24 TWELFTH ANNUAL REPORT. 



sensitiveness to changes in the environment, which is universal 

 among the Daphnidce. The compact Copepoda survive the vicissi- 

 tudes of confinement with comparative immuity, but the first taint 

 in the water destroys the delicate organism of Daphnia, The cause 

 for this may be found in the exposure of the most vital and delicate 

 parts of the organism to the influences of the surrounding aqueous 

 medium. In particular the circulatory and respiratory systems, 

 which here are not easily to be distinguished one from the other, 

 constitute a relatively very large area of close contact with the 

 water. It thus happens that the central organs are influenced in 

 a very short time by whatever deleterious substances may be dis- 

 seminated in the water. 



Notwithstanding this lack of centralization, the structure of 

 these animals is of a very considerable degree of complexity and, 

 presenting so many instructive modifications under circumstances 

 so favorable for study, has been verj^ thoroughly investigated. The 

 very transparency which has made it possible to clear up many 

 questionable points in crustaceolosfy from the lessons learned in 

 Daphnia, has rendered the investigation of certain sets of organs 

 extremely difficult, and among these may be mentioned the circula- 

 tory system. The circulation of the nutritive fluid and the gen- 

 eral facts connected with the heart were indeed early understood; 

 but there remains many a detail and some important relations 

 which are as yet either imperfectly known or entirely misunder- 

 stood. The following notes are offered as a contribution to the, as 

 yet incomplete, knowledge of the circulatory apparatus. 



The observations were confined for the most part to Daphnia 

 schcefferi and Simocephalus vetulus^ with occasional comparisons 

 with Eurycercus^ Pleuroxis, Pasithea and others. It is greatly 

 to be desired that the study might be carried to the Sididae, 

 in which the larger size and superior transparency would doubt- 

 less reward the search with several, as yet doubtful details. 

 The circulation of the nutritive fluid in the Daphnidge, then, is 

 somewhat complicated, but may be divided into a superficial and a 

 deep system. It must be remarked that this distinction is arbitrary 

 and only used for its convenience. The one extends over the entire 

 inner surface of the carapace, while the latter is in close relation 

 with the vegetative organs, and extends into the branchial vessels 

 of the feet. The nutritive fluid which is normally colorless and 

 supplied with corpuscles of organized nutriment, (it seems doubt- 

 ful if they should be called blood corpuscles) is confined for the 

 most, if not its entire, course within membranous walls of connec- 





