STEtJCTtJRE OF EYE. 



157 



or stalk-eyed) they are situated on more or less 

 elongated pedestals. In some of the lower forms, 

 though less complex, they are very large, occupying, 

 as in the curious Leptodora (Fig. 105) of our deep 

 lakes, the whole front of the head ; while in Corycseus 



Fig. 106.— Eye of Mysis (after Grenacher). n, Nuclei; Lf, facets; Rk, crystalline 

 cones ; n\ cells of the retinula ; Rl, retinula ; Em, rhabdom ; Cp, blood-vessels ; 

 iV. fibres of the optic nerve ; iV', iV", N^^\ JV^"', decussations of the fibres of the 

 optic nerve; G, G\ (?", (?"S ganglia; M, muscles for the movement of the 

 eye-stalk- Zm', Zni", nuclei. 



(Fig. 107) they extend to more than one -half of the 

 whole length of the body. 



The higher Crustacea possess no ocelli. In the 

 lower species, on the contrary, a central ocellus is often 

 present, especially in the young state. 



