34 AVEJ?S. [Vol. VI. 



In the Mammalia at last we find the two chambers in the 

 adult entirely separated so far as the sacculo-utricular canal is 

 concerned. The two chambers are connected indirectly by the 

 splitting of the ductus endolymphaticus, whose proximal portion 

 is involved in the division of the ear chamber and really repre- 

 sents the utriculo-cochlear canal. It should be emphasized that 

 the division of the internal ear is not into upper and lower 

 portions, so far as the primitive auditory vesicle and its sense 

 organs are concerned, but into anterior and posterior portions, 

 the latter of which very soon assumes a ventral position, which 

 is constantly magnified and increased ever after during the de- 

 velopment of the phylum. It is very important to keep this 

 distinction in mind when trying to unravel complications of 

 structure of the mammalian ear which were puzzling enough to 

 gain the name of auditory labyrinth. 



LAGENA (cochlea). 



The lagenar tube of the Torpedo is a short, curved, blind pouch 

 given off from the sacculus. It forms a continuation of this 

 chamber downwards, backwards, and outwards. The evagina- 

 tion is developed from the floor of the chamber, but owing to 

 secondary changes involved in the separation of the lagenar 

 papilla from the saccular or parent sense organ, the pocket 

 many times appears to be an evagination of the lateral walls of 

 the chamber. In the Sting Ray such is the case (Fig. i), while 

 in the ear of Carcharias (Fig. 3) the lagena is a quite shallow 

 depression in the floor of the sacculus near its posterior wall. 

 A cochlea in the restricted sense of the term has not yet made 

 its appearance. 



As already stated, Myxine shows no trace of a lagenar pocket, 

 but Petromyzon has acquired a distinct chamber and sense 

 organ in the manner described, which are homologous with the 

 parts of the lagena in the Elasmobranchs. 



The position occupied by the lagenar opening in the wall of 

 the sacculus in the Petromyxon ear is a peculiarity of the group. 

 The lagena appears to open upward into the common utriculo- 

 saccular chamber, its opening passing through the plane of the 

 utriculo-saccular partition. On the completion of this partition, 

 which is scarcely indicated in Myxine, but which is well grown 



