136 THE ORIGIN OF VERTEBRATES 



material around the invading chondroblasts, and that the difference 

 in the resulting cartilage is mainly due to the difference in chemical 

 composition of the matrix of the connective tissue which is invaded. 

 Thus the difference may be formulated as follows : — 



The hard cartilage is formed by the invasion of chondroblasts 

 into a fibrous tissue, which contains a gelatinous rather than a mucoid 

 substratum, in contradistinction to the soft cartilage which is formed, 

 probably also by the invasion of chondroblasts, in a tissue — the 

 muco-cartilage — which contains a specially mucoid substratum. 



Such, then, is the very clearly defined starting-point of the ver- 

 tebrate skeleton — two distinct formations of different histological 

 and chemical structure,— the one forming a segmented branchial 

 skeleton, the other a non-segmented basi-cranial skeleton. 



The Cartilaginous Skeleton of Limultjs. 



Among the whole of the invertebrates at present living on the 

 earth, is there any sign of an internal cartilaginous skeleton that 

 will give a direct clue to the origin of the primitive vertebrate 

 skeleton ? The answer to this question is most significant : only 

 one animal among all those at present known possesses a cartilaginous 

 skeleton, which is directly comparable with that of Ammocoetes, and 

 here the comparison is very close — only one animal among the 

 thousands of living invertebrate forms, and that animal is the only 

 representative still surviving of the palseostracan group, which was 

 the dominant race when the vertebrate first made its appearance. 

 The Limulus, or king-crab, possesses a segmented branchial internal 

 cartilaginous skeleton (Fig. 53, A), made up of the same kind of cartilage 

 as the branchial skeleton of Ammocoetes, confined to the mesosomatic 

 or branchial region, just as in Ammocoetes, forming, as in Ammoccetes, 

 cartilaginous bars supporting the branchiae, and these bars are situated 

 externally to the branchiae, as in Ammocoetes. In addition this 

 animal possesses a basi-cranial internal semi-cartilaginous unseg- 

 mented plate known as the entosternite or plastron situated, with 

 respect to the oesophagus, similarly to the position of the trabecular 

 with respect to the infundibulum in Ammocoetes. Moreover, the 

 cartilaginous cells in this tissue differ from those in the branchial 

 region, in precisely the same manner as the hard cartilage differs from 

 the soft in Ammoccetes. 



