200 MARINE BIOLOGICAL LABORATORY. 



is very important, for it introduces us to a series 

 of changes which have led to the production of the 

 gnathostome type of ear. 



The reptile, bird, and mammal ears furnish us with a 

 progressive series in which the parts already present in 

 the Torpedo are carried to successively higher degrees 

 of perfection. The most noticeable change from the 

 Torpedo condition is the much elongated lagenar re- 

 gion, or, as it is called in the mammalia, the cochlea, 

 which by means of the high differentiation of its con- 

 tained sense-organ has risen to first rank among the 

 auditory sense-organs. 



The utricular and saccular regions are relatively 

 much reduced. The semicircular canals have under- 

 gone a similar reduction, and among the birds and mam- 

 mals appear more nearly semicircular or more evenly 

 curved than in either the reptiles or the fishes. 



All of these ears retain traces of the primitive division 

 into anterior and posterior chambers, and the relation 

 of the nerves described for the Cyclostome and the Tor- 

 pedo obtains throughout the group, except, of course, 

 where a given sense-organ has divided, when, as is fre- 

 quently the case among the higher types, two or more 

 nerve branchlets are present where only one existed in 

 the lower type. 



In Fig. 6, which is a figure of the developing human 

 ear seen from the outside, the anterior and external 

 ampullce are intimately related to each other and to the 

 utriculus U, while the posterior is given off from the 

 sacculus. A line passed through the space between U 

 and 5 of the figure, and continued so as to pass out just 

 above the ampulla of the posterior canal, divides the ear 



