504: Mr. Brayley on the ocular points oftht Helicidce^ 



for touch j at least they display little if any sensibility of the 

 presence of light, while their existence obviously increases the 

 irritability of the tentacula as organs of touch." And whilst ex- 

 plaining the passage from the Invertebrata to the Vertebrata by 

 means of the MoUusca and the Cephalopoda^ in the next page, 

 prior to adverting to the senses of sight and bearing as indubitably 

 possessed by the latter group of animals, he makes this additional 

 remark : '' Hitherto we have seen but few animals endowed with 

 the organs of sight ; and when the eyes existed, or rather when we 

 supposed these organs to exist, we have found them merely black 

 points, affording no trace of that peculiar organization which we 

 are led from analogy to conceive necessary for the purpose of 

 vision." In both places, however, it will be observed, (and no 

 circumstance could have better shewn our deficiency in actual 

 knowledge upon the subject,) that even Mr. Macleay has em- 

 ployed terms indicative rather of doubt than of certainty : but 

 lest any one should imagine, on the other hand, that although 

 the conflicting and vague evidence of previous writers might have 

 induced some degree of hesitation in Mr. M's mind, yet that the 

 fact he records of the black points being devoid of the organiza- 

 tion necessary for the purpose of vision, must have removed all 

 doubts from the minds of subsequent describers of the MoUusca^ 

 I shall terminate my citations with an extract or two from one of 

 the latest and most elaborate general works upon them; the 

 " Histoire Naturelle des Mollusques Terrestres et Fluviatiles," 

 by the Baron de Ferussac. 



In the Supplement to the history of the Limaces in this work, 

 the author gives a minute account of the anatomy of the Vagi^ 

 nulus Taunaisii^ communicated to him by M. De Blainville ; and 

 as it is manifest that the ocular {sic dicta) points of the naked and 

 the testaceous MoUusca are of precisely the same nature, an ex- 

 tract from this eminent comparative anatomist's description of the 

 tentacula of that animal, will serve to shew his views on the sub- 

 ject immediately before us. " Je n'ai pas fait," he says, " Panatomie 

 de I'oeil ou point noir qui se trouve porte a I'extremite des tenta- 

 cules posterieurs; j'ai seulement remarque qu'il est fort sensible, 

 et qu'il est place a la face dorsale d'une sorte de petit renflement 



