SEA-FISHERIES LABORATORY. 335 



Canalis externus (e. s. c). — Sometimes called the 

 horizontal semicircular canal to distinguish it from the 

 other two vertical canals. Swells into the ampulla 

 externa (amp. e.) immediately behind its connection with 

 the utriculus in front. The crista acustica ampulla 

 externae (ext. cr.) is situated on the floor of the ampulla 

 behind, but in front it ascends upwards and outwards, so 

 as to invade its external wall. The canal passes hori- 

 zontally downwards and backwards, external to the rest of 

 the ear, to communicate with the utriculus behind. 



Canalis posterior (p. s. c). — The large ampulla pos- 

 terior (amp. p.) occupies a similar position to that of the 

 anterior canal. Its outer wall contains a transverse sense 

 organ and ridge, the crista acustica ampullae posterioris 

 (p>ost. cr.). Above, the canal passes upwards and forwards 

 into the vertical utricular chamber. 



The Sacculus (sac.) appears at first to be an absolutely 

 closed bag in the Plaice, closely opposed to the utriculus 

 below, but having no connection with it. From its inner 

 wall it sends upwards, however, a blind finger shaped 

 process which must represent a vestigial ductus endo- 

 lymphaticus (d. end.). There is no saccus endolymphati- 

 cus. Into the base of the ductus there open two very 

 minute capillary tubes (see figure), and as each arises 

 from, and communicates with, the utriculus, they must 

 together represent the canalis utriculo-saccularis. There 

 is thus only a very slight communication between the 

 sacculus and utriculus, and this of a very curious nature. 



The sacculus contains the large hard saccular otolith 

 deposited in concentric laminae (oto.), and often called the 

 otolith.* There is, however, a small but quite conspicuous 



* See Reibisch, Wiss. Meeresuntersuch, Abth. Kiel, N.F., Bd. iv., p. 

 233 for an interesting discussion of the relation of the otolith to the age of 

 the fish. Reibisch finds that it is possible to deduce the age of a Plaice 

 from the conformations of the otolith. 



