SENSE ORGANS OF AMIURUS. 385 
urged into the cavum with full force. Secondly, the fluid is imbedded 
in reticular tissue ; and thirdly, any impulse communicated to the 
transverse ductus will be deadened by the close apposition of the 
saccular nerves. But in Am/wrus the fluid in the atria and cavum 
is not imbedded in the meshes of the reticular tissue, the wall of 
the saceus endolymphaticus is so thin that any motion in the sur- 
‘rounding fluid must disturb its contents, and the currents so pro- 
duced must certainly affect the neuroepithelium as much if not far 
more than the currents produced by ordinary sound waves. I should 
be inclined to look upon the dorsal reservoir which I have described 
above rather as a safety-valve to prevent too great a disturbance of 
the neuroepithelium by the violence of currents produced by sudden 
expansions of the air-bladder. 
It is interesting to consider, in the light of Moreau’s researches!, 
what advantage it is to the fish to be provided with an apparatus 
which records the varying states of distension of the air-bladder de- 
pendent on the greater or less weight of the superincumbent column 
of water. The chief function of the air-bladder, according to Moreau, 
is to enable its possessor to alter its specific gravity so as to be in 
equilibrium in one particular plane where it may remain with little 
or no muscular effort, but from which it can only displace itself ver- 
tically upwards or downwards by muscular effort. 
In Physoclystous fishes (those with no air-duct), this complete ac- 
commodation to a new level takes place slowly, for the volume of air 
in the air-bladder is not altered by muscular contraction but is re- 
duced in amount through absorption and increased in amount through 
excretion by the walls of the bladder, the retia mirabilia there 
being probably the organs engaged in this physiological process. In 
Physostomous fishes, on the other hand, accommodation to a new 
higher level is more quickly effected by the ejection of bubbles of air 
through the air duct, while the additional amount of air necessary to 
produce equilibrium under increased pressure is slowly formed by the 
walls of the air-bladder. The Physostomi are therefore possessed of 
greater freedom of movement than the Physoclysti under artificially 
diminished pressure or ata higher level than that in which they were 
1 Recherches experimentales sur les fonctions de la vessie natatoire. 
Ann. des Sci. Nat. T. 4, 1876. 
It would be extremely interesting to examine the morphological nature of the ‘safety-valve’ 
described by Moreau in Caranz truchurus. 
