Further, not only is this cartilage exactly similar to parenchymatous cartilage, as it occurs in the branchial cartilages of Ammocœtes, but also its matrix stains a brilliant purple with thionin in striking contrast to the exceedingly slight light-blue colour of the surrounding perichondrium. In its chemical composition it shows, as might be expected, that it is a cartilage containing a very large amount of some mucin-body.
The Muco-cartilage of Limulus.
The resemblance between this structure and that of the branchial bars of Ammocœtes does not end even here, for, as already mentioned, the cartilage originates in a peculiar connective tissue band, the entapophysial ligament, and this tissue bears the same relation in its chemical reactions to the ordinary connective tissue of Limulus, as muco-cartilage does to the white fibrous tissue of Ammocœtes. The white connective tissue of Limulus, as already stated, resembles that of the vertebrate more than does the connective tissue of any other invertebrate, and, similarly to that of Ammocœtes, does not stain, or gives only a light-blue tinge with thionin. The tissue of the entapophysial ligament, on the contrary, just like muco-cartilage, takes on an intense purple colour when stained with thionin. It possesses a mucoid substratum, just as does muco-cartilage, and in both cases a perfectly similar soft cartilage is born from it.
Fig. 59.—Diagram of Limulus, to show the Nerves to the Appendages (1-13) and the Branchial Cartilages.
The branchial cartilages and the entapophysial ligaments are coloured blue, the branchiæ red. gl., generative and hepatic glands surrounding the central nervous system and passing into the base of the flabellum (fl.).
One difference, however, exists between the branchial cartilages of these two animals; the innermost axial layer of the branchial bar of Limulus is very apt to contain a specially hard substance, apparently chalky in nature, so that it breaks up in sections, and gives the appearance of a broken-down spongy mass; if, however, the tissue is first placed in a solution of hydrochloric acid, it then cuts easily, and the whole tissue is seen to be of the same structure throughout, the main difference being that the capsular spaces in the axial region are much larger and much more free from cell-protoplasm than are those of the smaller younger cells near the periphery.
I have attempted in Fig. [53] to represent this close resemblance between the segmented branchial skeleton of Limulus and of Ammocœtes, a resemblance so close as to reach even to minute details, such as the thinning out of the cartilage in the subchordal bands and entapophysial ligaments respectively between the places where the branchial bars come off.
Fig. 60.—Diagram of Ammocœtes cut open to show the Lateral System of Cranial Nerves V., VII., IX., X., and the Branchial Cartilages.