16 TWELFTH ANNUAL REPORT. 



The Cladocera or Daphuoidea are characterized by the more or 

 less leaf-like feet, and the lamina of thin chitine which encloses the 

 greater part of the body, or at least forms a sac for the protection 

 of the eggs. This so-called shell springs as a fold from the maxil- 

 lary segment and is the most conspicuous and variously formed, 

 while really least important, of the structural peculiarities. 



All Cladocera begin life with a single median eye, but some lose 

 it during later life. In one case it remains the only visual organ. 



The outer covering is in most cases changed by frequent moults. 

 The period of the moult is one of the most precarious in the life 

 history of the animal. 



Although figures and brief descriptions of animals belonging to 

 this group are to be found in the works of Swammerdam, Leewen- 

 hoek, Trembley and other of the older authors, Mueller^ was the 

 first to produce a systematic work upon these in common with 

 other minute fresh-water Crustacea. He may be called the father 

 of the study of micro-crustacea. Jurine,^ an eminent Swiss natur- 

 alist, was the next to contribute important discoveries relating to 

 these interesting animals, though Ramdohr had given anatomical 

 details of several species. Gruisthuisen, a little later gives farther 

 details of Daphnia sima (Simocephalus). The work of Milne 

 Edwards gives a resume of what was known regarding these ani- 

 mals in that period. Soon afterwards the work of Baird became 

 the beginning of a new era, and the study of the minute Crustacea 

 sprang into importance at once. The Scandinavian peninsula 

 being the birth-place of the science, it is proper that the most 

 exhaustive work on the group should be performed there. 



The most important of the later writers are Leydig, Schoedler, 

 Fischer, Lilljeborg, P. E. Mueller, Sars, Weismann, Claus and 

 Kurz. 



The complete bibliography of the subject up to Mueller's time is 

 found in Baird's British Ento7nostraca; the greater part of the 

 later bibliography is to be found in P. E. Mueller's Danmark's 

 Cladocera. A few only of the more important works are here 

 mentioned. 



Koch, C. L., Deutcblands Crustaceen, etc. 



Schoedler, J. E., Ueber Acanthocercus ligldus, etc. 



Dana, J. D., Crustacea of the Wilkes' Exploring Expedition. 



Lievin, Die Brancbiopoden der Danziger-Gegend. 



Fischer, Leb., Ueber die in der Umgegend von St. Petersburg vorkommenden Crus- 

 taceen, etc., 1851. 



lAlloehorg, W., De Crustaceis ex ordinibus tribus, (or) Om de Inom Skane forekom- 

 mande Orustaceer af ordningarne Cladocera, Ostracoda ocb Cope- 

 poda. 



