XIV SENSE-ORGANS 395 
blind. Amongst other blind Fishes Amblyopsis and Typhlichthys 
(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 MMyxine 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.—lIt is 
only in the Cyelostomes that 227M towns cs of Oi 
this structure can have any ‘species of a new family of Teleosts from 
claim to be regarded as a visual aera Ocean (B). Nat. size. (From 
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- 
cells 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 
1 Higenmann, Arch. f. Entwickelungsmech. viii. 1899, p. 545. 
2 Studnitka, Sitzber. k. bohm. Ges. Wiss., 1899, No. xxxvii. 
