1908.] Structure and Relationships of the Labyrinth. 523 
by which it does so is illustrated by a transition stage which may be seen 
in the case of the echidna, and which has hitherto escaped recognition, 
Though it has not hitherto been suspected, the echidna possesses a definite 
perilymph recess which communicates with the scala tympani of the cochlea 
by an oval opening corresponding exactly to that which exists in the same 
position in birds and in some reptiles, eg., the alligator. The perilymph 
recess of the echidna is not so large as in the bird or reptile, and the neck 
which unites it to the cochlea is not relatively quite so constricted as in 
these divisions of the vertebrata. Its reduction in size is owing to the fact 
that it has become a shallower pouch, the sides of the sack having become 
gathered up as it were. Thus the sack assumes the shape, roughly speaking, 
of an egg with its long axis horizontal, and that portion of the aqueduct of 
the perilymph which unites the perilymph recess with the cranial cavity 
in birds and reptiles tapers off from the posterior end of the perilymph 
recess in the echidna. Further, this portion of the aqueduct becomes a 
much longer tube owing to the deposition of bone round about it. 
By the same process the round window, which in reptiles and _ birds 
occupies only the lower portion of the outer wall of the perilymph recess, 
becomes raised up in the echidna and occupies the whole breadth of the 
outer wall of that cavity. It is interesting to note, moreover, that the round 
window in the echidna still retains the oval shape as seen in the reptile and 
the bird. Further, by the same process of raising of the round window, the 
anterior margin of the latter comes almost into contact with the wall of the. 
cochlea at the outer margin of the oval opening between the cochlea and 
the perilymph recess. 
The next step is seen in the kangaroo and wallaby. The oval opening 
between the scala tympani of the cochlea and the perilymph recess has. 
become much wider, so much so indeed, that the floor of the cochlea sweeps. 
gradually round to join the walls of the perilymph recess. Thus the latter 
cavity now forms a cone-shaped bulging on the floor of the scala tympani, 
there being little or no distinction as to the line at which the cochlea ends 
and the perilymph recess begins. The cone-shaped bulging, which is all that 
is now left of the latter cavity, has, of course, its base in the cochlea wall, 
while its apex tapers off into the aqueduct of the perilymph, passing inwards. 
and backwards to open into the cranial cavity.* 
By this process of the merging of the perilymph recess into the floor of the. 
scala tympani, it is clear that the round window may be said now to open 
into the scala tympani, for the bulging on the floor of the latter is now all that. 
is left of the perilymph recess. 
* Gray, op. cit., vol. 2, p. 55. 
