270 MICROSCOPY FOR BEGINNERS. 



posterior coxse are in contact only as described in the 

 Key. They are all moderately narrow. The mouth is 

 round, ciliated, and with a kind of hood which the be- 

 ginner may have some trouble to recognize. 

 The ventral plates are curved, almost cres- 

 centic, and narrow, one being on each side 

 of the ventral opening, and just behind them 

 are two small tracheal apertures. The in- 



Fi<?- 174. Coxae ... i .c -MI , ,1 .1 



testmal orifice is visible at the rear in the 



middle line, with a tracheal opening on each 

 side. The eyes are four, in two pairs, rather close to- 

 gether. A large red, nearly spherical JEyldis is quite 

 common in our ponds. The young are described as 

 being red, transparent, with four eyes wide apart, and 

 six legs. 



5. ARRENTJRUS (Figs. 175, 176). 



The coxse form two groups on each side, the two an- 

 terior coxae being in contact by their entire length, as 

 are also the two posterior. Occasionally the anterior 

 groups on opposite sides are in contact at 

 the median line, as in Fig. 175. The ventral 

 plates are oval, their greatest length gener- 

 ally being from before backward, and usu- 

 ally close together. The two oval, lateral 



Fig. 175. -Coxae J 



of Arreuurus plates are oval from side to side, and some- 



times curved. The mouth is small, round, 



and encircled by a ring-like hood. The skin is usually 



hard and roughened, or covered by a deep net -work 



of strongly elevated lines, which give it a beautiful 



