126 



THE ORIGIN OF VERTEBRATES 



that the primitive cartilaginous skeleton of the vertebrate consisted 

 of the following structures (Fig. 53, B), viz. the branchial bars 



forming a basket-work, the 



A 



PL 



Ent 



\ 



trabecule and parachordals, 

 the auditory and nasal cap- 

 sules — a clear proof that the 

 cranial skeleton is older than 

 the spinal. Of these struc- 

 tures the branchial bars are 

 the only evidently segmented 

 parts. 



The Soft Cartilage of the 

 Branchial Skeleton of 

 Ammoccetes. 



The study of Ammoccetes 

 gives yet another clue to the 

 nature of the earliest skeleton, 

 for these two marked groups 

 of cartilage — the branchial and 

 basi-cranial — are characterized 

 by a difference in structure as 

 well as a difference in topo- 

 graphical position. J. Miiller 

 was the first to point out that 

 these two sets of cartilages 

 differ in appearance and con- 

 stitution, and he gave to them" 

 the name of yellow and grey 

 cartilage. Parker has described 

 them fully under the terms 

 soft and hard cartilage, terms 

 which Schaffer has also used, 

 and I shall also make use of 

 them here. The whole of the 

 branchial cartilaginous skele- 

 ton is composed of soft cartilage, while the basi-cranial skeleton, con- 

 sisting of trabecule, parachordals, and auditory capsule, is composed 



y 



Fig. 53. — Comparison of Cartilaginous 

 Skeleton of Limulus and Ammoccetes. 



A, Diagram of cartilaginons skeleton of 

 Limulus. Soft cartilage, entapophysial liga- 

 ments, deep black ; branchial bars simply 

 hatched; liard cartilage, lateral trabecule 

 of entosternite, netted ; Ph., position of 

 pharynx. 



B, Diagram of cartilaginous skeleton of 

 Ammoccetes. Soft cartilage, sub-chordal 

 cartilaginous bands, deep black ; branchial 

 basket-work (first formed part), simply 

 hatched ; hard cartilage, cranio-facial skele- 

 ton, trabecule, parachordals and auditory 

 capsules, netted; Inf., position of tube of 

 infundibulum (old oesophagus). 



