SENSE-ORGANS OF THE INTEGUMENT 193 



off from the rest of the tube by radial folds of connective tissue, 

 and containing the nerve-endings. The tubes are rilled with a 

 gelatinous substance. 



The function of the nerve eminences is doubtful, but it appears 

 that they are concerned with the perception of mechanical stimuli 



FIG. 153. DISTRIBUTION OF THE LATERAL SENSE-ORGANS IN A SALAMANDER 



LARVA. 



from the surrounding water, and thus are important as regards the 

 appreciation of the direction of these stimuli. 



The horny wart-like structures arising periodically during the breeding 

 season in Cypriiioids and known as "pearl organs," are due to a modification 

 of the reduced nerve-eminences. Similar structures occur in Anura. 



1). End-buds. 



The nerve eminences pass through a stage in development in 

 which they clearly resemble end-buds, and the latter may be 

 looked upon as the phyletically older organs, which do not become 

 so highly differentiated as the former. No sharp line of demarca- 

 tion can, however, be drawn between the two, as all kinds of inter- 

 mediate forms are met with : they are here described separately 

 merely for the sake of clearness. 



In contrast to the nerve-eminences, which tend to sink below 

 the surface, the end-buds usually form a dome-like elevation pro- 

 jecting above the general level of the epidermis. A central 

 sensory epithelium, provided with sensory hairs, and peripheral 

 supporting cells can be recognised, but the former are as long as 

 the latter. 



In Lampreys and most Elasmobranchs they remain at a primi- 

 tive stage of development, but become of great importance in 

 Ganoids and Teleosts, in which they are scattered irregularly over 

 the whole body and are particularly numerous in the fins, lip- 

 folds, barbules, and mouth. From the Dipnoi onwards they become 

 limited to the oral and nasal cavities. In Dipnoi and Amphibia 

 they occur on the papillaB of the oral and pharyngeal mucous 

 membrane and tongue. In Reptiles their distribution is somewhat 

 more limited, and this is still further the case in Mammals, in 

 which, however, they are still found on the soft palate, on the walls 

 of the pharynx, and even extend into the larynx ; but here they 

 are most numerous on the tongue, where they occur, situated more 

 deeply, on the circumvallate and fungiform papillae as well as on 

 the papilla foliata, and function as organs of taste. 



o 



