150 



GENERAL PRINCIPLES OF ZOOLOGY. 



conclusions upon anatomy. But the anatomy of many 

 sensory organs, like those of smell and taste, is by no 

 means so characteristic that it alone is sufficient to deter- 

 mine the physiological significance. 



Tactile Organs. The skin of animals functions as a 

 tactile organ, usually over the whole area, although not 

 everywhere with equal intensity. Prominent parts, like 

 the crown of tentacles of polyps and of many won/is, the 

 feelers of arthropods and snails, should 'be taken up as a 



FIG. 74. FIG. 7S . 



FlG. 74. Skin of an insect with an ordinary hair (h) and a tactile hair (/) ; , nerve ; f, sen- 

 sory cell ; e, epithelium; c, cuticle. (After v. Rath.) 



FlG. 75. Tactile corpuscles from the bill of a duck. (From Wiedersheim). A, nerve-fibre 

 with terminal bulb (JK) ; Q, inner, L, covering lamellae ; ZZ, nucleus of the latter ; A 1 , 

 axis cylinder ; MS, medullary sheath ; AY, nerve-sheath. 



special subject. Special epithelial cells serve for touch ; 

 these are provided with stiff hairs projecting above the sur- 

 face, the tactile bristles or tactile hairs (Fig. 74). Only 

 in the vertebrates do the nerves of touch terminate in spe- 

 cially modified end organs (Pacinian corpuscles, corpuscles 

 of Meissner, etc.) (Fig. 75); these usually lie under the 

 epithelium. 



Organs of Smell and of Taste are known with certainty 

 to exist only in vertebrates. The olfactory organ of fishes 

 consists of two small pits in the skin, above and in front of 

 the mouth-opening; here the epithelium is rich in sensory 



