THE ARTHROPODA. 



415 



that it appears as a mere continuation of the hypodermal layer 

 ■(Fig. 195, hyp, gl, rt). Over the eye lies the lens which has arisen 

 by the thickening of the outer chitinous covering of the body, and is 

 secreted by the hypodermis (lentigen or vitreous body-layer). From 

 such a simply constructed eye we have to derive the complicated 

 eyes found among the Arthropoda, but in so doing we must dis- 

 tinguish sharply between the various phyletic ontogenetic series of 

 the Arthropoda, and we must remember that it is not possible to 

 regard as directly related one to another the various forms of com- 

 pound eyes found in the separate divisions such as the Crustacea and 

 the Insecta, although the eyes in these groups are very similar in 

 structure. 



7KO; 



LI . w* 



Fig. 196.— Three ommatidia of the lateral eye of Limulus (after Watase). In A the retinula is 

 supposed to be cut through medianly, in B and C it is retained whole, c, central ganglion- 

 cells; eh, chitinous covering; hyp, hypodermis; I, lenticular sphere; mes, mesodermal 

 tissue; n, nerve; rh, rhabdom ; rt, retinula. 



It may appear at first sight unreasonable not to regard the compound eyes of 

 the Crustacea and the Insecta, which are so remarkably similar in organisation, 

 as directly related one to the other, but when the phylogenetic course of develop- 

 ment of the two divisions is taken intoaccount weshall have to takeup this position. 

 It can only be assumed that the development of compound eyes is a character of 

 the Arthropodan organisation, and that it takes place in the different divisions 

 (Crustacea, Arachnida, Myriopoda, and Insecta) independently, and yet may 

 lead, as in the Crustacea and the Insecta, to almost the same result. 



The eyes of Peripatus differ altogether in structure from those of 

 other Arthropoda. The eyes of this form also, indeed, originate as 



