191 
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 ampulle 
externe (evt. 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. ¢.)—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 ampulle posterioris 
(post. 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 utricults 
below, but having no connection with it. I’rom 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 lamin (ofe.), 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. 
