AUDITORY ORGAN. 199 



St, Fov). In several Urodeles (e.g. Cryptobranchus, Menopoma, 

 Spelerpes) the stapedial plate becomes elongated by the addition 

 of a distal element, and thus forms a " columella." A tympanic 

 cavity, with a tympanic membrane lying on a level with the skin, 

 and a Eustachian tube opening into the pharynx, are met with first 

 in the Auura, in which also the columella is more perfect, con- 

 sisting of a bony and cartilaginous chain, expanded distally to fit 

 against the tympanic membrane. The whole columella probably 

 corresponds to the upper element of the hyoid arch (pharyngo- 

 hyal) : the fact that it is sometimes continuous with the wall of 

 the auditory capsule at an early stage may probably be explained 

 as a secondary modification. 



Reptiles and Birds. In the Chelonia, the auditory organ 

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

 Keptiles and Birds, the chief modifications are confined to the 

 cochlea, which shows gradually a higher condition of development 

 as we pass from Chelonians and Snakes to Lizards and Crocodiles. 

 In the Chelonia, where, as already mentioned, the auditory organ 

 remains in a lower stage of development, the cochlea grows out in 

 the form of a short canal (ductus cochlearis, lagena) ; in Croco- 

 diles and Birds this canal is considerably longer, and at the 

 same time it becomes slightly coiled (Figs. 163 165). A more 

 marked differentiation also gradually takes place in the mem- 

 brana basilaris and the papilla acustica basilaris. Both 

 become more and more elongated, and, at the same time, distinct 

 indications of a scala tympani and vestibuli are seen. (Compare 

 the description of these parts on p. 204.) 



In the Lacertilia the most varied types of auditory organ are met with ; 

 in many (Phrynosoma, Pseudopus, Anguis), the membrana basilaris is hardly 

 more highly developed than in Ophidia. Tn Iguana, an advance towards 

 Lacerta and the other higher Lizards is to be noticed ; the membrana 

 basilaris is longer, though the lagena with its papilla is not so much marked. 

 In Acantias and Platydactylus this state of things is carried still further, 

 and finally the more highly developed auditory organ of Plestiodon.and 

 Egerina leads up to that of Crocodilia. Thus there is a continuous 

 and unbroken series from the lower forms to the higher. 



The structure of the auditory organ of Hatteria shows many striking 

 peculiarities : it thus, like that ofChamaeleo, occupies an isolated position. 



Whilst the cochlea gradually becomes more independent of the 

 sacculus, the latter shows the greatest variety both as to form 

 and size in the different types (Figs. 163, 164, S). Thus, for 

 instance, it is usually very small in Birds, while in Lizards (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 the latter may be drawn out to form a canal (canalis 

 reunions), and this is particularly the case in Birds (Fig. 165) ; 



