[Illustration: FIG. 60.--DIAGRAM OF AMMOCOETES CUT OPEN TO SHOW THE LATERAL SYSTEM OF CRANIAL NERVES _V., VII., IX., X._, AND THE BRANCHIAL CARTILAGES.
The branchial cartilages and sub-chordal ligaments are coloured blue, the branchiæ red. _gl._, glandular substance surrounding the central nervous system and passing into the auditory capsule with the auditory nerve (_VIII._).]
In Fig. 59 I have shown the prosoma and mesosoma of Limulus, and indicated the nerves to the appendages together with the mesosomatic cartilaginous skeleton.
In Fig. 60 I have drawn a corresponding picture of the prosomatic and mesosomatic region of Ammocoetes with the corresponding nerves { 142}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 prosomatic 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 vertebrate ancestor, that they do possess a remarkable internal semi-cartilaginous 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 preponderance 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 hæmatoxylin and picric acid, while the fibres between the cell-nests stain a blue-brown colour, partly from the hæmatoxylin, partly from the picric acid.