Eyes of MoUuscs and Arthropods. 607 



inuscular contraction of tlie mantle, and may, or may not be present. 

 Carrière , after au examination by means of sections , concludes tliat 

 tlie Organs in question are not eyes, but peculiar glauds. 



Toward the expiration of the time at my disposai for the pre- 

 paration of this paper, I was enabled to examine hastily some mace- 

 rated preparations , and^found the eyes (?) coraposed of an immense 

 number of conical cells, expauded at the outer extremity aud drawn 

 out to a poiut at the inner; they were filled with a mass of refractive, 

 closely packed. globules, — which, indeed. gave them the appearauce 

 of glaud cells. — and were surrounded by narrow, ciliated cells, occa- 

 sionally faintly colored at their expanded outer ends. Upon the external 

 surface , the large cells were often provided with several, longitudinal 

 fibres, whieh appeared like the nerve fibres seen in the retinophorae of 

 Area, but I could not decide whether they really were so or not. These 

 Organs present several peculiarities which render a more accurate study 

 of them highly desirable, Their position on the edge of the mautle, 

 and their hemispherical shape, show, at first sight, considerable resem- 

 blance to the faceted eyes qì Arca^ to whose retinophorae and cover 

 cells. the large colorless cells, drawn out to a point at their inner ends 

 and surrounded by smaller ciliated ones, might be compared. The ab- 

 sence of pigment does not offer any serious difficulty, aud, if it could 

 be shown that the Innervation and composition of the large cells was 

 the same as in the retinophorae of the faceted eye of Arca., we should 

 be obliged to considcr these problematical organs of Pinna as eyes, 

 rather than glands. It seems, however, that in the absence of such 

 observatious , the question must for the present be left open. That 

 Pinna has some organs of vision, is shown by the same simple experi- 

 ments as bave been tried upon other Mollusca ; but whether the seat of 

 this seusitiveuess to light lies in the so-called eyes of Will, or in the 

 masses of dark pigment so abundant upon the edge of the mantle, is 

 of course difficult to decide. 



Avicula. 



Avicula was examined very hastily, just before sending the proof 

 sheets to the printer. It is extraordinarily sensitive to changes in the 

 amount of light, the least shadow causiug it to dose the shell quickly, 

 and with such force as to indicate that even this slight chauge acts 

 as a considerable irritant. The simple, diffuse ommatidia, the only 



