ANATOMY AND LIFE HISTORY OF MOLLUSCA. 149 



terms, which implied that they were the homologues of well-known 

 ophthalmic constituents, a perfectly distinct terminology has been 

 resorted to. We have for instance Ommatidium, Retinophora, 

 Retinula, Retinidiu?n, Omr)iatium, Ommerythrine and Retineum. 

 These are the terms suggested by Mr. Patten and others, and as 

 it is necessary to understand them in order to comprehend the 

 literature of the subject and the terms I may be obliged to use, I 

 shall here give their definitions, observing that we are dealing with 

 the highest or ultimate sub-divisions of structure revealed by the 

 microscope. 



Ommatidium denotes the structural elements of all eyes, 

 consisting of two to four colourless cells. 



Retinophora indicates the aforesaid cells. 



Retinula is the circle of pigmented cells surrounding the 

 retinophora. 



Retinidium is the specialised part of the Retia-terminalia 

 contained in the cuticular secretion of each cell which forms a rod. 



Ommatium is a group of ommatidia in which the retinidia are 

 completely isolated, as in the compound eyes of Arthropoda and 

 Mollusks. 



Ommerythrine is the red pigment in all eyes; whether confined 

 to the rods alone, or to the retinube, or to the underlying tapetum. 



Retineum is a collection of ommatidia in which the retinidia of 

 both retinulse and retinophorse form a continuous layer ; the 

 retinula? retaining their pigment and primitive arrangement round 

 the retinophora • such as the invaginate eyes of all Mollusks. 



It should be added that in many places upon the molluscan 

 hypodermis, especially where exposed to the light, the cuticula is 

 found to be divided into two layers, an outer, structureless one 

 which is the corneal cuticula of the visual organs, and an inner 

 layer ; the retinidial cuticula filled with the ultimate ramifications 

 of the hypodermic nerves. 



It is probable then, that where the so-called eyes in reality 

 consist of hardly more than pigment-spots, it is useless to expect 

 in these simple organs, structures which would deserve the titles 

 of lens, vitreous body, choroid iris, and so forth. Nevertheless 

 they are organs of sense and may be justly termed visual organs ; 

 though the mode of their operation may be concealed from us as 

 yet, or may never be revealed, because they belong to functions 

 which are utterly outside the reach of our experience. To use 

 the words of Mr. Patten (op. cit. p. 570) " If we study the structure 

 of the eyes of Pecten we shall find that the parts really have the 

 function that their names and composition indicate. We see a 

 constant purpose in view, a concentration of the rays of light and 

 the formation of inverted images of external objects upon a sensitive 

 nervous layer, the arrangement of whose elements shows a definite 

 relation to the direction of the rays of light." 



