171 



CHORDATA. 



branches ending in tactile discs. Though long known among terrestrial 

 vertebrates, these have recently been found in fishes. 



The club-shaped corpuscles (also known as Vater-Pacinian corpuscles, 

 fig. 81) are allied to the tactile corpuscles by form and mesodermal 

 position, although they differ materially in structure and are not easily 

 to be regarded as ectodermal in origin, since they occur not only in the 

 skin but in deeper structures like the mesentery of the cat. The latter 

 position renders their function problematical. 



While in the terrestrial vertebrates typical sensory epithelium is lack- 

 ing, it attains a high development in the skin of fishes. The dermal 

 nerves pass into the epidermis and end in oval corpuscles, which, while 

 imbedded in a stratified epithelium, consist of a single layer of sense and 

 supporting cells. According to structure, nerve hillocks (neuromasts} and 

 nerve-end buds are distinguished. The first are the specific organs of the 



? I 



feflk^' 



V "-" 



J-VW '.--'*> 



S 

 R, 



% 



3I 



'^ 



7.V 



&1^' 



Rj 



. 



FIG. 529. 



FIG. 530. 



FIG. 529. Section of olfactory epithelium of a fish Belone (from O. Hertwig, after 

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



FIG. 530. Diagram of nose of lizard (after Wiedersheim). AN, outer nasal 

 cavity: C, olfactory sac; Ca, Stenson's duct; Ch, choana; IN, inner nasal cavity; MS, 

 roof of mouth; P, Jacobson's organ; |, connexion between nasal cavities. 



lateral line, to be mentioned later, of fishes and branchiate amphibians 

 and amphibian larva-, and therefore appear to subserve special and im- 

 portant sensations connected with aquatic life. 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 

 tin- circumvallate papilla? at the base of the tongue; in rodents on the large 

 foliate papilkc, etc. 



Whether the end buds are related to the olfactory organs is questionable. 



