482 



ARTHROPODA. 



The crystalline cones are formed from the coalescence of cuticular 

 deposits in several cells, the nuclei of which usually remain as Semper's 

 nuclei. These cells are probably simple hypodermis cells, but in some 

 forms, e.g. Phronima, there may be a continuous layer of hypodermis cells 

 between them and the cuticle. In various Insect eyes the cells which 

 usually give rise to a crystalline cone may remain distinct, and such eyes 

 have been called by Grenacher aconous eyes, while eyes with incompletely 

 formed crystalline cones are called by him pseudoconous eyes. 



The rhabdom of the retinulae is, like the crystalline cone, developed by 

 the coalescence of a series of parts, which are primitively separate rods 

 placed each in its own cell : this condition of the retinulae is permanently 

 retained in the eyes of the Tipulidee. 



The development of the compound eye has so far only been 

 satisfactorily studied in some Crustacea by Bobretzky (No. 367) ; 

 by whom it has been worked out in Palaemon and Astacus, but 

 more fully in the latter, to which the following account refers : 



The eye of Astacus takes its 

 origin from two distinct parts, (i) 

 the external epidermis of the pro- 

 cephalic lobes which will be spoken 

 of as the epidermic layer of the 

 eye, (2) a portion of the supra- 

 cesophageal ganglia, which will be 

 spoken of as the neural layer of 

 the eye. The mesoblast is more- 

 over the source of some of the 

 pigment between the two above 

 layers. The epidermic layer gives 

 rise to the corneal lenses, the 

 crystalline cones, and the pigment 

 around the latter. The neural 

 layer on the other hand seems to 

 give rise to the retinulae with their rhabdoms, and to the optic 

 ganglion. 



After the separation of the supra-cesophageal ganglia from the superficial 

 epiblast, the cells of the epidermis in the region of the future eye become 

 columnar, and so form the above-mentioned epidermic layer of the eye. 

 This layer soon becomes two or three cells deep. At the same time the 

 most superficial part of the adjoining supra-cesophageal ganglion becomes 

 partially constricted off from the remainder as the neural layer of the eye, 

 but is separated by a small space from the thickened patch of epidermis. 



FIG. 283. DIAGRAMMATIC RE- 

 PRESENTATIONS OF PARTS OF A COM- 

 POUND ARTHROPOD EYE. (From 

 Gegenbaur.) 



A. Section through the eye. 



B. Corneal facets. 



C. Two segments of the eye. 



c. corneal (cuticular) lenses ; r. 

 retinulse with rhabdoms ; n. optic 

 nerve ; g. ganglionic swelling of optic 

 nerve. 



