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84 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 niesosoniatic 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 

 Ammoccetes. could easily be conceived as remaining - at the surface, and so giving 

 rise to the lateral line org'ans. 



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 tills 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 till up the encephalic region of Limidus. 



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. 



