366 THE ORIGIN OF VERTEBRATES 
not, however, attempted to examine the whole sense-organ for the 
purpose of estimating their number and arrangement. 
As is seen in Fig. 149, they possess a fine tubule of the same 
character as that of the neighbouring sense-organs, which apparently 
terminates at the apex of the projecting spike. They appear to 
belong to the same group as the other poriferous sense-organs, and 
are of special interest, because in their appearance they form a link 
between the latter and the poriferous sense-organs which charac- 
terize the pecten of the scorpion (ef. Fig. 152, C). 
Such, then, is the structure of this remarkable sense-organ of the 
flabellum, as far as I have been able to work it out with the materials 
at my disposal. It is 
evident that the flabellar 
organs, apart from the 
spike-organs, are of the 
same kind as those de- 
scribed by Patten on the 
mandibles and chele of 
Limulus, and therefore it 
is most probable that the 
nerve - terminals in the 
chitinous tubules, and 
the origin of the latter, 
are similar in the two sets 
of organs. 
Fic, 149.—SprKe-OrGAN OF FLABELLUM. These organs, as Patten 
ch.t., chitinous tubule. has described them, are 
situated in lines on the 
spines of the mandibles of the prosomatic locomotor appendages, 
and are grouped closely together to form a compact sense-organ 
on the surface of the inner mandible (Lankester’s epicoxite) (i.m. 
in Fig. 155), so that a surface-view of the organ here gives 
the characteristic appearance of these poriferous sense-patches. 
Precisely similar organs are found on the chilaria, which are, in 
function at all events, simply isolated mandibles, to use Patten’s 
terminology. 
On the digging appendage (ectognath), as the comparison of 
Fig. 155, A and C, shows, the mandibular spines are almost non- 
existent, and the inner mandible or epicoxite is not present, so that 
