[Illustration: 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. {141}I have attempted in Fig. 53 to represent this close resemblance between the segmented branchial skeleton of Limulus and of Ammocoetes, 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. [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?