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 LIMULUS. 
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 Ammoccetes, 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 paleostracan 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 Ammoceetes, confined to the mesosomatic 
or branchial region, just asin Ammoceetes, forming, as in Ammoccetes, 
cartilaginous bars supporting the branchie, and these bars are situated 
externally to the branchie, as in Ammoccetes. In addition this 
animal possesses a basi-cranial internal semi-cartilaginous wnseg- 
mented plate known as the entosternite or plastron situated, with 
respect to the cesophagus, similarly to the position of the trabecule 
with respect to the infundibulum in Ammoccetes. 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. 
