302 THE DERMAL SKELETON. 



retained, and which it is a menace to remove. In its place, it is producing an 

 armor that is permeable, capable of indefinite expansion, and one that cannot, 

 and need not, be shed at frequent intervals. 



The new conditions under which this skeleton is developed, and especially 

 its permanent retention within the foreign mesodermic tissues, were no doubt im- 

 portant factors in bringing about a permanent change in its chemical compo- 

 sition. 



III. Summary and Comparison. 



I. We have shown that Limulus possesses a remarkable dermal skeleton, and 

 that in either coarse or minute structure there is nothing resembling it known in 

 any other invertebrate. The nearest approach to it is found in the pteraspidian 

 division of the ostracoderms. No other known animal, vertebrate or invertebrate, 

 resembles Pteraspis in the structure of its exoskeleton so closely as Limulus. If 

 Limulus were an extinct animal, it would be exceedingly difficult to discover any 

 differences between the minute structure of its deeper lying exoskeleton and 

 that of Pteraspis. 



II. The surface ornamentation of the exoskeleton of the ostracoderms may 

 be resolved into a series of alternating ridges and grooves that are either very 

 uniform in size and nearly parallel, as in Pteraspis; or sinuous and with a tendency 

 to break up into rows of tubercles, as in Bothriolepis; or finally forming tubercu- 

 late- polygonal areas, as in Cephalaspis. 



III. The outer surface is also marked by a special series of shallow grooves 

 in Bothriolepis (Fig. 247), Ateleaspis (Fig. 242), and Tremataspis (Fig. 236), that 

 probably mark the location of rows of sensory organs. 



Another set of deep lying canals, probably representing enclosed surface 

 grooves, are present in Pteraspis and Tremataspis. They are of even caliber and 

 open outward by narrow slits, or pores, or by very short, chimney-like canals, 

 (Figs. 193, 196, s.c.) that probably contained sensory organs or mucous glands. 

 They differ from the vascular canals in that they are generally filled with a matrix 

 like that outside the shell, showing that they communicated freely with the out- 

 side, while the vascular canals and even the cancellse, may be quite empty. 



IV. The surface ornamentation of the exoskeleton of the ostracoderms 

 may be regarded as a further specialization of the ridges and grooves seen on the 

 outer surface of the exoskeleton in the marine arachnids. It would require but 

 a slight modification of the scale-like markings in Pterygotus, or of the zigzag 

 ridges and grooves, or the polygonal areas, in Limulus, to produce the character- 

 istic markings of Pteraspis, Bothriolepis, or Cephalaspis. 



V. The surface ornamentation of the cephalic buckler in both arachnids and 

 ostracoderms may be regarded as the external expression of internal irregulari- 

 ties in growth which ultimately lead to the breaking up of the continuous shell into 

 separate plates. 



