AUDITORY ORGAN 299 



ring of cartilage lying on the level of the skin, and a Eustachian 

 tube opening into the pharynx, are met with in most Anura, 1 in 

 which also the columella is more perfect, consisting of a bony and 

 cartilaginous rod expanded distally to fit against the tympanic 

 membrane. The columella is wanting in certain Urodeles (e.g. 

 Triton). A membranous fenestra coclitew (rotunda) in the outer 

 wall of the auditory capsule is present in many Amphibians and 

 in all higher Vertebrates in addition to the fenestra vestibuli. The 

 ear of the Gymnophiona essentially resembles that of the Urodela. 



Reptiles and Birds. In Chelonians, the auditory organ 

 shows many points of resemblance to that of Urodeles ; and in all 

 the Sauropsida the chief modifications are confined to the cochlea, 

 which gradually shows a higher condition of development in passing 

 from Chelonians and Snakes to Lizards and Crocodiles. 



In Chelonians the lagena and pars basilaris are almost com- 

 pletely united in the cochlea,' 2 and, in contrast to the Anura, they 

 now communicate with the sacculus by a single aperture. The 

 cochlea grows out in the form of a short canal (" ductus cochlearis)," 

 which in Crocodiles and Birds is considerably longer and slightly 

 coiled (Figs. 219 and 220). A more marked differentiation also 

 gradually takes place in the membrana basilaris and the papilla 

 acustica basilaris (Fig. 218, B). Both become more and more 

 elongated, and, at the same time, distinct indications of scales 

 tympani and vestibuli are seen (cf. Fig. 223). In the Lacertilia the 

 most varied types of auditory organ are met with, gradually leading 

 up to that seen in the Crocodilia. Thus there is a continuous and 

 an unbroken series from the lower forms to the higher. 3 



Thus the cochlea gradually becomes more independent of the 

 sacculus, which shows the greatest variety both as to form and size 

 in the different types. It is usually very small in Birds, for in- 

 stance, while in Lizards (e.g. Lacerta) it is of considerable size. 

 The aperture of communication between the utriculus and sacculus 

 persists, though it gradually becomes narrowed, as does also that 

 between the sacculus and cochlea. The connection between these 

 latter may be drawn out to form a canal (canalis re-uniens), and this 

 is particularly the case in Birds ; in Crocodiles an intermediate 

 condition between Birds and Lizards is seen. The membranous 

 labyrinth of Birds, however, is always specially characterised by the 

 peculiar arrangement of the anterior and posterior canals, which 

 are greatly arched and the position of their openings into the sinus 

 superior (canal commissure) is reversed. In lower types (Swimming 

 Birds) this peculiarity is less marked than in the higher forms. 



1 Except, e.g. the Pelobaticlfe. 



2 At the same time even here a posterior portion (pars basilaris) can be 

 plainly distinguished from an anterior portion (lagena) by a ridge extending into 

 the lumen (e.g. Lacerta). 



'The structure of the auditory organ of Hatteria shows many striking 

 peculiarities : like that of Chamteleo, it occupies an isolated position. 



