48 CLASS XIY. 



eye-ball, which m such cases is small, is covered by a production 

 of the skin neither attenuated nor transparent : in the most the skin 

 forms round the anterior margin of the eye-ball a circular fold, and 

 then becoming thin and transparent passes over it as conjunctiva. 

 Only few have eye-lids with free edges ; some sharks have also a 

 third eye-lid, the memhrana nictitans. Lachrymal glands are want- 

 ing. The external coat [sclerotica) is elastic and fibrous ; it has on 

 the inside a cartilaginous layer, which sometimes partially ossifies. 

 The cornea is flat, or at least not very convex. On the inside next 

 the sclei'otica is situated the external lamina of the vascular coat 

 [clioro'idea), a glistering silvery or gold-coloured membrane, which 

 jDasses into the iris ; to it succeeds the vascular layer, formed of a 

 network of blood-vessels and covered by a layer of black pigment ; 

 this last passes at the posterior sm'face of the iris into the uvea. At 

 the entrance of the optic nerve into the eye-ball is situated in most 

 osseous fishes a vascular body, which surrounds the nerve like an 

 incomplete ring [glandula choroidalis, comp, above, p. 34). The 

 place where the optic nerve enters the eye-ball is frequently not in 

 the axis of the eye. The retina arises from a streak or irregular 

 white spot, where the optic nerve, mostly in form of a folded 

 band, is expanded. In many osseous fishes a production of the 

 choroidea as a sickle-shaped band [processus falciformis) penetrates 

 the vitreous humour, opposite the entrance of the optic nerve, and 

 attaches itself to the margin of the capsule of the crystalline lens ; 

 generally the attachment is effected by means of a transparent but- 

 ton [camjyamda Halleri), of which the structure is not yet suffi- 

 ciently known. The vitreous humour is more fluid than in the rest 

 of the vertebrates. The crystalline lens is almost spherical and 

 very large ; it projects through the pupil at its anterior part. The 

 aqueous humour is present in small quantity alone \ 



The auditory organ of fishes consists exclusively of that part 

 which in the higher vertebrate animals constitutes the labyrinth. 

 The external auditory passage, the cavity of the tympanum, the 

 Eustachian tube and the ossicles of the ear are wanting. The 



1 Compare Rosenthal Zergliederung des Fiscliaufjes in Reil's Archiv, x. s. 393 — 

 414; D. W. S(EMMERRiNG De OcuJoi'uiii hominis animaliumque sectione horizontali. 

 Gottingas, 1818, fol. pp. 62 — 71 ; Gottsche Ueher die Retina, im Auge der Gratenfische, 

 Mueller's ^wA/v, X834, I. s. 457— 466. 



