FISHES. 49 



membranous labyrinth in the bony fishes and sturgeons lies for 

 Ihe most part free in the cavity of the cranium at the side of the 

 brain, and surrounded by the same fatty substance ; in the Plagio- 

 stomes and Cyclostomes the auditory apparatus is situated on each 

 side of the cranium in a special cavity, and is included in a carti- 

 laginous labyrinth. In almost all fishes three very large semi- 

 circular canals are present, of which the two perpendicular (the 

 anterior and the posterior canal) have the part between the two 

 in common, and thus together open into the vestibule by three 

 apertures, whilst the outermost, horizontal canal, opens into it by 

 two apertures. This vestibule (vestibulum, alveus communis) is 

 situated below the semicircular canals, and has an elongated sac 

 as an appendage, which is separated from it by a constriction. 

 The posterior part of this sac has a small vesicular appendage 

 (cysticula BRESCHET), whilst at the anterior part of the alveus 

 communis there is also a small expansion (utriculus, appendix 

 clavata vestibuli). In each of these three parts there lies a small 

 stone, of which that which lies in the anterior part of the sac 

 is the largest. These ossicles are hard in the bony fishes, 

 generally grooved, crimped at the margin and sometimes very 

 glistering and of a pure white; in the Plagiostomes and sturgeons 

 they are softer and granular ; they consist of carbonate of lime 1 . 

 In the Cyclostomes neither the stones nor any such sand has been 

 met with. In these the auditory apparatus is also more simple. 

 In Petromyzon and Ammoccetes two semicircular canals alone 

 are found. They are short and thick, and lie towards the mem- 

 branous vestibule, into which they open, as well by their broad 

 extremities (the two ampullae], as also by a common aperture at the 

 part where they unite. In Myxine and Bdellostoma MUELL. the 

 entire membranous labyrinth is a single circular canal, which 

 may be regarded in part as a canalis semicircular is, in part as 

 vestibulum. In the Plagiostomes the cartilaginous labyrinth is 

 prolonged beneath the skin and extends to the upper part of the 

 cranium; in the rays also the membranous labyrinth is pro- 

 longed upon the cranium and leads to a membranous sac, which, 

 situated between the cranium and the skin, opens on the skin 

 by one or by three small apertures. These parts are filled with 



1 Compare E. KRIEGER De Otolithis, Dissert, inaug. physiol. Berolini, 1840, 4to. 

 VOL. II. 4 



