296 



THE DERMAL SKELETON. 



II. DERMAL SKELETON OF LIMULUS. 



We shall describe in some detail the dermal skeleton of Limulus because it 

 has the usual arthropod characters, and at the same time several other very im- 

 portant ones that are not found in any other animal, so far as I know, outside the 

 vertebrates. 



The outer surface of the shell of half grown Limuli (Fig. 199, B.) is marked 

 by broad zig-zag ridges separated by shallow grooves. In some places, notably 



FIG. 201. Dermal skeleton of Limulus. A, Cross- section through the posterior median portion of the thoracic 

 shield; B, through the pineal eye chamber; C, surface view of the trioculate parietal eye; D, bony trabeculae 

 on the inner surface of the shield, in the lateral eye region; E, Cross-section of the lateral eye chamber. 



in the region about the lateral eyes, the ridges break up into polygonal areas, 

 each of which contains a crater-like depression with radiating grooves extending 

 toward the base of the cone, Fig. 200, D. The distinctness of the ornamentation 

 is accentuated by the deeper color and other optical properties of the matrix in 

 the grooves and craters; but the same figures can be obtained in wax impressions 

 of the outer surface, except that they are fainter. 



FIG. 202. A, Inside margin of the thoracic shield, showing the mode of growth of the trabeculse; B, mass of bony 

 trabeculae from the inner surface of the thoracic shield. 



The inner surface of the shell at certain places gives rise to great masses of 

 interwoven chitenous bars, or trabeculae, separated by irregular spaces filled 

 with loose connective tissue, blood-vessels and nerves. (Figs. 203, 204.) 



The bars are concentrically laminated and contain numerous fine canals and 

 spindle-shaped cavities, so that the whole mass of tissue resembles in a very 

 striking way the cancellous bony tissue of vetebrates. 



