PHYLUM CHORDATA 



109 



oyer almost the whole surface of the body. In higher Craniates, 

 with the exception of Birds, they are chiefly confined to the epi- 



FiG. 7S3. — A, vertical section of one of the papillas of the tongue of a Mammal, rf, siib- 

 niucosa; e. epithelium ; «. nerve-fibres; t. taste-buds. B, two taste-buds. c. covering cells 

 shown in lower bud ; J, sub-mucosa ; e. epithelium of tongue ; vi, sensory processes ; n. 

 internal sensory cells shown in upper bud. (From Foster and Shore's Physiolofjy.) 



thelium of the tongue and soft palate, and are supplied by the 

 gustatory branches of the trigeminal, facial, and glossopharyngeal. 



The olfactory organ is typically a sac-like invagination of the 

 skin of the snout, anterior to the mouth, and communicating with 

 the exterior by an aperture, the external 

 nostril. It is paired in all Craniata, ex- 

 cept Cyclostomes, in which there is a 

 single olfactory sac, supplied, however, by 

 paired olfactory nerves. The sac is lined 

 by the olfactory mucous membrane or 

 Schneiderian membrane, the epithelium of 

 which contains peculiar, elongated sensory 

 cells (Fig. 783), their free ends often pro- 

 duced into hair-like processes. In the 

 Dipnoi and all higher groups the posterior 

 end of each sac communicates with the 

 cavity of the mouth by an aperture called 

 the posterior nostril, and an analogous 

 communication occurs in the case of the 

 unpaired organ of the Hags (vide infra). 



In many air-breathing Vertebrates there 

 is formed an offshoot from the olfactory 

 organ, which, becoming separated, forms 

 a distinct sac lined with olfactory epi- 

 thelium and opening into the mouth. This 

 is Jacohson's organ : it is supplied by the olfactory and trigeminal 

 nerves. 



The paired eye is a more or less globular structure, lying in 

 the orbit, and covered externally by a thick coat of cartilage or of 



Fig. V83.— Epithelial cells of 

 olfactory mucous membrane. 

 A, of Iiamprey; B, of 

 Salamander. E. inter- 

 stitial cells ; R, olfactory 

 cells. (From Wiedersheim's 

 Vertehrata.) 



