SENSE-ORGANS 



395 



blind. Amongst other blind Fishes Amllyopsis and TyphlicMiys 

 (Amblyopsidae) ^ and Lucifuga (Zoarcidae) may be mentioned, the 

 first two inhabiting the cave streams of North America, while the 

 third has a similar habitat in Cuba. When the eyes degenerate 

 they dwindle in size and recede from the surface. The lens and 

 the iris wholly or partially disappear, and although it is generally 

 recognisable the retina loses certain of its characteristic layers, or 

 the latter are but imperfectly 

 formed. In Myxine even the 

 eye-muscles are absent. 



The eyelids of Fishes are 

 little more than marginal folds 

 of skin, capable of little if 

 any movement, and leave the 

 eyes largely uncovered. Some 

 Sharks have a third eyelid or 

 " membrana nictitans " at the 

 anterior corner of the eye. 

 Lachrymal glands are un- 

 known. 



The Parietal Eye.— It is 

 only in the Cyclostomes that 

 this structure can have any 

 claim to be regarded as a visual 

 organ. In the Lamprey (Fig. 

 228) the parietal eye is a slightly flattened vesicle lying 

 directly over the pineal vesicle, and connected by a slender 

 stalk or nerve with the right ganglion habenulae. The dorsal 

 or more external half of the vesicle is bi-convex, and forms the 

 "pellucida," while the inner half or retina is said to consist 

 of supporting cells with interspersed deeply pigmented sense- 

 ceUs and ganglion cells.^ The external skin over the parietal 

 eye is partially transparent in the living animal. 



In many of the oldest known Fishes, such as the Ostracodermi, 

 the Antiarchi, and the Crossopterygian Osteolepida, there are 

 indications of the existence either of one or of two median sense- 

 organs on the upper surface of the skull, in the shape of one or 

 two foramina, or hollow protuberances, or pit -like grooves or 



^ Eigenmann, Arch. f. Entwickehmgsmech. viii. 1899, p. 545. 

 ^ Studniika, Sitzber. k. bbhm. Ges. Wiss., 1899, No. xxxvii. 



Pig. 227. — The telescopic eyes of Opistho- 

 proctus soleatus, Vaill. (A), and of a 

 species of a new family of Teleosts from 

 the Indian Ocean (B). Nat. size. (From 

 Chun. ) 



