152 GEOL. AND NIT. HIST. SURVEY OF MINNESOTA. 



with about ten teeth. The claws are short and curved and have one 

 basal spine and fine teeth. The palp of the male antenna has two 

 setae. 



This species has been found near Madison by Birge and by Forbes 

 in Lake Michigan. 



FAMILY DAPHVID^. 



The family Daphmdm contains^^the genera Moinadaphnia^ Moina, 

 Cenoda2)hnia, Scapholeberis, Simocephalus and Daphnia, which include 

 the commonest, as well as some of the largest, Gladocera. The genera 

 may be distinguished by the following table: 



I. Head rounded, not beaked; antennjiles long in both sexes, shell 

 not covering the end of the abdomen. 

 a. Abdomen with process of abdomen to retain ova, . Moiiiadaphnia, 166 

 h. Abdomen vrithout the process in ordinary females, . . . Moiua, 160 

 II. Head rounded; antennules rather short; shell inclosing whole 



body Ceriodapbuia, 167 



III. Head somewhat beaked below; shell angled below or extending in 



long spines from lower angle, pigment fleck roundish, Scai>holeberis, 174 



IV. Head beaked below; shell rounded below, with a blunt spine 



above; pigment fleck elongate, Simocephalus, 177 



V. Head beaked below; shell extending in a sharp spine at the upper 



posterior angle; pigment fleck small, Daplinia, 181 



The Circulatory System of the Daphnklse. 



Plate LI. 



In the Daphnidce, and, indeed, the Cladocera in general, we meet an 

 instance of great development of surfaces at the expense of solidity of 

 form and compactness of organs. The whole body is composed of an 

 aggregate of laminte, and the appendages all approximate more or 

 less toward this fundamental modification. Thus, for example, the 

 head is a leaf like body with a laminate shield above and a pair of flat 

 organs beneath. The abdomen terminates in a knife-like post-abdo- 

 men, while the thorax, with its narrow form, foliaceous feet and, far 

 more, the enormous development of the outer wall to inclose, more 

 or less fully, the entire body, is the typical illustration of this fact. 

 Necessarily this structural modification exerts a formative influence 

 on the internal organs which are all more or less influenced by it; and 

 this is peculiarly the case with the more external and, in general, the 

 paired organs. Thus the ''shell glands," so called, which in Copepoda 

 are generally coiled tubes, become here greatly flattened organs closely 

 united with the shell. The physiological result of this modification 

 is the sensitiveness to changes in the environment, which is universal 



