SENSE ORGANS. 323 



crocodiles and Sphenodon, and slightly bent, thus resembling 

 that in birds and foreshadowing the spiral cochlea of mammals. 

 In many reptiles the ductus endolymphaticus (aqueductus 

 vestibuli) ends in the cranial cavity just beneath the skull roof, 

 and in the Ascalabota it extends from the skull cavity into the 

 neck, swelling into a large lobed sack in the neighbourhood of 

 the shoulder girdle. It is rilled with a soft otolithic mass. 



The patches of sensory epithelium in the membranous labyrinth, where 

 the branches of the auditory nerves end, are as follows : (1) in each ampulla 

 there is a projection of sensory epithelium known as the crista acustica ; 

 (2) there is a patch in the saccule and utricle known as the macula 

 acustica of the saccule and utricle respectively ; (3) on the floor of the 

 utricle there is an additional patch, the macula acustica neglecta ; (4) in the 

 cochlear there are two patches, the papilla acustica basilaris and the 

 papilla acustica lagenae. In the Chelonia and Ophidia the cochlear process 

 is hardly differentiated into a pars basilaris next the utricle and a peripheral 

 lagena, but in the Crocodilia the pars basilaris is well developed and forms 

 the main part of the ductus cochlearis, the lagena being merely a terminal 

 end-sac. The papilla acustica basilaris which is contained in the pars 

 basilaris is not differentiated into the organ of Corti, but in crocodiles the 

 membrane on which it is placed is called the membrana basilaris and there 

 is an indication of the scala vestibuli, scala tympani and membrane of 

 Reissner. * 



A tympanic cavity, membrane and eustachian tube are present 

 except in snakes and apodal lizards. There is a fenestra rotunda 

 as well as a fenestra ovalis, and the columella auris passes from 

 the latter to the tympanic membrane (for details see account of 

 orders). A cutaneous fold above the tympanic membrane of 

 crocodiles represents the first trace of an external ear. 



The olfactory organ presents, particularly in the Chelonia 

 and Crocodilia, a considerable augmentation of the surface of 

 the mucous membrane, the folds of which are supported by the 

 single cartilaginous turbinal. The lacrymal duct usually opens 

 beneath the turbinal, but it may open into the posterior nares 

 (Ophidia) or into the pharynx (Ascalabota). 



Jacobson's organs are absent in Crocodilia and Chelonia. 

 In Lacertilia and Ophidia they are present between the nasal 

 sacs and the roof of the mouth (between the turbinals and 

 vomer) as a pair of sacs 'lined by olfactory epithelium and open- 

 ing into the mouth just in front of the choanae. They develop 

 as outgrowths of the nasal sacs, are innervated by the olfactory 



* G. Retzius. Das Gehorogan d. Wirbelthiere, vols. 1 and 2, Stockholm, 

 1881 and 1884. 



