384 THE ORIGIN OF VERTEBRATES 
other air-breathing arthropods. Many of these organs, such as the lyriform 
organs of arachnids, and the ‘halteres’ or balancers of the Diptera, are usually 
regarded as auditory and equilibration organs. 
On all the mesosomatic appendages of Limulus very remarkable sense-organs 
are found, apparently for estimating pressures, which, when the appendages 
sank into the body to form with their basal parts the branchial diaphragms of 
Ammoceetes, could easily be conceived as remaining at the surface, and so giving 
rise to the lateral line organs. 
Further confirmation of the view that an organ, such as the flabellum, must 
be looked upon as the originator of the vertebrate auditory organ, is afforded by 
the extraordinary coincidence that in Limulus a diverticulum of the generative 
and hepatic mass accompanies the flabellar nerve into the basal part of the digging 
appendage, while in Ammoccetes, accompanying the auditory nerve into the 
auditory capsule, there is seen a mass of cells belonging to that peculiar tissue 
which fills up the space between the brain and the cranial walls, and has already, 
on other grounds, been homologized with the generative and hepatic masses 
which fill up the encephalic region of Limulus. 
For all these reasons special sense-organs, such as are found in the flabellum 
of Limulus and in the pectens of scorpions, may be looked upon as giving 
origin to the vertebrate auditory apparatus. In such case it is highly probable 
that the parachordals, with the auditory capsules attached, arose from a second 
entochondrite of the same nature as the plastron; a probability which is 
increased by the fact that the scorpion does possess a second entochondrite, 
which, owing to its special relations to the pecten, is known as the supra-pectinal 
entochondrite. 
