1884.] NATURAL SCIENCES OF PHILADELPHIA. 303 



In Patella^ as shown by Fraisse,^ there is a simple sphere, made 

 up of pigmented cells, similarly formed as those described for the 

 Lamellibranchiata. This sphere is open in front and allows the 

 entrance of the external media. 



Haliotis ^ gives us an advance ; here we have an open sphere as 

 in Patella^ but instead of the refractive cuticula to each cell, 

 they afe physiologically combined into one mass, forming a 

 lens. This lens is the product of the cells of the e^-e, and is 

 purely a secretion — a simple cuticular lens, as is found in all the 

 eyes of the invertebrata — while the lens of the vertebrata, where 

 it exists, is always cellular. The cellular lens-like bodies found 

 in the so-called eyes of Pecten and Spondylvs, and the dorsal eyes 

 of Onchidium, are exceptional, and will be treated of elsewhere. 



Fissurella ^ gives us an eye that goes practically as far as any 

 gastropod eye, the higher forms merely carry out, a little more 

 in detail, this plan. This results in a closed eye containing a lens, 

 the transparent epidermal covering acting as a cornea. The 

 pigmented layer is as in Haliotis^ namely, the cells composing 

 it are devoid of a transparent cuticula, the lens and cornea 

 serving as the refractive bodies. 



The phylogenetic development of the moUuscan eye, therefore 

 (cephalopoda excepted), is as follows: (1), a pigmental surface 

 of epithelial cells ; (2), pigmented invaginated grooves for pro- 

 tection, at centralized points of the body, each visual cell having 

 a cuticular body ; (3), this groove contracting to an open sphere 

 which closes ; (4), the refractive bodies of each cell being cen- 

 tralized into a cuticular lens. A distinct nerve, specialized for 

 sight, is developed {Haliotis and Fissurella), which connects the 

 eye with the superior cephalic ganglia. 



Now, let us see how the ontogenetic development agrees with 

 the phylogenetic. 



Bobretzky^ and Haddon ^ have given us the development of 



' Fraisse, Paul, Ueber MoUusken Augen mit embryonalem Typus. Zeit- 

 schr. f. wis. Zool., Bd. xxxv, 1881. 



^ Fraisse, Paul, Ueber Mollusken Augen mit embryonalem Typus. 

 Zeitschr. f. wiss. Zool., Bd. xxxv, 1881. 



^Bobret^ky, N., Studien iiber die embryonale Entwickelung der Gastro- 

 poden, Arch. f. m'kr. Anat. Bd., xiii, 1877. 



* Haddon, A. C, Note on the Development of Mollusca, Quart. Jour. 

 Mic. Sci., n. s., vol. xxii, 188'i. 



