QQ2 William Patten 



a Constant stream of energy , but if this stream is broken by tlie Inter- 

 vention of any object, a special irritation will be produced giving rise to 

 muscular contractions. The lieliophags of Arca are so perfectly con- 

 structed and so sensitive, that the slightest change in the light received 

 causes an irritation sufficient to produce strong muscular contractions; 

 it is, therefore, a heliophag of a high stage of development , and 

 might very properly be called an eye. 



If the heliophag became so sensitive that the slightest changes in 

 the amount of light caused muscular contractions, we would bave the 

 first Steps towards the formation of an eye. If , by association, and by 

 the high development of the heliophag, certain light combinations i. e. 

 Images of certain objects, caused contraction, and others, equally in- 

 tense, not, then the heliophag would bave ali the essential character- 

 istics of an eye, but would not necessarily bave lost its primitive func- 

 tion of absorbing energy. 



The 80-called eyes of Pecfen, Arca, Chiton^ and Onchidium repre- 

 sent various stage s of transition from heliophags to true eyes, but in ali 

 probability with their primitive functions stili unimpaired. 



Nerve Endings. 



In order to comprehend more fully the nature of the nerve endings 

 in the retinal layer, it is necessary to study the similar process on the 

 unmodified epithelium. Sections show that in the Mollusca indifferent 

 epithelial cells are usually very high and narrow, being constricted in 

 the middle, and ending at the base in several root-like fibres attached 

 to the basai membrane. In certain regions, notably on the Ophthalmie 

 fold, the cells are deeply pigmented at the outer ends ; they are covered 

 by a cuticula varying in thickness, and divisible into two layers, a thin, 

 refractive outer, and an inner one passing insensibly into the outer ends 

 of the cells. When these cells bave been successfully isolated, one 

 may see, in nearly ali, several nerve fibres extending along the wall of 

 the cells towards their outer ends. These fibres generally cling quite 

 firmly to the sides of the celi, but examples may be found in which they 

 bave been separated from the whole length of the wall, remaining attached 

 only at the outer extremity (PI. 30, figs. 50 and 53). I bave seeu cases 

 in which, to one celi, as many as five or six such fibres, several times as 

 long as the celi, were attached. In sections of the mantle edge pre- 

 served either in chromic acid, or picro-chromic osmic acid (Fol; and 

 mounted in acetate of potash, a perfect network of fibrillae, the general 



