466 CEPHALOPODA. 



every indication of inferiority of construction when compared with 

 that of the Dibranchiate tribes. Encased in no orbital cavity, and 

 consequently unprovided with any other muscular apparatus than 

 the fleshy pedicle whereby it is connected with the head ; unpro- 

 tected by eyelids and devoid of lachrymal appendages ; without 

 either transparent cornea, aqueous humour, iris, or crystalline lens ; 

 and, moreover, coated internally with a dark pigment, apparently 

 situated in front of the nervous expansion which represents the 

 retina, instead of behind it in the usual position of the choroid 

 tunic, all these are facts calculated to arrest the attention of the 

 physiologist, and excite the surprise of every observer who studies 

 on a large scale this part of the animal economy. 



(507.) The eyes of the Dibranchiate Cephalopoda are not less 

 remarkable in their construction than those of the Nautilus, and 

 from their greater complexity will require a more elaborate descrip- 

 tion. In order to simplify the details connected with this portion 

 of our subject as much as possible, we shall describe separately, as 

 forming distinct parts of the ocular apparatus met with in the com- 

 mon Cuttle-fish (Sepia cfficinalis), first, the orbit ; secondly, the 

 globe of the eye ; thirdly, the chamber of the optic ganglion ; and 

 fourthly, the muscles of the visual organ. 



(508.) The orbit differs from that of all other classes of ani- 

 mals, inasmuch as it is a cavity circumscribed on all sides and 

 covering even the front of the eye.* The bottom of the orbital 

 cavity is cartilaginous, being partially formed by a process derived 

 from the cranial cartilage ; but elsewhere it is made up of the com- 

 mon fleshy integument of the body (fig. 13, d, d, e) : becoming 

 gradually attenuated, the skin (b) passes over the anterior portion 

 of the eye, where, being transparent (/), it represents the cornea, 

 although it has no connection with the eye-ball itself. Beneath 

 the cornea the integument again becomes opaque, and forms a 

 thickened fold (a), which might be considered as the rudiment of 

 an under eyelid. The orbit, therefore, forms a complete capsule, 

 enclosing the whole of the apparatus of vision. 



(509.) The globe of the eye fills up the anterior part of the 

 orbital chamber, and is remarkable from having no cornea properly 

 so called ; so that, on raising the transparent skin (/) which forms 

 the exterior wall of the orbit and supplies the place of the cornea, the 



* Descriptive and Illustrated Catalogue of the Physiological series of Comparative 

 Anatomy contained in the Museum of the Royal College of Surgeons, London, vol. iii. 

 part i. plate 52. 



