PHYLUM CHORDATA 



101 



branch surrounded by a complex laminated sheath. Touch- 

 corpuscles and Pacinian bodies are found only in the higher 

 forms. 



In Fishes, characteristic sense-organs are present, known as the 

 organs of the lateral line. Extending along the sides of the trunk 



B 



FIG. 735. A, tactile spot from skin of Frog, a, touch-cells ; l>, epidermis ; N, nerve. B, tactile 

 corpuscle from dermal papilla of human hand, a, connective-tissue investment ; b, touch- 

 cells ; ,>, ??,', n", ri", nerve. C, Pacinian corpuscle from beak of Duck. A, A', neuraxis ; JK, 

 central knob and surrounding cells; L,Q, investing layers ; AS, medullary sheath of nerve. 

 (From Wiedersheim's Vertebrata.) 



and tail is a longitudinal streak, due to the presence either of an 

 open groove or of a tube sunk in the epidermis, and continued on 

 to the head in the form of branching grooves or canals (Fig. 736, A). 

 The organs are lined with epithelium, (B), some of the cells of 

 which (b) have the rod-like form characteristic of sensory cells, and 

 are produced at their free ends into hair-like processes (c) : they 

 are innervated by the lateral branch of the vagus, and, in the 

 head, by the seventh and sometimes also the ninth nerve. At 

 their first appearance in the embryo the organs of the lateral line 

 are distinct, segmentally-arranged patches of sensory epithelium in 

 intimate connection with the ganglia of the third, fifth, seventh, 

 ninth, and tenth nerves. Cutaneous sense-organs, having at first 

 a metameric arrangement, also occur in the aquatic Amphibia. 



The sense of taste is lodged in the tongue, the epithelium of 

 which contains end-buds (Fig. 737) similar to those of the skin and 

 supplied by the gustatory branches of the trigemirial and glosso- 

 pharyngeal. 



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

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



