iSyc)] Frvsli-Watcr lintomostraca. 623 



turc clothed in defensive armor of crystal, with an ovoid helmet 

 on the head, from beneath which protrude the secondary antennae, 

 which in this family are always the lar^^er. and the chief organs of 

 locomotion. On the back is a small shield-like plate from which 

 are suspended the two plates enclosin<; the body. Under the 

 .scuta of the back the heart can be seen i)ulsating re«;ularly, while 

 just below it is the intestinal canal, usually «;reen from the vege- 

 table matter contained therein. The jaws are susi)ended from the 

 upper part of the head and meet below, where their toothed, 

 grinding edges are placed in opposition, so that all the food must 

 necessarily pass between them to be comminuted. The existence 

 of a median dorsal plate in Daphnia seems to have been over- 

 looked, it will be readily found however in this species [D.vctula) 

 by placing a specimen on a slide and allowing it to dry and then 



tilting it up. 



The genus Daphnia is quite well represented in our waters, 

 both in variety of species and abundance of .specimens. Ihe 

 section of the genus separated by Dana and called Ccnodaphma 

 includes those members which have a cellularly reticulated shell, 

 but this character does not seem constant in the closely allied 

 smaller forms which it evidently ought to include, if indeed the 

 .same species may not embrace forms with both reticulate and 

 non-reticulate shells. 



One of the most interesting of all Daphnia-like species is Sida, 

 Plate III. A species of Sida and also a new species of the allied 

 genus Daphndla occurs in Minnesota waters. The body of Sida 

 is highly transparent, rendering the study of the inner parts less 

 diffic'lilt than in most of these animals. The movement of blood 

 corpuscles in the head and the currents caused by the branchial 

 feet (indicated by arrows in the drawing) can be readily traced. 



The curious Polyphemus .ilso is represented, an animal in which 

 the body is much curved upon itself, and the last joint of the 

 abdomen is greatly elongated and bears two long flagella. The 

 single staring eye, occupying the whole of the head, is the most 

 conspicuous organ, while the apparently undeveloped branchial 

 feet at first suggest the young of some other .species. 



Hut the most curious of all these minute, shelled forms, is the 

 single species of Bosmina (Plate iv), which constitutes the family 

 Bosminidce of Dana. The idea at once suggested is a strange bur- 

 lesque on the elephant, though the animal is not by any means 

 of elephantine proportions, being not two hundredths of an inch 



