{ 138}[Illustration: FIG. 58.--TRANSVERSE SECTION THROUGH THE MESOSOMA OF LIMULUS, TO SHOW THE ANTERIOR (A) AND THE POSTERIOR (B) SURFACES OF A MESOSOMATIC OR BRANCHIAL APPENDAGE.
In each figure the branchial cartilaginous bar, _Br.C._, has been exposed by dissection on one side. _Ent._, entapophysis; _Ent.l._, entapophysial ligament cut across; _Br.C._, branchial cartilaginous bar, which springs from the entapophysis; _H._, heart; _P._, pericardium; _Al._, alimentary canal; _N._, nerve cord; _L.V.S._, longitudinal venous sinus; _Dv._, dorso-ventral somatic muscle; _Vp._, veno-pericardial muscle.]
At the base of each of these appendages, where it is attached to the body of the animal, the external chitinous surface is characterized by a peculiar stumpy, rod-like marking, and upon removing the chitinous covering, this surface-appearance is seen to correspond to a well-marked rod of cartilage (_Br.C._), which extends from the body { 139}of the animal well into each appendage. This bar of cartilage arises on each side from the corresponding entapophysis (_Ent._), which is the name given to a chitinous spur which projects a short distance (Fig. 58, B) into the animal from the dorsal side, for the purpose of giving attachment to various segmental muscles. These entapophyses are formed by an invagination of the chitinous surface on the dorsal side and are confined to the mesosomatic region, so that the mesosomatic carapace indicates, by the number of entapophyses, the number of segments in that region, in contradistinction to the prosomatic carapace, which gives no indication on its surface of the number of its components.
Each entapophysis is hollow and its walls are composed of chitin; but from the apex of each spur there stretches from spur to spur a band of tissue, called by Lankester the entapophysial ligament (_Ent.l._) (Fig. 58), and in this tissue cartilage is formed. Isolated cartilaginous cells, or rather groups of cells, are found here and there, but a concentration of such groups always takes place at each entapophysis, forming here a solid mass of cartilage, from which the massive cartilaginous bar of each branchial appendage arises.
Further, not only is this cartilage exactly similar to parenchymatous cartilage, as it occurs in the branchial cartilages of Ammocoetes, 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.