No. I.] EVES OF MOLLUSCS AMD ARTHROPODS. 71 



known, contain double rods only, the crystalline-cone cells, or 

 retinophorae, of the compound eye are usually quadruple, 

 although in Amphipods and related Crustacea they are double. 

 On the other hand, the rod-bearing cells, or retinophorae, of 

 Scorpio and Limiiliis are respectively five and ten or fifteen 

 fold. 



It is probable that ommatidia are present in various modifica- 

 tions in all eyes. In Vertebrates the axial nerve-fibre of the 

 rods, and the presence of two nuclear-like bodies in the rod- 

 bearing cells, afford good reasons for supposing that the rods, 

 and the cells which bear them, are double. The ommatidia are, 

 therefore, essential elements, and a classification of eyes 

 must be founded mainly on the modifications which they have 

 undergone. I distinguish three principal kinds of light-sensi- 

 tive layers, according to the modification of the ommatidia. 



A retmeum is a collection of ommatidia in which the rods of 

 both retinulas and retinophorae, or of the latter alone, form a con- 

 tinuous layer, the retinulae retaining their pigment and primi- 

 tive arrangement around the retinophorae ; e.g., invaginate eyes 

 of all Molluscs, except Pecten, and worms (?). 



An ommateum is a group of ommatidia in which the rods, 

 produced only by the retinophorae, are completely isolated; e.g., 

 faceted eyes of Molluscs and Arthropods, and some Arthropod 

 ocelli. 



A retina is composed of ommatidia whose retinulae, having 

 lost their rods, are transformed into colorless ganglionic cells ; 

 e.g., Pecten and Vertebrates. 



Part I. 



Molluscs. 



Area Nocp. is extremely sensitive to slight changes in the 

 intensity of light. In a normal condition it never fails to close 

 its shell when any shadow is cast upon it This perception of 

 light gradations may be so delicate that if a small object, such 

 as a lead-pencil, is brought with extreme caution within two and 

 a half or three inches of the open shell, and in such a manner 

 that no perceptible shadow falls upon the animal, it at once 

 closes its shell, and with the same energy as when a deep shadow 

 is cast upon it. On examining the exposed parts of the mantle 



