THE EVIDENCE OF 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 trabecule and 
the branchial cartilaginous system, which of itself indicates a position 
for the auditory capsule between the prosomatic trabecule and the 
mesosomatic branchial cartilaginous system. 
The auditory capsule and parachordals when formed are made of 
the same kind of cartilage as the trabecule, i.c. 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 
trabecule, 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 
