266 



AQUATIC MICROSCOPY FOR BEGINNERS. 



k. Frontal appendages not twisted nor coiled; body 

 stout. Branchipus, 17. 



entom6straca 



I. DAphnia (Fig. 178). 



There are several species of Ddphnia, all of which 

 may be known by the presence, on the posterior 

 border, of a sharp spine, which is never on the lower 

 angle. It varies in length in the different species, 

 sometimes being nearly as long as the shell, 

 and extending obliquely upward. It also varies in 

 lemgth and in position on the same individual, being 

 longest in the young, and 

 becoming short with age. 

 In the species figured 

 {Ddphnia ptilex) it is usually 

 on the upper angle, but not 

 rarely as shown in the cut. 

 In very old specimens it 

 may be entirely absent, but 

 it is always present at some 

 time in the animal's life. 

 The shell is oval and 

 slightly flattened. The an- 

 tennse are prominent, and 

 are usually divided into two 

 parts at the free end, each 

 division bearing several 



feathery bristles. The feet a're flattened, and gener- 

 ally in rapid motion, so as to bring food to the mouth, 

 and ogygen to the blood. The heart is noticeable as 

 a small colorless organ under the shell of the back 

 near the head. It pulsates rapidly. The eye is large 



Fig. 178. — Ddfihnia. 



