THE EVIDENCE OE THE AUDITORY APPARATUS 377 



The Origin of the Parachordals and Auditory Cartilaginous 



Capsule. 



In addition to what I have already said, there is another reason 

 why a special sense-organ such as the pecten is suggestive of the 

 origin of the vertebrate auditory organ, in that such a suggestion 

 gives a clue to the possible origin of the parachordals and auditory 

 cartilaginous capsules. 



In the lower vertebrates the auditory organ is characterized by 

 being surrounded with a cartilaginous capsule which springs from 

 a special part of the axial cartilaginous skeleton on each side, known 

 as the pair of parachordals. The latter, in Ammoccetes, form a 

 pair of cartilaginous bars, which unite the trabecular bars with the 

 branchial cartilaginous basket-work. They are recognized throughout 

 the Vertebrata as distinct from the trabecular bars, thus forming 

 a separate paired cartilaginous element between the trabecular and 

 the branchial cartilaginous system, which of itself indicates a position 

 for the auditory capsule between the prosomatic trabecular and the 

 mesosomatic branchial cartilaginous system. 



The auditory capsule and parachordals when formed are made of 

 the same kind of cartilage as the trabecular, i.e. of hard cartilage, and 

 are therefore formed from a gelatin-containing tissue, and not from 

 muco-cartilage. Judging from the origin already ascribed to the 

 trabecular, viz. their formation from the great prosomatic entochon- 

 drite or plastron, this would indicate that a second entochondrite 

 existed in the ancestor of the vertebrate in the region of the junction 

 of the prosoma and mesosoma, which was especially connected with 

 the sense-organ to which the auditory organ owes its origin. This 

 pair of entochondrites becoming cartilaginous would give origin to 

 the parachordals, and subsequently to the auditory capsules, their 

 position being such that the nerve to the operculum would be 

 surrounded at its origin by the growth of cartilage. 



On this line of argument it is very significant to find that 

 the scorpions do possess a second pair of entochondrites, viz. the 

 supra-pectinal entochondrites, situated between the nerve-cord and 

 the pectens, so that if the ancestor of the Cephalaspid was sufficiently 

 scorpion-like to have possessed a second pair of entochondrites and 

 at the same time a pair of special sense-organs of the nature either of 



