I30 THE PINEAL ORGAN 



directed forward and laterally, while the axes of the posterior pair are 

 directed backwards and laterally. Approximately the same directions of 

 the optic axes are present in Diplodontus, Eylais, and Hydrodroma as in 

 Limnesia. These four species have, therefore, fields of vision corres- 

 ponding to the four quarters of the compass. 



In Curvipes carneus the anterior and posterior eyes on each side are 

 united in a double eye, of which the posterior eye on each side is the 

 smaller. The same type of fusion, of the anterior and posterior eyes of 

 each side, takes place sometimes in Hygrobates longipalpis. 



Fig. 92. 



A — The dorsal-shield of Hydrodroma, seen from above, and showing the pigmented 

 sense-organ p.s.o., called the " fifth " or " median eye ". On each side of the 

 shield are the lateral eyes enclosed in a chitinous capsule. 



B — The " median eye " highly magnified, showing seven radially arranged pigment- 

 cells, surrounded by a circle of papillae, of the cutis. 



In Eylais extendens all four eyes, which are close together near the 

 median plane, are enclosed in a so-called " spectacle frame " ; while in 

 the genus Hydrodroma the two eyes on each side are enclosed in a chitinous 

 capsule, which is separated from its fellow of the opposite side by the whole 

 width of the dorsal shield. In the centre of this shield is an organ which 

 has been described as a " fifth median unpaired eye " (Fig. 92). It 

 consists of a minute, disc-like body containing seven radially disposed 

 pigmented areas in which, according to Schaub's description, there is to 

 be seen a rhabdome formation. Lang, however, who examined specimens 

 of the organ prepared by modern technique and under an immersion lens, 



