370 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 plaque 
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 
Fic. 150 (from GraBeR).—A, Section or SuscostaL NeRVURE oF Hinp WING oF 
Dytiscus TO SHOW PATCH OF PoRIFEROUS ORGANS (s.0.). B, SURFACE VIEW oF 
PoRIFEROUS ORGANS; THE WHITE SPACE ROUND EACH ORGAN INDICATES THE 
DEEPER LYING REFRINGENT Bopy 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 plaque 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 
