354 AMPHIBIA xn. 27- 



ossicle, the operculum, working in parallel, with its inner end in the 

 oval window caudal to the columella and its outer end attached by a 

 muscle to the scapula. In terrestrial forms the columella becomes fused 

 with the window at metamorphosis and its function is taken over by 

 the operculum, probably receiving vibrations from the fore-legs. The 

 more aquatic forms (Cryptobranchus) retain the larval condition and 

 never develop an operculum. The tympanum and columella are also 

 reduced in some terrestrial anurans {Bombinator) but in the aquatic 

 Xenopus and Pipa the operculum and its muscle are lost, perhaps a 

 paedomorphic feature. 



The sense of hearing is certainly well developed, especially in 

 Anura, which respond to vibrations from 50 to 10,000 a second. The 

 hearing is used especially in the breeding-season, when the croaking 

 serves to attract both sexes to the water. The prey may also be located 

 by sound. Urodeles have been shown to give no response to the ring- 

 ing of a bell suspended from the ceiling, which, however, produces 

 reactions in Rana and Bufo. 



A peculiar feature of many Anura is an immense backward develop- 

 ment of the perilymphatic space of the inner ear, forming a sac 

 extending above the brain and on either side of the spinal cord as far 

 back as the sacrum. Portions of this sac emerge between the vertebrae, 

 showing as whitish masses on account of the granules of chalk they 

 contain. The calcium salts in these sacs diminish greatly during 

 metamorphosis and they then refill. The system may serve as a 

 calcium reserve also for the adult. 



28. Behaviour of Amphibia 



The habits of amphibia, like their special structures, enable them 

 to deal with the various emergencies that threaten the continuation of 

 life on land. Frogs and toads have a strong sense of place and they 

 show distinct 'homing' reactions. They are able to learn to find their 

 way out of mazes and to remember the way for periods of at least 

 thirty days. 



Complex migrations are made by many species; nearly all migrate 

 to the water in spring. In this migration the males usually precede 

 the females, then attract the latter by their calling. The receptors for 

 the orientation towards the water are known in the osmoreceptors in 

 the mouth of the frog. This orientation is particularly clear in uro- 

 deles, in which sound plays no part in the migration. The power to 

 find water is obviously of first importance for any animal living on 

 land, and further study of the receptors involved would be interesting. 



