538 



CHORD AT A. 



tions connected with aquatic life; hence the idea of a 'sixth 

 sense/ lacking to man (cf. p. 125). The end buds are especially 

 collected in the neighborhood of the mouth, on the lips and bar- 

 bels. Since they also occur in the mucous membrane of the mouth, 

 especially in the palatal regions, they connect with the taste organs. 

 The taste buds have the same structure as the end buds of fishes. 

 They occur in all classes of vertebrates, and are most abundant in 

 man in the walls of the circumvallate papillae at the base of the 

 tongue; in rodents on the large foliate papillae, etc. 



The end buds also lead to the olfactory organs. The olfactory 

 epithelium of many fishes and amphibia is a stratified epithelium 

 with closely arranged end buds (fig. 572), By disappearance of 



FIG. 572. Section of olfactory epithelium of a fish (Belone). (From O. Hertwig, after 

 Blaue.) e, epithelium fc, olfactory buds ; n, nerves. 



the isolating parts of the ordinary epithelium the end buds form 

 a continuous sensory epithelium, which is the rule in most ver- 

 tebrates. 



The olfactory organ, the nose, lined with its sensory epithelium, 

 .acquires a special interest both from its grade of development and 

 from the important systematic distinctions it affords. Except the 

 cyclostomes, which have an unpaired nasal sac, all vertebrates have 

 paired olfactory organs. In adult fishes and in the embryos of 

 higher forms are two pits which lie in front of or dorsal to the 

 mouth; they are either distinct from it or only connected with 

 it by an oronasal groove in the skin (fig. 599). If the animal 

 be terrestrial and replace branchial by pulmonary respiration, a 

 respiratory canal is developed in connexion with the nose. The 

 oronasal groove closes to a tube which begins with an opening 

 (nostril) on the surface and ends with a second opening (choana) 

 in the mouth cavity. The olfactory sac proper is included in the 

 wall of this tube, usually on its dorsal surface (fig. 573). In Am- 



