656 William Patten 



invariably overlooked by Grenacher ; the ease with which it may be 

 demonstrated in such differeut genera of both Insects and Crustacea 

 as 3Ia?itts, Masca, Penaeus, OrcJiestia, BrancJtipus, Galathea^ Pagurus 

 and Palaemon r enders it highly probable, even if not beyond ali doubt, 

 that it occurs in ali faceted eyes. Wherever the corneal hypodermis 

 Ì8 present, the corresponding configuration of its cells and of the corneal 

 facets , or when these are absent the corneal markings , prove that 

 to this layer the corneal cuticula owes its origin. In the absence of 

 positive evidence , it w^ould be unreasonable to suppose that in other 

 compound eyes the cornea was due to a different source. We are there- 

 fore compelled to admit that the corneal hypodermis is univers- 

 ally present in the compound Arthropod eye, and always 

 gives rise to the corneal facets. It is probable that the corneal 

 cells may in some cases be regarded as more than simple constructive 

 Clements; their radiating fibres and their general shape in Galathea 

 render it almost certain that they play a physiological part in re- 

 gulating the amount of light, and therefore acting as a sort of iris. 



The general structure of the ommatidia remains about the same 

 as in the Mollusca; there is a teudency, however, towards an increase 

 in the number of retinophorae, which reach four, or very rarely five, 

 in the compound eye, while in certain aberraut forms , as Limulus^ 

 they may increase to as many as 8 or 10. 



In the Mollusca, we bave seen that a naturai result of the evaginate, 

 convex arrangement of the ommatidia is an expansion of the outer ends 

 of the latter , and a reduction of the retinulae to protective purposes. 

 An exactly similar series of chauges follows the evagination of the 

 Arthropod eye. The retinophorae of each ommatidium bave increased 

 to four equi valent cells, showing no trace of the Subordination of 

 one, accompanied by the predominance of others , as is the case in 

 Mollusca. The terminal, cuticular secretions, orrods. by a 

 series of changes to be shortly enumerated, bave been 

 transferred to the axial faces of the outer ends of the reti- 

 nophorae; they there unite to form the crystalline cones, to 

 accommodate which the outer ends of the retinophorae are 

 enlarged into a cup-like expansion, the calyx, while their 

 inner ends are reduced to a slender tube, or style, serving at 

 once as a support for the calyx, and as a protective canal for 

 the axial nerve. 



The compound eye of Arca is extremely valuable for comparison 

 with that of Arthropods: firstly, because it is very simple, and may be 



