226 COMPARATIVE ANATOMY 



In Chimseroids, Ganoids, Teleosts and Dipnoans, the auditory 

 capsules are not completely surrounded by cartilage or bone, the 

 perilymphatic and cranial cavities only being separated by a fibrous 

 partition. 



In certain Teleosts (Siluroidei, Gymnotidse, Characinidre, Gymnarchidra, 

 Cyprinoidae) the auditory organ comes into relation with the air-bladder by 

 means of a chain of bones (" Weberian ossicles") derived from certain parts 

 of the four anterior vertebrae and corresponding pairs of ribs, and by this 

 means the relative fulness of the air-bladder can be appreciated by the Fish. 

 Connections between processes of the air-bladder and the internal ear are 

 also met with in several other Teleosts. 



The auditory organ of. the Dipnoi most nearly resembles that 

 of Elasmobranchii, and more particularly that of Chimsera. 



Amphibia. The membranous labyrinth of Amphibians re- 

 sembles that of Fishes and Dipnoans in many respects, but impor- 

 tant differences are seen, more particularly as regards the lagena, 

 which, especially in the Anura, becomes further constricted off 

 from the sacculus and reaches a higher stage of development. 

 Traces of a papilla acustica lagenye lying within the lagena are met 

 with in the Myctodera, and even in Menopoma and Siredon. In 

 the Anura (Fig. 181) a higher condition is seen in the presence of 

 a small ridge-like outgrowth in the interior of the thickened lagena 

 on which a definite region, supported by cartilage, corresponds to 

 the basilar membrane of higher types ; this bears another patch of 

 nerve endings the papilla acustica basilaris. 



The ductus endolymphaticus, as in certain Teleosts, may give 

 rise to large sac-like enlargements containing calcareous matter 

 and lie close to its fellow, either on the dorsal surface only, or 

 on both dorsal and ventral sides of the brain. The latter is the 

 case in Anura, for instance, in which the sac extends as an unpaired 

 structure along the whole vertebral canal dorsally to the spinal cord, 

 giving rise to paired outgrowths extending through the inter- 

 vertebral foramina and forming the characteristic calcareous bodies 

 situated close to the spinal ganglia. These are lined by pavement 

 epithelium and are plentifully supplied with capillaries : they are 

 riot glandular, as was formerly supposed. 



A further advance in structure as compared with Fishes is seen 

 in the gradual differentiation of a middle ear. In the outer wall 

 of the auditory capsule is a membranous space, the fenestra ovalis, 

 which is plugged by a cartilaginous stapcdial plate ; and from the 

 latter a rod-like cartilage or bone, the columella, usually extends 

 outwards towards the quadrate (p. 84). A tympanic cavity, with 

 a tympanic membrane supported by a ring of cartilage lying on 

 the level of the skin, and a HJustackian tube opening into the 

 pharynx and corresponding phylogenetically to the hyoid cleft 

 of Fishes, are met with in most Anura, in which also the colu- 

 mella is more perfect, consisting of a bony and cartilaginous rod 



