SECT. VI 



THE SENSORY ORGANS 



97 



Fig. 24 is a diagram of these water-sacs. A small 

 pore, in the shape of a fine transverse slit (see 

 Figs. 24 and 69, p. 303) in front of the eyes, is, in 

 large specimens, visible to the naked eye. This leads 



Fig. 24.— Diagram to show the water-sacs over the eyes of Apus. s, water-sacs, 

 c, canal leading into the same ; /, pore ; e, paired eye ; d, bram ; a-, cesophagal 

 commissure ; o^; optic ganglion ; ;;/, eye-muscles. The eye is drawn in the 

 section ; in reality a median section passing through the pore passes between the 

 eyes ; ^Oj unpaired eye receiving a branch from the water-canal. 



into a canal, which runs along the dorsal surface of 

 the unpaired "eye." Here it widens out con- 

 siderably, its upper and lower chitinous membranes 

 being in close contact, except in the median line 



