not necessarily carry 

 with it the inference that it is regarded by the writer as valid or 

 well founded. It merely represents in our enumerations a more or 

 less continuous succession of organisms which conform approxi- 

 mately to the descriptions and figures of the species designated by 

 the name in question. Inferences regarding the rank or validity 

 of the species reported will be given whenever the statistical data 

 or my observations on the variability of the organism seem to 

 afford data bearing on the standing of the species. While not a 

 few of the species reported may justly be regarded as synonyms, an 

 effort has been made to use only names which represent valid 

 species or at least a variety or a seasonal form. 



COMPARISON OF FRESH-WATER AND MARINE PLANKTON. 



The plankton of fresh water is very generally composed of an 

 assemblage of organisms, of plants and animals, principally crypto- 

 gams and invertebrates. Not all orders are represented, and those 

 that do occur vary greatly in the number of their representatives. 

 The fresh-water plankton differs from that of the sea in the almost 

 universal absence of larval forms, in the smaller number of inverte- 

 brate groups represented, and in the smaller size of its component 

 organisms. Fresh-water plankton has almost no limnetic coelen- 

 terates, Hydra fusca being the only representative as yet discovered 

 in our locality. The absence of the larger Crustacea, of limnetic 

 mollusks and worms, and of tunicates and Radiolaria robs limnetic 

 life of the diversity found among pelagic organisms of the sea. 

 The only larval stages found in our locality are the glochidia of the 



