No. r.] EYES OF MOLLUSCS AND ARTHROPODS. 79 



allel cross-lines ; the thick outer layer consists of longitudinal 

 fibres, which may contain a few nuclei. 



There are numerous refractive fibres which arise from the 

 periphery of the eye-stalk, and, converging toward the base of 

 the eye, penetrate the sclerotica and the superimposed layers as 

 far as the inner ends of the rods. In the sclerotica they expand 

 into refractive spindle-shaped bodies, often of a faint pink 

 color. 



Development of the Eyes of Pecten. 



On the branchial wall of the ophthalmic fold of Pectens, 

 2 mm. long, are a few minute, pigmented and transitory cups, 

 undoubtedly homologoas with the invaginated eyes of Area. 



The stalked eyes first appear as oval optic thickenings at the 

 base of the ophthalmic fold. By a continued proliferation of 

 the cells on the outer side of the optic thickening, an oval 

 knob-like papilla is formed, containing a solid core of hypo- 

 dermic cells. At first the core is ill-defined ; several of the 

 more deeply situated cells separate from the rest to form the 

 ganglionic cells, which later provide the eye with nerve-fibres. 

 The whole papilla then elongates, and a disc-shaped cavity ap- 

 pears in the centre of the core, transforming it into an optic 

 vesicle. 



In the following stages the posterior wall becomes more 

 sharply defined, and there, for the first time, the cells of the 

 optic vesicle are provided with distinct cell-walls. The inner 

 wall of the optic vesicle divides into two layers, — the inner one 

 giving rise to the tapetum, and the outer one to the argentea. 

 The latter is formed by the transformation of cells into superim- 

 posed plated membranes, the nuclei being retained, in some 

 cases, until the eye has completed its development. The outer 

 wall of the optic vesicle divides into three zones, consisting of 

 the fibrous, the ganglionic, and the retinophoric layer of the 

 retina. 



Some of the connective-tissue cells surrounding the optic 

 vesicle give rise to the retinal-sac, a nucleated membrane, the 

 anterior wall of which develops into the septum^ and the pos- 

 terior into the sclerotica. 



The connective-tissue cells above the septum give rise to the 

 lens. The cells of the tapetum are at first filled with coarse. 



