37o 



THE ORIGIN OF VERTEBRATES 



by Graber, is the bulging of the porous canal near its termination 

 (Fig. 150, C). This bulging is filled with a homogeneous, highly 

 refractive material, from which, according to Lowne, a chordotonal 

 thread passes, to be connected with a ganglion-cell and nerve. 

 This sphere of refractive material he calls the ' capitellum ' of the 

 chordotonal thread. The presence of this material produces in a 

 surface view an appearance as of a halo around the terminal placpie 

 with its central pore ; Graber has attempted to represent this by the 

 white area round the central area (in Fig. 150, B). A very similar 

 appearance is presented by the surface view of the flabellum in 

 those parts where the tube runs straight to the surface, so that the 



5.0 



i \ 



B 



Fig. 150 (from Graber). — A, Section op Subcostal Nervure op Hind Wing op 

 Dytiscus to show patch op Poriferous Organs (s.o.). B, Surface View op 

 Poriferous Organs ; the White Space round each Organ indicates the 

 deeper lying Refringent Body which fills the bulging of the Canal 

 seen in Transverse Section in C. 



refractive material which fills the oval bulging shines through the 

 overlying chitin and appears to surround the terminal placpue with a 

 translucent halo. 



Such a peculiarity must have a very definite meaning, and sug- 

 gests that the canals in the flabellum of Limulus and in the hind 

 wings of insects belong to the same class of organ, the chitinous 

 tubule with its nerve-terminal in the former corresponding to the 

 chordotonal thread in the latter. One wonders whether this sphere 

 of refractive material or ' capitellum ' (to use Lowne's phraseology) 

 is so universally present in order to act as a damper upon the 

 vibrations of the chordotonal thread in the one case and of the 



