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 Lase 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 ectocmath, 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 Bohon'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 Ammoccetes. 



The method by which such a sense-organ, situated externally on 



