1879. |’ Eresh-Water Entomostraca. 623 
ture clothed in defensive armor of crystal, with an ovoid helmet 
on the head, from beneath which protrude the secondary antenne, 
which in this family are always the larger, and the chief organs of 
locomotion. On the back is a small shield-like plate from which 
are suspended the two plates enclosing the body. Under the 
scuta of the back the heart can be seen pulsating regularly, while 
just below it is the intestinal canal, usually green from the vege- 
table matter contained therein. The jaws are suspended 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. vetula) 
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. The 
section of the genus separated by Dana and called Ceriodaphnia 
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. 
ne of the most interesting of all Daphnia-like species is Sida, 
Plate 11. A species of Sida and also a new species of the allied 
genus Daphnella occurs in Minnesota waters. The body of Sida 
is highly transparent, rendering the study of the inner parts less 
difficult 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 also 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. 
But the most curious of all these minute, shelled forms, is the 
single species of, Bosmina (Plate 1v), which constitutes the family 
Bosminidz 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 | 
