362 



THE ORIGIN OF VERTEBRATES 



of the nerve-fibres turn towards the flat, uppermost side, where the 

 sense-organ is situated. Between the nerve-fibres (n.) and the chi- 

 tinous surface containing the special sense- tubes masses of cells (gl.) 

 are seen, as in Fig. 146, apparently nerve- cells, which form a broad 

 border between the nerve-fibres and the pigmented chitinogenous 





r l 



^ 



V 







-,3o 



b*%. 



=- - h $m> r <*^ 

 ■J » \ %^ L/^"^ 



<=. «"/ 



Fig. 147. — Section parallel to 

 the Surface op Flabellum, 

 showing the porous termi- 

 NATIONS of the Sense-Organs 

 and the Arrangement op the 

 Canaliculi round them. 



ch p bfn g 1 P ch 



Fig. 146. — Section through Flabellum. 



ch., chitiuous layers; s.o., sense-organs; sp., 

 spike-organ; p., pigment layer; gl., ganglion 

 cell layer; bl. and n., blood-spaces and nerves. 



layer (p.). On the opposite side, nothing of the sort intervenes 

 between the pigmented layer and the blood- spaces and nerve-fibres 

 which constitute the central mass of the flabellum. 



At present I am inclined to look upon this mass of cells as 

 constituting a large ganglion, which extends over the whole length 

 and breadth of the upper surface of the flabellum. At the same 



