EYES. 



329 



of the outer world may be conveyed to the brain. From the 

 small number of the cells in the recipient layer, however, it would 

 appear that the perception of surrounding objects thus conveyed 

 must be of a low order. 



The eyes of Myriapods and the Thysanura consist of a number 

 of such ocelli, col- 

 lected into groups. 



Although we 

 have to conclude 

 that the Insecta 

 and Crustacea be- 

 long to two distinct 

 divisions of the 

 phylum; the highly 

 developed c o m - 

 pound eyes of each 

 agree closely in the 

 plan on which they 

 are formed. They 

 may be derived 

 from a group of 

 ocelli, the number of which is greatly increased, while the 

 number of cells in each is decreased (Grenacher). Each ocellus, 

 or single eye, which thus with its fellows makes up the com- 

 pound eye, is known as an ommatidium (Fig. 232). They are 

 usually disposed so as to present to the exterior an even convex 

 surface. 



The minute structure of the eyes of Branchipus and Palaemon 

 may be taken as typical of the compound eyes of Arthropods 

 (Fig. 232). Each ommatidium is limited externally by a corre- 

 sponding portion of the transparent cuticle which may be thick- 

 ened in the centre to form a biconvex lens (cl in A), as in the 

 facetted eyes of Insects, Decapods, Isopods, etc., or maintain a 

 uniform thickness, as in Branchipus (c in E). Under it lie the two 

 corneagen or lentigen cells by which it is secreted. Beneath are 

 four (2 in the Isopod Sphaeroma) cells, the vitrellae or crystal 

 cells (kz) grouped about the crystalline cone, the product of their 

 secretion. In Branchipus its shape is fusiform, but it is often 

 conical, with the point directed inwards. In Palaemon (Fig. 

 232, A k and k f ) it is represented by two distinct refractive 



Rz 



FIG. 231. Section through the ocellus of a Dytiscus larva 

 (after Grenacher, from Claus). CL cornea! lens ; Gk the 

 subjacent hypodermis cells forming the " vitreous body " ; 

 P pigment in the peripheral cells of the latter ; Rz retinal 

 cells ; St cuticular rods of the latter. 



