THE EVIDENCE OF THE SKELETON 1 39 



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 meso- 

 somatic carapace indicates, by the number of entapophyses, the 

 number of segments in that region, in contradistinction to the pro- 

 somatic 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. I.) (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 enta- 

 pophysis, 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 parenchy- 

 matous cartilage, as it occurs in the branchial cartilages of Ammoccetes, 

 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 Ammoccetes 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 Ammocu'tes. 

 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 Ammocn'tes, does not 

 stain, or gives only a light-blue tinge with thionin. The tissue of 



