XIII 



PHYLUM CHORDATA 



107 



occurring in the dermis at the termination of a sensory nerve : touch- 

 corpuscles (B) are formed of an ovoidal mass of connective-tissue 

 containing a ramified nerve, the terminal branches of which end 

 in touch-cells : Pacinian corpuscles (C) consist of a terminal nerve- 

 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 

 ncuromast-organs or organs of the lateral line. Extending along 

 the sides of the trunk and tail is a longitudinal streak, due to the 

 presence either of an open groove or of a tube sunk in the 



-A 



Fn;. 7 SO.- A, tactile spot from skin of Frog, a, touch-cells ; b, epidermis ; N, nerve. B, tactile 

 corpuscle from dermal papilla of human hand, a, connective-tissue investment ; 6, touch- 

 cells ; n, n', n", n'", nerve. C, Pacinian corpuscle from back of Duck. A, A', neuraxis ; ./A, 

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

 (From Wiedersheim's Vertebrata.) 



epidermis, and continued on to the head in the form of branching 

 grooves or canals (Fig. 781, A). These organs and also certain others 

 in the form of pits or of unbranched canals, are lined with 

 epithelium (B), some of the cells of which are arranged in groups, 

 the ncuromasts, and have the form characteristic of sensory 

 cells (I) 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 of the lateral- 



