Netvoiis Si/slcm and Senses, 



531 



nerves moi-e considerable * ; arguments of too general a nature 

 to have any influence when adduced for a special purpose. 

 He might have found, perhaps, a better proof in their position, 

 for in very many genera the tentacula do not support the 

 eyes, nor are they, nor can they be, employed in tracking the 

 way. This is the case in the genus Dori^, in which the teii- 

 taenia are situated on the back, point directly upwards, are 

 remarkably large, and of curious and complex organisation. 



4. Sight.— The, tunicated and bivalved Mollusca are desti- 

 tute of eyes, but, with the exception of the Pteropoda, all the 

 others have organs which have been generally considered as 

 subservient to the purposes of vision. There can be no ques- 

 tion of their function in the cuttle-fish, for in them the eyes 

 are very large, and similar, in all material points, to those of 

 vertebrate animals. They are two in number, one on each 

 side of the 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 

 107 ...s^^^^^^ _ pronounce them to be 



optical instruments of 

 considerable power. {Jig. 

 107.) 



The case is greatly dif- 

 ferent with the reputed 

 eyes of the other Mol- 

 lusca. From their mi- 

 nuteness it is difficult to 

 unravel their structure; 

 and in many instances 

 they are so situated, that, 

 were they organs of vi- 

 sion, the creature, it is 

 presumed, could be little benefited by them. Moreover, it 



. * Manuel, p. 107., or more particularly his excellent Principes d'Anato- 

 mie Comp., i. 341. 



f The figure is a view of the eye of Octopus vulgaris, copied from Cuvier. 

 a a, A cellular and muscular tunic, the latter for opening the lids ; b b, the 

 conjunctiva; c, another tunic, enveloping the globe of the eye, and a 

 pouch situated behind it (e\ containing (/) the optic ganglia, and the 

 glands which surround it (g). The pouch (e) is a transparent membrane, 

 which occupies all the spacfe between the globe of the eye and the tunics 

 which go to its lids ; so that the former actually fills only about a third of 

 the greater globe, which, at first view, appears to be the eye itself, h h. The 

 external coat of the proper eye, perforated with an infinite number of 

 minute holes, for the passage of the filaments from the optic ganglion ; and 

 if another coat, formed apparently by the expansion and netting of these 

 nervous filaments, m, The crystalline lens. 



M M 2 



