192 THE EYE OF GASTEROPODS. 



Swammerdam's description of tlie organ is, indeed, in 

 some degree inaccurate. He mistook the large nerve con- 

 tained in the interior of the cylindrical tentaculum of the 

 snail, and which, according to the variable condition of the 

 tentaculum, is sometimes bent or waved and sometimes 

 straight, for the optic nerve, — a mistake in which he has 

 been followed by all subsequent anatomists, until Professor 

 Miiller, of Bonn, proved that this strong nerve does not go 

 to the eye, but to the extremity of the tentacula, where it 

 terminates in the form of a papilla. The true optic nerve 

 is a very fine filament running alongside this larger one, * 

 diverging from it about a line and a half from its extremity 

 at an acute angle, and passing forwards to the eye, which 

 is situated, not on the apex of the tentaculum, but a little 

 to a side, and is of very small size. It "is almost spherical, 

 a little flattened anteriorly. It is covered in front by a 

 very thin transparent layer of the external skin, and is sur- 

 rounded, laterally and posteriorly, by an entirely black cho- 

 roid. This black globule contained, in all the individuals 

 examined by Miiller, a transparent and semi-fluid substance, 

 apparently entirely filling the eye ; at the bottom it seemed 

 more fluid, and appeared to contain many brilliant particles 

 when the eye was dissected under the microscope. In the 

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

 perfectly clear and transparent, and composed of the same 

 semi-fluid matter which filled the bottom of the eye, differ- 

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

 specimens of the snail which Miiller examined, the trans- 

 parent matter was not solid, and the discoid crystalline itself 

 was semi-fluid and compressible. In the Murex Tritonis 

 this lenticular portion is quite hard, and of an amber colour." j- 



All doubt, then, relative to the reality of the eyes of the 

 mollusca may be said to be removed ; and when I consider 

 their structure, and their very general existence in the class, 

 I am far from joining with those who believe them to be of 



au travel's des coriics ; toutcs choses si dclicatcs, qu'ellcs parurent une sorte 

 de merveille, de miracle, tant dc la part de celui qui les avait observees, que 

 de la nature qui les a faites." — Hist, de Sc. Nat. ii. 429. 



* " With regard to the optic nerve of Gastoropods, anatomists were formerly 

 in error, mistaking for it the great nerve of the tentacula, which is the nerve 

 of touch ; the optic nerve is much more minute, and looks like a branch of 

 that larger nerve, hut may be traced backwards to the cerebral ganglion." — 

 MuLLEii's Eleni. of' Phys. trans, p. 1117. Roget's Bridgew. Treat, ii. 

 481. 



t Edinb. Journ. Nat. and Geogr. Science, iii. 283. An historical account of 

 the discoveries in the anatomy of the eye of Gasterojjods, is given in the 

 Ann. des Sc. Nat. xxii. 7—19. 



