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 Amiurus the fluid in the atria and camim 

 is not imbedded in the meshes of the reticular tissue, the wall of 

 the saccus 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 1 , 

 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 Physostoiai are therefore possessed of 

 greater freedom of movement than the Physoclysti under artificially 

 diminished pressure or at a higher level than that in which they were 



1 Recherehes experimentaies sur les fouctious de la vessie natatoire. 

 Ann. des Soi. Nat. T. 4, 1876. 

 It would be extremely interesting to examine the morphological nature of the ' safety- valve ' 

 described by Moreau in Caraa.o traahums. 



