142 THE ORIGIN OF VERTEBRATES 
aud cartilages. In this figure the animal is supposed to be slit open 
along the ventral mid-line and the central nervous system exposed. 
THE PROsOoMATIC SKELETON OF LIMULUS, COMPOSED OF HARD 
CARTILAGE. 
The rest of the primitive vertebrate skeleton arose in the proso- 
matic region, and formed a support for the base of the brain. This 
skeleton was composed of hard cartilage, and arose in white fibrous 
tissue containing gelatin rather than mucin. 
Is there, then, any peculiar tissue of a cartilaginous nature in 
Limulus and its allies, situated in the prosomatic region, which is 
entirely separate from the branchial cartilaginous skeleton, which 
acts as a supporting internal framework, and contains a gelatinous 
rather than a mucoid substratum ? 
It is a striking fact, common to the whole of the group of animals 
to which our inquiries, deduced from the consideration of the structure 
of Ammoceetes, have, in every case, led usin our search for the verte- 
brate ancestor, that they do possess a remarkable internal semi-carti- 
laginous skeleton in the prosomatic region,called the entosternite or 
plastron, which gives support to a large number of the muscles of 
that region,; which is entirely independent of the branchial skeleton, 
and differs markedly in its chemical reactions from that cartilage, in 
that it contains a gelatinous rather than a mucoid substratum. 
In Limulus it is a large, tough, median plate, fibrous in character, 
in which are situated rows and nests of cartilage-cells. The same 
structure is seen in the plastron of Hypoctonus, of Thelyphonus, 
and to a certainty in all the members of the scorpion group. Very 
different is the behaviour of this tissue to staining from that of the 
branchial region. No part of the plastron stains purple with 
thionin; it hardly stains at all, or gives only a very slight blue 
colour. In its chemical composition there is a marked preponder- 
ance of gelatin with only a slight amount of a mucin-body. In 
some cases, as in Hypoctonus (Fig. 57, B) and Mygale, the capsules 
of the cartilage-cells stain a deep yellow with hematoxylin and 
picric acid, while the fibres between the cell-nests stain a blue-brown 
colour, partly from the hematoxylin, partly from the picric acid. 
All the evidence points to the plastron as resembling the basi- 
cranial skeleton of Ammoccetes in its composition and in the origin 
