34 



THE DERMAL SKELETON. 



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

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



1 



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, showing the cancellated exoskeleton; the trabeculae, with 

 their axial lacunae; and the dentine-like chitenous matrix, with its parallel pore canals and concentric lamellae; 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 



