No. I.] THE VERTEBRATE EAR. 29 



UTRICULUS. 



The utriculo-sacculus of the Hagfish (PI. IX, Fig. 2 and Cut 

 2) includes all the body of the internal ear, exclusive of the 

 relatively large ampullae {a. and «./.), and represents the little 

 modified remnant of the auditory vesicle after the production 

 of the two ampullar organs and their confluent canals (c.a. and 

 c.p). The utriculo-saccular sense organ (the maculae acusticae 

 utriculi and sacculi) covers a considerable portion of the floor of 

 this chamber, and is drawn out into an oval band ; its division 

 into discreet sense organs is at least indicated by the manner in 

 which the nerve branches are distributed to it. The walls of 

 the chamber, especially in the region of the nerve entrance, on 

 its ventral face, show the beginnings of the groove which be- 

 comes so prominent in the Petromyzon ear, caused by a merid- 

 ianal constriction of its wall corresponding to the internal per- 

 forated partition. This partition in Myxine does not separate 

 the sense organ into two parts. 



Petromyzon (PL IX, Fig. 2 ; PI. X, Fig. 2). 



The Lamprey has developed along the line marked out in 

 Myxine, and as a result the utriculo-saccular cavity is well sep- 

 arated into two distinct chambers communicating by means of 

 the large sub-central perforation of the membranous partition. 

 The partition is incomplete above, where the semicircular canals 

 open into a common chamber, and below, where their ampullae 

 open out onto its floor. This partition has been described as a 

 pair of folds, one of which occupies the inner face of the ex- 

 ternal wall of the ear, and the other of which occupies the 

 inner face of the medial or inner wall. The position of both of 

 these vertical folds is indicated on the exterior by grooves, 

 that of the outer fold being the deepest and serving to mark off 

 very distinctly the anterior chamber or the utriculus from the 

 posterior chamber or the sacculus. 



As the folds approach the bottom of the chamber, they be- 

 come lower and indistinct. The floor of the chamber adjacent 

 to the middle of the posterior wall is depressed, or pushed 

 downwards, inwards, and backwards, forming what I shall desig- 

 nate the lagena. This pocket lies entirely within the territory 

 of the sacculus, although the anterior third of it appears to com- 



