ENTOMOSTKACA OF MINNESOTA. 185 



backwards. It is well known that male Baphnke often have the spine 

 while the females may have none, and here again it is possible that 

 the narrower cavity over the abdomen requires this assistance, while 

 this is not the case with the females. 



The shell gland is early formed and the branchial lamellte of the 

 feet appear almost simultaneously with the feet themselves as distinct 

 lobes. The branchial chamber is not a simple chamber, but is essen- 

 tially a curved tube, as can be very well seen in the last foot of the 

 adult. This tube doubles upon itself and crosses in the manner of a 

 loop and a constant stream flows rapidly through it. 



The nervous system is, at first, paired from beginning to end and 

 first unites anteriorly, the ocular ganglia fuse after the union of the 

 two pigment flecks in the compound eye, then the cephalic ganglion 

 is formed by the union of the two preoesophageal ganglia, and the com- 

 missures passing about the oesophagus. I have not been able to de- 

 termine if the suboesophageal ganglia become fused. From the anterior 

 ganglia spring the nerves to the antennte and jaws, which latter are 

 larger in the embryo. 



It is greatly to be regretted that no sufficient revision of Daphnia 

 can now be attempted. This is partly due to the very imperfect 

 nature of the descriptions and partly to the scattered literature of 

 the group. We here supply nothing more than a provisional key 

 and synoptical descriptions. I take pleasure in acknowledging the 

 substantial assistance, in respect to recent literature of the group, 

 received from my friend. Professor Birge, as also for valuable sugges- 

 tions respecting Cladocera in general. 



KEY TO THE GENUS DAPHNIA. 



I. Pigment spot present. 



A. Anal margin of the post-abdomen deeply excavated. . . schaeflferi, 187 



B. Anal margin slightly if at all sinuous. 



a. Head very short and regularly curved from the heart to the 



beak psittacea, 187; atkinSGuii, 188 



h. Head not very short or regularly curved. 



* Claws of post-abdomen with a distinct comb of spines. 

 t A strong toothed process from the dorsal margin of 

 the head at some time of life. 



t Spine long, straight clathrata, 188 



Xt Spine short, curved miuneliaha, 189 



tt No strongly toothed dorsal process at any period in 

 either sex. 

 t Head with a'helmet-like crest. 



§ Spine long, springing from the dorsal angle, 



curved dorsad exilis, 190 



