168 CRUSTACEA. 



be made of the observations of Leydig and Grobben, according to 

 which this eye has a paired rudiment. 



According to Urbanowicz (No. 23), the eye in Cyclops is formed 

 "of three ectoderm-cells, each of which secretes pigment and becomes 

 a refractive sphere." 



The development of the paired compound eye has been best studied 

 in the Decapoda (Bobretzky, No. 41 ; Eeichenbach, No. 65 ; 

 KiNGSLEY, No. 52 ; Herrick, Nos. 48 and 49 ; and Parker, No. 

 62). It has also been observed in Mysis (Nusbadm, No. 39}, Para- 

 podopsis (BouTCHiNSKY, No. 37a), and BrancJiipus (Glaus, Nos. 8 

 and 9). The description of the development of the compound eye 

 cannot be separated from that of the optic ganglion. 



The simplest example of the development of a compound eye 

 is that of Branchip7is. The rudiment of the compound eye, as well 

 as of the optic ganglion, can here be traced back to a pad-like 

 growth of hypodermis, the superficial parts of which become trans- 

 formed into the eye, while the deeper parts contain the material 

 for the optic ganglion, which is connected with the brain. The 

 several layers of cells which represent the rudiment of the eye, and 

 which must be regarded as a simple thickening of the hypodermis, 

 soon show an arrangement of the elements into a superficial layer 

 (which yields the corneal cuticle and the crystalline cones) and a 

 deeper pigmented layer for the formation of the retinulae, the latter 

 layer being connected with the rudiment of the optic ganglion by 

 fibrous strands. In the lateral parts of the whole rudiment there 

 soon occurs a histological difi'erentiation of the optic ganglion and 

 of the ommatidia which compose the eye, whereas, in the anterior 

 and more median portions, a proliferating hypodermis of embryonic 

 character (Fig. 85) is retained until a later stage ; this constantly 

 yields new elements for the enlargement of the whole rudiment. 

 Strictly speaking, two budding zones (/.•', A:"), which arc distinct 

 from, but in contact with one another, can be distinguished at this 

 point ; one of these (A:"), by the production of new ommatidia, adds 

 to the size of the eye itself, while the other and more proximal 

 (A;') yields the elements corresponding to the optic ganglion. During 

 these ontogenetic processes, the movable eye-stalks have arisen by 

 simple outgrowth from the lateral parts of the head. 



The development of the eye in the Schizopoda and the Decapoda 

 is very similar to that in Brancliipus. Here also the compound 

 eye arises from a thickening of the hypodermis, which from the 

 first is in close connection with the rudiment of the optic ganglion. 



