No. I.] EYES OF MOLLUSCS AND ARTHROPODS. 83 



proof of the intercellular nature of the nerves and of the ner- 

 vous structure of the rods was obtained by dissolving the 

 cuticula, leaving the net-work of nerve-fibrillae free and unin- 

 jured. It is then seen that the fibres of the optic nerve pen- 

 etrate the basal membrane and pass outwards between the cells 

 of the ommatidia. As they pass beyond the outer end of the 

 cells the nerves break up into innumerable branches, which do 

 not end freely, but unite with each other in all directions to 

 form a network of continuous fibrillse. 



On the upper side of each tentacle of Haliotis is a longitudi- 

 nal groove, the floor of which consists of thick columnar cells, 

 filled with a dark-brown or black pigment ; the cuticula is not 

 especiall}^ developed, neither could any colorless cells be seen. 

 The similarity of these pigmented bands to those on the sipho 

 and mantle edge of the Lamellibranchiata is apparent. 



11. 



Crustacea. 



Each square corneal facet of PincBus is secreted by two under- 

 lying oblong cells belonging to the corneal hypodermis. 



The centre of each ommatidium is occupied by four colorless 

 cells, — the retinophorae, — united to form an inverted pyramid, 

 whose base abuts against the corneal hypodermis, while the 

 apex rests upon the basal membrane. The inner end of the 

 pyramid is reduced to a slender, hollow stalk, — the style, — 

 whose inner end enlarges into a solid, pyramidal thickening, the 

 pedicle ; the latter rests upon the basal membrane by a delicate 

 stalk composed of the attenuated, inner ends of the four 

 retinophorae, two of which are united with each other. Each 

 leg of the stalk is divided at its inner end into several fibres by 

 which it is united to the basal membrane. This fact is impor- 

 tant, for it shows that the segments of the so-called rhabdom 

 of Grenacher are not secretions of the retinulse, but merely the 

 inner ends of the retinophorae (or crystalline-cone cells), 

 which terminate in the same root-like fibres seen in nearly all 

 hypodermic cells. 



The pedicle, whose abaxial walls are very thick, entirely 

 obliterating the central canal, is composed of plates varying in 

 thickness in different parts. Each plate is marked by a set of 



