116 FISHES. 



refractive power of the aqueous humour is compensated by 

 the greater convexity of the lens. 



Organ of Hearing. — No trace of an organ of hearing has 

 been found in Branchiostoma. In the Cydostomes the laby- 

 rinth is enclosed in externally visible cartilaginous capsules 

 laterally attached to the skull ; it consists of a single semi- 

 circular canal in the Myxinoids, whilst the Petromyzontes 

 possess two semicircular canals with a vestibulum. 



In all other iishes the labyrinth consists of a vestibule 

 and three semicircular canals, the vestibule dilating into one 

 or more sacs wliich contain the otoHths. A tympanum, tym- 

 panic cavity, and external parts, are entirely absent in the 

 class of fishes. 



In the Cliondro'pUrygians and Dipnoi, the labyrinth is 

 enclosed in the cartilaginous substance of the skuU. In the 

 former the excavation in the cartilage is larger than the 

 membranous labyrinth, but nearly corresponds to it in form ; 

 the part which receives the membranous vestibulum is called 

 Vestibulum cartilagirieum, from which a canal issues and 

 penetrates to the surface of the skull, where it is closed by 

 the skin in Sharks, but opens by a minute foramen in Eays. 

 The otolithic contents are soft and chalklike. 



In the Holocephali part of the labyrinth is enclosed in the 

 cartilage of the skull, another part being in the cranial cavity, 

 as in Ganoids and Teleosteans. The membranous vestibulum 

 is continued by a canal to a single opening in the roof of the 

 skull, from which two smaller canals are continued to two 

 small foramina in the skin covering the occipital region. 



In the Teleosteans the sac which contains the otohths 

 lies on each side of the base of the cranial cavity and is 

 often divided by a septum into two compartments of unequal 

 size, each containing a firm and soHd otolith; these bodies 

 (Fig. 51), possess indented margins, frequently other impres- 



