336 



PHYLUM ARTHROPODA. 



type to which the name monostidious * has been applied (Lan- 

 kester). They are all to be regarded as developments of a 

 single layer of the hypodermis. 



The median eyes of Limulus and Scorpio (Fig. 237), and the an- 

 terior median or " principal " eyes of Spiders (Fig. 237, A) belong 

 to a different type, the diplostichous. In these eyes two layers of 

 hypodermis are concerned in the formation of the optical appa- 

 ratus (Fig. 236). They are formed in ontogeny by the folding in 

 of the hypodermis from the side of the area which will be occupied 

 by the eye. The outer of the two layers thus involuted becomes 



a. 





FIG. 236. Section through the median eye of Scorpio in three stages of development. From 

 Korschelt and Heider. A after Parker, B and C diagrammatic, g ? brain (?) ; gl (g in C) 

 vitreous body ; h hypodermis ; I lens ; n optic nerve ; pr postretinal layer ; r retina ; rh 

 rhabdom. 



the retina (r, Fig. 236), its columnar cells secreting rhabdoms 

 (rh), and the inner, when it persists, the post-retinal membrane. 

 The layer of hypodermis external to the fold becomes the vitreous 

 body and secretes the lens. The nerve, by secondary shifting 

 of its position, enters the under surface of the retina. There are 

 thus three layers of hypodermis concerned in the development 

 of the diplostichous eye, though the dioptric layers are formed 

 from only two of them. The involutions from which these eyes 

 are formed are closely associated with those forming the brain, 

 but the morphological significance of this mode of development 

 is quite obscure. 



* o-rt'xos, a row, line. 



