TOO THE APODID^E PARTI 



gradual diminution of the shield and head, and the 

 sinking in of the sides of the latter till the eyes, 

 with their cones of long muscle-bands (cf. Fig. 24, ;;/), 

 became movable lateral ridges running dorso-ventrally. 

 Their gradual complete articulation at their bases 

 would then easily follow. This method of trans- 

 forming the eyes of Apus into movable stalked eyes 

 is well exemplified in Branchipus stagnalis, which is 

 nearly related to Apus. The head region seems in 

 it to be reduced to a small base for carrying the 

 enormous second antenna and the stalked eyes. 



THE UNPAIRED "EYE." 



This organ, which is just visible as a dark spot be- 

 tween the paired eyes of Apus, is often called the 

 " rudimentary median eye of Apus." Closer study 

 of it, however, reveals that it is a highly developed 

 sensory organ with definite functions of its own. In 

 describing the water-sacs over the eye, we have 

 already had occasion to refer to it, and we there found 

 that through the canal which leads into these water- 

 sacs its interior is also in open communication with 

 the external world, that is, if the chitinous fold which 

 runs into it is really open at the end. It is better to 

 defer a discussion as to the use of this organ until after 

 an examination of its general structure and probable 

 origin. 



Beyond the account of the middle eye of a Cope- 

 pod by Grenacher, this organ has received very little 



