142 THE ORIGIN OF VERTEBRATES 



and cartilages. In this figure the animal is supposed to be slit open 

 along the ventral mid-line and the central nervous system exposed. 



The Prosomatic 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 Ammocoetes, have, in every case, led us in 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 ha^matoxylin and 

 picric acid, while the fibres between the cell-nests stain a blue-brown 

 colour, partly from the ha?matoxylin, partly from the picric acid. 



All the evidence points to the plastron as resembling the basi- 

 cranial skeleton of Ammocoetes in its composition and in the origin 



