Structure of the Eyes of the Gasteropodous MolliLsea, 283 



laterally situated eye, but into the round extremity of the 

 tentaculum, and there terminating in a papilla, which is covered 

 by the external skin. 



The eye, situated at the side of the obtuse extremity of the ten- 

 taculum, (fig. 6.) is almost spherical, a little flattened anteriorly. 

 It is covered in front by a very thin transparent layer of the 

 external skin, and is surrounded, laterally and posteriorly, by an 

 entirely black choroid. This black globule contained, in all the 

 individuals examined by MuUer, a transparent and semifluid 

 substance, apparently entirely filling the eye ; at the bottom 

 it seemed more fluid, and appeared to contain xndcay brilliant 

 particles, when the eye was dissected under the microscope. 

 In the anterior part of the eye is a small discoid, or lenticular 

 body, (fig. 7. a. b.) perfectly clear and transparent, and composed 

 of the same semifluid matter which filled the bottom of the eye, 

 differing only from it in being a little more dense. In all the 

 specimens of the snail which Muller examined, the transparent 

 matter was not solid, and the discoid crystalline itself was semi- 

 fluid and compressible. In the Murex Tritoms^ this lenticular 

 portion is quite hard, and of an amber colour. 



One principal fault in the investigations which have hitherto 

 been made is, that the great nerve of the tentaculum has been 

 taken for the optic nerve itself; this nerve, though very small, 

 is, however, larger than the eye, which is remarkably delicate ; 

 it passes at the obtuse extremity of the tentaculum into a great 

 white pillar, which has been erroneously considered to be the 

 ganglion of the optic nerve. The true optic nerve is, however, 

 but a very minute branch of the tentacular nerve, and is given 

 off to the eye at an acute angle, about a line and a half from the 

 extremity of the greater nerve, (fig. 6.) This branch, which is 

 extremely small, will escape observation without careful exami- 

 nation ; but, after repeated dissection, the accuracy of the fact 

 is established. 



Some authors, in consequence of different experiments, have 

 supposed, that the snail cannot see. These experiments, how- 

 ever, seem to be inaccurate, as MM. Leuchs and Steifensand' 

 have remarked, that the snail turns away its ten taenia when a 

 straw is held at the distance of from two to four lines from its 

 eye, but without touching it. M. Steifensand states, that the 

 animal can thus be made to move from one side to the other. 

 He also remarked, that when a transparent piece of glass was 

 presented, the animal touched it, but not when the glass was 

 coloured. 



We thus see that the snail is provided with the rudiment of 

 an eye containing transparent parts ; and that it is not the only 

 organ of sense in the tentaculum, being situated at the side of a 

 large papilla, which performs the function of touch, and is pro-' 

 vided with a distinct and much larger nerve than the eye. 



It is known that the eyes of the murex are situated externally 

 to the tentacula, on a small eminence; so that their axis is 

 almost in the same direction as that of the tentacula. The 



