SENSES — SIGHT. 180 



head ; they are capable of being moved to a slight extent ; 

 they are formed with coats, humours, and nerves, so arranged 

 that, on physical grounds alone, we may confidently pro- 

 nounce them to be optical instruments of considerable 

 power (Fig. 34). The structure is most perfect in the 

 Loligo and Sepias ; it is less so in the Octopus ; and in 

 the Nautilus has become so simplified that there is some 

 reason to suppose the sense of sight in it to be reduced 

 to the simple consciousness of the reception of light. 



The case is greatly different with the reputed eyes of 

 the Gasteropod mollusca. From their minuteness it is diffi- 

 cult to unravel their structure ; and, in many instances, they 

 are so situated that, were they organs of vision, the creature, 

 it is presumed, could be little benefited by them. More- 

 over, it has been asked of what avail it would be for an 

 animal to discover distant objects, which could neither over- 

 take them if necessary for food, nor avoid them if inimical 

 to its existence ; and it has been asserted that the eyes of 

 snails, at least, are in every respect insensible to light, for 

 the creatures creep and climb as correctly in the dark as 

 by daylight ; they do not at any time perceive obstacles, 

 placed on purpose in their way, until they touch them ; and 

 when deprived of the organs they crawl on as unconcernedly, 

 and guide themselves as safely, as they did previously to 

 the mutilation. On these grounds some naturalists of emi- 

 nence have denied that the little black points, denominated 

 eyes by the vulgar and the learned, are ocular bodies, and 

 find in them nothing more than the organs of an exquisite 

 sense of touch. * 



Organs of touch they may be in Snails (Helix, Limax, &c), 

 in which they are elevated on movable and filiform tubes 

 capable of being directed to all points ; but organs of touch 

 they surely cannot be in the greater number of the class, 

 on which they are placed, as it were purposely, to be re- 

 moved from the contact of external objects. Such, for ex- 

 ample, is their position in the Whelks (Buccinum) and Rock- 

 shells (Murex), in the Lymnafidae, Turboniclae, and others. 

 Seeing, too, how amply the mollusca are otherwise furnished 

 with tactic organs, additional ones might be deemed super- 

 fluous ; and, notwithstanding the facts opposed to it, I am 

 firmly of opinion that what have been called the eyes have 

 been properly so designated, — organs susceptible not merely 

 of the impressions of light, but capable of distinguishing ob- 



* Gaspard in Zool. Journ. i. 171). Also, Lister in Phil. Trans, abritlg. ii. 

 139. 



