284 Structure of the Eyes of the Gasteropodous Mollusca. 



surface of the eye is convex; but this convexity is surrounded 

 by a little elevated margin, formed of the substance of the 

 tentaculum. It is not very difficult to raise the eye with a 

 sharp needle. We then procure a globule, of a grayish black 

 colour, whose longitudinal diameter in the axis of the eye is 

 somewhat greater than the transverse diameter. The substance 

 of the tentaculum forms the external circular margin of the eye : 

 a very delicate membrane passes over the surface of the 

 globule ; and this, which may be considered as the cornea, is 

 distinctly separated from the surface of the globule. Muller 

 is convinced, that there is between this cornea and the rest of 

 the eye, a small space, which advances to the anterior third of 

 the eye. Posteriorly, the eye is imbedded in the substance of 

 the tentaculum. It is probable that, during life, a fluid may be 

 contained in this contracted space. 



The eye, situated under this transparent lamina, is composed 

 externally of a grayish black membrane, which passes on the 

 anterior surface into an iridiform black ring, and which is per- 

 forated in the middle by a very distinct pupil ; hence it results 

 that the eye seems more opaque externally. The cornea-form 

 epidermis passes a little farther than the external circurr^erejice 

 of the obscure margin. Lri^^Jiuj Ui-^^vju ii ^nii sj^uj 



At the posterior surface of the' ej^^ thie dp tfc nerve feiit^r's," 

 being a branch of the nerve which supplies the tentaculum. On 

 opening the eye with a needle, under the microscope, the 

 internal surface of the choroid is seen to be almost entirely 

 black, and containing a hard round body, which entirely fills the 

 choroid. The central body of the eye is of a somewhat irregular 

 figure, very hard, and of the colour of amber ; to its surface is 

 attached here and there a whitish film, probably the rest of the 

 retina which could not be seen separately in the interior of 

 the eye. The crystalline, when cleared, is semitransparent, 

 resembling generally the crystalline as described by Muller, in 

 the eyes of the Aracknida, when they have been preserved in 

 spirits ; it is difficult to say whether it should be called a crys- 

 talline or a vitreous body. 



Plate VIII. Fig. 8. Situation of the eye of 3Iurex Tritonis at 

 the side of the tentaculum, of its natural size, a. The proboscis ; 

 i, the anterior part of the tentaculum, endowed with the sense 

 of touchy; c, the eye on an eminence at the base of the tenta- 

 culum;;*^/' ''' 



Fig.'9', A portion of the preceding figure much magnified, 

 and showing in its situation the eye in the substance of the 

 tentaculum, and under the thin skin. «, The margin of the 

 tentaculum raised round the anterior convexity of the. eye; 

 6, the epidermis or cornea ; c, the choroid ; d, its anterior black 

 iridiform margin, with the pupil ; e, optic nerve. 



Fig. 10. The choroid opened, to shew its internal black 

 surface. The crystalline is removed. 



Fig. II. The crystalline alone. 



Fig. 12. The crystalline contained in the choroid.. . 



