126 



GENEEAL PRINCIPLES OF ZOOLOGY. 



equal intensity. Prominent parts, like the tentacles of polyps 

 and of many worms, the antennae of arthropods and the snails, 

 need only mention. Special epithelial cells with stiff hairs pro- 

 jecting above the surface, the tactile bristles or tactile hairs, are 



- 71 



FIG. 77. FIG. 78. 



FIG. 77. Skin of an insect with an ordinary hair (h) and a tactile hair ((); n, nerve; 



*, sensory cell; e, epithelium; c, cuticle. (After vom Rath.) 

 FIG. 78. Vater-Pacinian corpuscle of the mesentery of a cat. a, axis cylinder; /, fat; 



0, blood-vessel; i, inner bulb; /c, capsule with nuclei; n, medullated nerve-fibre. 



tactile (fig. 77). Only in the vertebrates do the nerves of touch 

 terminate in specially modified end organs (Vater-Pacinian cor- 

 puscles, corpuscles of Meissner, etc., fig. 78); these usually lie 

 under the epithelium. 



Organs of Smell and of Taste are accurately known only in 

 vertebrates. The olfactory organ of fishes consists of two small 

 pits in the skin, above or in front of the mouth. 



In the air-breathing vertebrates this pair of pits which here 

 also arise from the skin are taken into the dorsal wall of the two 

 respiratory canals leading from the outside to the pharynx. Now 

 since the olfactory cells distributed in these pits (fig. 37, /?) are 

 frequently characterized by bundles of olfactory hairs, while the 

 surrounding epithelium is often ciliated, one is inclined to regard 

 as organs of smell sensory organs of invertebrates (e.g., medusas, 

 cephalopods), which have the form of ciliated pits and lie near the 

 respiratory apparatus (e.g., the osphradium of molluscs). Yet 

 there are exceptions. Experiments seem to show that in the 

 arthropods the antennae probably serve for smelling. Here the 

 sensory perception can be connected only with certain modified 



