450 COMPARATIVE ANATOMY AND PHYSIOLOGY. 



definite groups, each of which may be known as a 

 retinula. The next stage in complication is brouglit 

 about by the cells of the sensitive layer becoming 

 divisible into an outer (vitreous) portion, and an 

 inner retinal part ; this may be effected without the 

 elements passing through the second stage, or that 

 of segregation into retinulse ; as happens, for example, 

 with the lateral eyes of the scorpion, and the more 

 simple eyes of adult insects (Fig. 191 c). In the 

 more complex cases the retinal cells form retinulse 

 (Fig. 191 D). 



While the sensory parts thus become more compli- 

 cated, the refractive element, or cornea, which is formed 

 by the epidermis, may likewise lose its primitive sim- 

 plicity as a continuous investment to the eye, and be- 

 come divided into a number of facets, each of which 

 is in connection with its proper set of sensitive cells ; 

 and of these lenses there may be several hundreds. 

 We may distinguish, therefore, an eye with one lens 

 from an eye with many by calling them respectively 

 monomeniscous and polymeniscous. 



The eyes of Arthropods (Fig. 191) are, therefore, 

 in the classification of Lankester and Bourne : 



A. IVIonosticnoilS (formed by a single layer of 



cells). 



a. Non-retinulate, as in the larvae of insects. 

 )8. Retinillate. i. Lateral eyes of scorpions. 



ii. Lateral eyes of Limulus. 



B. DiplosticllOiis (formed by a double layer of cells, 



one vitreous, and one retinal). 



a. Non-retinulate. Dorsal eyes of spiders, and 

 simple eyes of adult insects. 



j8. Retinillate. Central eyes of scorpions, com- 

 pound eyes of insects and Crustacea. 

 i. MonomeniscoilS (with a single lens). 

 ii. PolymeniSCOUS (with a number of lenses). 



1. Separate vitreous bodies. 



2. Aggregated vitreous bodies. 



Among the Invertebrata the highest type of eye 



