No. I.] THE VERTEBRATE EAR. 33 



and the crista acustica posterior do in the case of the posterior 

 canal. 



Among the forms occurring between the Elasmobranchs and 

 the MammaHa we find almost every conceivable gradation be- 

 tween the simple sacculus of the Cyclostomes and lower Elasmo- 

 branch species and the very complicated and highly differentiated 

 structure known in the Mammalia as the cochlea and its organ 

 of Corti. 



The Hagfish can be said to possess a sacculus in its simplest 

 condition in the first stages of development. The auditory 

 chamber, designated sacculus by Retzius, is clearly the utriculo- 

 sacculus of other forms. The separation being indicated by the 

 very meagre transverse furrow in the bottom of the basal or 

 ventral portion of the ear sac, and by the branches of the 

 auditory nerve distributed to the compound sense organ, which 

 covers more or less completely the floor of the double chamber. 



Petromyzon presents us with an advance on the Myxine con- 

 dition, for here the median raphe has acquired a greater size, 

 but the anterior and posterior halves of the chamber are still 

 in open communication with each other. They have, however, 

 developed pocket-like processes, which represent the first traces 

 of the recessus utriculi and of the legena of the higher forms. 



Alligator. 



In this reptile the sacculus has become distinctly marked off 

 from the utriculus, and is only connected by a much reduced 

 canalis sacculo-utricularis. The avian sacculus communicates 

 even less freely than the reptilian, though within the group 

 there is a constant reduction of the connecting canals progress- 

 ing from the lower to the higher types. In other particulars 

 there is no essential difference between the saccular regions of 

 the two groups. 



The sacculus of the Mocking-bird is a minute chamber placed 

 alongside the utriculus, with which it communicates by means 

 of an opening, all there is to represent the longer canalis utric- 

 ulo-sacculus, so well developed in some of the lower vertebrates, 

 and by means of the canalis sacculo-cochlearis it opens out into 

 the cochlear tube. This canal is a long, curved tube of relatively 

 large dimensions, and is formed by the secondary constriction 

 of the neck of the cochlea. 



