1004 



PISCES. 



rior is much the larger compartment, to which 

 the posterior chamber seems a superadded ap- 

 pendix. Both of these compartments are filled 

 with a pellucid fluid, and each contains a stony 

 mass or otolithe, of which that in the anterior is 

 the largest, that in the posterior being compara- 

 tively of small dimensions. In Orthagoriseus, 

 however, according to Cuvier, the saccus is 

 single, and instead of an otolithe only con- 

 tains a few granules apparently rather of 

 mucus than of cretaceous substance. 



Otolithes. Most Fishes are furnished with 

 three stony masses, which are intimately con- 

 nected with the function of hearing. Of these, 

 the otolithes or lapilli, one, generally the small- 

 est, is contained in the anterior extremity of the 

 vestibule; the other two are situated in the two 

 compartments of the saccus. The otolithe con- 

 tained in the anterior compartment of the saccus 

 is generally of remarkable size, forming a con- 

 siderable protuberance in the base of the occi- 

 pital bone, in which it is lodged ; this is con- 

 spicuously seen in the Gadidse and some of the 

 Perch tribe. 



The substance of these otolithes consists of 

 carbonate of lime, but they assume various de- 

 grees of hardness and considerable diversity of 

 colour in different Fishes. In most cases they 

 present a texture as hard and fragile as porce- 

 lain. In a few instances, as for example in the 

 Sturgeon (Accipenser Stitrio), there is only one 

 lapillus, which is soft and as easily crushed 

 and reduced to powder as a piece of chalk ; as 

 is likewise the case with the otolithes of the 

 Raid<e and Squalida. 



In shape the otolithes vary exceedingly in 

 different genera. For the most part they are 

 smooth and present this character in common, 

 that they are marked with asperities, fossae, and 

 grooves for the attachment or reception of nerv- 

 ous filaments. Those contained in the saccus 

 are frequently surrounded by a serrated margin, 

 which is rarely the case with the lapilli of the 

 vestibule. But whilst there is so much diver- 

 sity in the shape of the otolithes belonging to 

 different genera of Fishes, the form of those met 

 with in the species belonging to the same genus 

 is wonderfully constant, so much so, indeed, 

 that not only the general outline, but the most 

 minute fossules and grooves were found by 

 Weber accurately to correspond in different 

 specimens, so that it was difficult to distinguish 

 one from the other; from which circumstance 

 those otolithes might be employed with advan- 

 tage as affording excellent generic characters to 

 the zoologist. The connection of the otolithes 

 with the saccus or with the vestibule is so 

 difficult to be perceived, that they might be 

 thought to be loose in the contained fluid ; 

 when, however, we find them small in the 

 younger Fishes, and increasing in size as age 

 advances, it is evident that they must receive nu- 

 tritious vessels ; they are moreover attached by 

 nervous filaments of extreme delicacy, which 

 pass to them from the saccus. In many points 

 they touch the membranous walls of the cavity 

 in which they are lodged ; when, therefore, the 

 sac is but loosely connected with the bones of 

 the cranium, sonorous vibrations cannot be 



communicated immediately from the cranium 

 to the lapilli, but must first be communicated to 

 the surrounding fluid. 



Semicircular canals. All Fishes, with the 

 exception of the Petromyzonidae, have three 

 semicircular canals entering into the formation 

 of the internal organ of hearing, and the arrange- 

 ment of which is as follows. The anterior 

 arises by one extremity from the anterior part of 

 the vestibule, and, winding upwards and back- 

 wards, meets the posterior semicircular canal 

 derived from the hinder part of the vestibular 

 cavity ; at the point of meeting the two join to 

 form one common duct, which enters the vesti- 

 bule near its middle. Both these canals are 

 placed perpendicularly. The third or external 

 semicircular canal issues from the anterior part of 

 the vestibule, and winds horizontally outwards to 

 join the vestibule again at its posterior part near 

 the origin of the posterior canal. In this way 

 the three semicircular canals open into the mem- 

 branous vestibule by five orifices. In the Her- 

 ring, however, ( Clupea Harengus) not only do 

 the anterior and posterior canals unite, but the 

 external also joins the posterior, so that in this 

 fish there are only four apertures communicating 

 with the vestibule. 



Each of the semicircular canals near its com- 

 mencement from the vestibule swells into an 

 oval dilatation called the ampulla, so that three 

 of these ampullae exist, two at the anterior part 

 of the vestibule, and the third near its posterior 

 extremity. 



The connection between the semicircular 

 canals and the cranium is effected by the assist- 

 ance of osseous passages, in which one or two 

 (rarely all three) of the semicircular canals are 

 lodged, and in some Fishes, as for example in 

 Cobitis fossilis, these are entirely deficient. 

 The membranous canals are not at all adherent 

 to the osseous passages, but are only connected 

 with them by the intervention of a most delicate 

 cellulosity, or are merely suspended in a fluid, 

 with which all the osseous canals as well as the 

 entire cranium is filled up ; they are conse- 

 quently extracted without the employment of 

 the slightest force. 



Those canals which are not enclosed in bony 

 channels are simply annexed to the bones of the 

 cranium by a fine cellular web. 



From the above arrangement it may be 

 clearly understood that these parts are pur- 

 posely left but loosely connected to the surface 

 of the bones, for otherwise the bony canals 

 would not so greatly exceed the membranous 

 ones in size, but on the contrary would be 

 filled and lined by them throughout ; and that 

 sonorous vibrations most readily arrive at the 

 labyrinth through the fluid with which the 

 canals are surrounded. 



In Murana anguilla the anterior and poste- 

 rior semicircular canals mount so high towards 

 the vertex of the cranium that they are not 

 placed by the side of the brain, but absolutely 

 rise above it and approximate their fellows of 

 the opposite side. 



The length and calibre of the semicircular 

 canals vary very much, not only in different spe- 

 cies, but also when compared with each other. 



