382 THE ORIGIN OF VERTEBRATES 
or sensory knobs, to use Patten’s description. The middle one of 
these three sclerites enlarges greatly in the digging appendage, and 
grows over the coxopodite to form the base from which the flabellum 
springs. Thus, as they have pointed out, the flabellum does not 
belong to the coxopodite of the appendage, but to the middle sensory 
knob of the entocoxite. Upon opening the prosomatic carapace, 
it is seen that the cephalic generative and hepatic masses press 
closely against the internal surface of the prosomatic carapace and 
also of the entocoxite, so that any enlargement of one of the sensory 
knobs of the entocoxite would necessarily be filled with a protrusion 
of the generative and hepatic masses, This is the reason why the 
generative and hepatic material apparently passes into the basal 
segment of the ectognath, and not into that of the endognaths; it 
does not really pass into the coxopodite of the appendage, but into 
an enlarged portion of the entocoxite, which can hardly be considered 
as truly belonging to the appendage. Kishinouye has stated that 
a knob arises in the embryo at the base of each of the prosomatic 
locomotor appendages, but that this knob develops only in the last 
or digging appendage (ectognath) forming the flabellum. Doubtless 
the median sclerites of the entocoxites of the endognaths represent 
Kishinouye’s undeveloped knobs. 
I conclude, therefore, that the flabellum, together with its basal ~ 
part, is an adjunct to the appendage rather than a part of it, and 
might, therefore, easily remain as a separate and well-developed 
entity, even although the appendage itself dwindled down to a 
mere tentacle. 
The evidence appears to me very strong that the flabellum of 
Limulus and the pecten of scorpions are the most likely organs to 
give a clue to the origin of the auditory apparatus of vertebrates. 
At present both the Eurypterids and Cephalaspids have left us in the 
lurch ; in the former there is no sign of either flabellum or pecten ; 
in the latter, no sign of any auditory capsule beyond Rohon’s dis- 
covery of two small apertures situated dorsally on each side of the 
middle line in Tremataspis, which he considers to be the termination 
of the ductus endolymphaticus on each side. In both cases it is 
probable, one might almost say certain, that any such special 
sense-organ, if present, was not situated externally, but was sunk 
below the surface as in Ammoceetes. 
The method by which such a sense-organ, situated externally on 
