196 



SUB-CLASS (AND ORDER) TELEOSTEI. 



by drawing the lens nearer to the retina. The eyes of fishes 

 when at rest are accommodated for vision of near objects, i.e. 

 the opposite of the condition in the eyes of the terrestrial 

 Vertebrates. There is a layer of tissue between the choroid 

 and sclerotic which contains crystals, the argentea ; and round 

 the entrance of the optic nerve between the same coats there 

 is in many Teleosts (those with a pseudobranch) a vascular 

 plexus of unknown function, called the choroid gland. 



ass 



cp 



The eye-muscles are the 

 usual four recti and two 

 obliqui ; the former often 

 arise from a subcranial bony 

 canal the floor of which is 

 formed by the parasphenoid. 

 Movable eyelids are not 

 present, though there may 

 be a circular fold of skin round 

 the eye. In Anableps the 

 cornea is crossed by a hori- 

 zontal stripe which divides 

 the pupil, so that there 

 appear to be two pupils one 

 above the other. 



The auditory organ * 

 consists of the membra- 



FlG. 116. Membranous labyrinth of Perca fluvia- noUS labyrinth and is 

 tilis, inner view (from Wiedersheim). aa anterior .., 



ampulla; ac auditory nerve; ae external, ap WltllOUt accessory StrUC- 

 posterior ampulla ; ass apex sinus superioris ; 

 ca anterior, ce horizontal, cp posterior semi- 

 circular canal ; de ductus cndolyni.phaticus ; I 

 lagena cochleaeti mn macula acustica neglecta ; 

 ms macula acusca sacculi ; mu macula ac. re- 

 cessus utriculi ; o otoliths of the recessus utriculi, 

 the saccule and the lagena ; pi papilla acustica 

 lapenae ; raa, rap, rl, rs, nerves to the am- 

 pullae of the anterior, and posterior semi- 

 circular canals, to the lagena, and to the 

 saccule ; rec recessus utriculi ; s saccule ; ss 

 sinus utriculi superior ; u utricle. 



tures except in those 

 forms (Ostariophysi) in 

 which a chain of small 

 bones connects it with 

 the air bladder (see 

 p. 202). The membra- 

 nous labyrinth is con- 

 tained in the auditory region of the skull wall, but the cavity 

 in which it is placed is not shut off from the cranal cavity by 

 bone or cartilage. It is constructed on the usual plan, con- 

 sisting of a central chamber or vestibule and three semicircular 

 canals (Fig. 116). The vestibule is divided by a constriction 

 into two parts, an upper, the utricle (u) into which the semi- 



* G. Retzius, Das Gehororgan der Wirbelthiere, Bd. 1., Stockholm, 1881. 



