THE BREATHING APPARATUS, ETC., OF FISH 29 



lobe going to the left eye, and vice versa, but are not united ; 

 but in Palaichthyes — as in the sturgeon — the two nerves cross 

 and are fused together at the point of crossing, just as they are 

 in the higher vertebrates. 



The question whether fish can hear is often debated among 

 anglers as hotly as whether they can distinguish colours. As 

 The sense every kind of fish, except the lowest of all, the 

 o earing, i^j^j^elet {Brachiostoma lanceolatum)* is provided with 

 a special acoustic apparatus, it seems unreasonable to deny 

 that fish possess the faculty of hearing. Water surface is well 

 known to facilitate the transmission of sound horizontally 

 through air, but it remains in doubt in what degree atmo- 

 spheric sound-waves communicate themselves to, and are 

 transmissible within, the water.f At all events, the acoustic 

 apparatus of fishes is not on the same plan as that of terrestrial 

 vertebrates. The ear has no tympanum or drum, neither 

 is there any external orifice to the auditory chamber. In 

 Teleostean fishes there is a labyrinth consisting of a vestibule 

 connected with three semicircular canals, filled with fluid and 

 contained among the cranial bones or within the cranial cavity. 

 On each side of the base of the cranial cavity is placed a sac 

 communicating with the vestibule. Sometimes each sac is 

 divided into two unequal receptacles, each containing a stony 

 concretion termed an otolith^ and in the vestibule is another of 

 these concretions, in contact with the ends of the acoustic nerve. 

 In some species communication between the ear and the outer 

 world is facilitated by openings through the skull, closed 

 externally only by skin or very thin bone. In the perches, 



* A creature with colourless blood and no brain, excluded on that 

 account by Haeckel from the branch {p/iyia) of fishes, and isolated among 

 vertebrates as the branch Acrania, or brainless animals. With less 

 apparent reason Haeckel denied a place among Fishes to the Cyclostomata 

 (lampreys), which he constituted a separate branch, Monorrhina. 



t An instance in point from my own observation will be found on 

 page 56. 



