304 



THE DERMAL SKELETON. 



right angles to them. The differences are mainly ones of degree, i.e., the number 

 and size of the canals, and the density and composition of the matrix. 



Fig. 208 — Semi-diagrammatic sections illustrating the evolution of the chitenous epidermal exoskeleton of arach- 

 nids into the bony sub-dermal exoskeleton of vertebrates. A , Exoskeleton of immature Limulus showing the begin- 

 ning of the trabecular ingrowths; B, mature Limulus,_§howing the cancellated exoskeleton; the trabeculas, with 

 their axial lacunas; and the dentine-like chitenous matrix, with its parallel pore canals and concentric lamellee; C, 

 the exoskeleton of an ostracoderm, consisting of dentinal, vascular, cancellous, and basal layers. In the outer layer 

 "are "shown several forms of tubercles, denticles, and dentinal ridges, with various forms of dentinal canals; D, the 

 final stages showing the conversion of the vascular, cancellous, and basal layers into subdermal bone; and the con- 

 version of the isolated remnants of the primitive epidermal skeleton, into dermal denticles. 



When two such similar structures as enamel and dentine appear to arise, one 

 from the mesoderm, the other from the ectoderm, it probably means that either 



