SUMMARY. 303 



VI. From a vertebrate standpoint, the continuous dermal armor of the ostra- 

 coderms appears to be a very primitive exoskeleton, and one from which that 

 of the true fishes has been produced by breaking up the surface ornamentation 

 into isolated dermal denticles and bony plates. From the invertebrate stand- 

 point, the ostracoderm skeleton is a highly specialized one, produced by an exag- 

 geration of the type of exoskeleton seen in Limulus. 



VII. It may be urged that the bony plates of the ostracoderms are meso- 

 dermal in origin, while those of Limulus are epidermal. But we have no means 

 of knowing whether the bony plates of the ostracoderms were developed entirely 

 inside, or outside the ectoderm, and we cannot class them as subdermal, or 

 mesodermal, unless we beg the whole question and assume that the ostracoderms 

 are typical fishes. 



VIII. The structure of the exoskeleton of the ostracoderms is as much like 

 that of Limulus as that of vertebrates. The dermal skeleton of vertebrates can 

 be derived as readily from that of Limulus as from that of the ostracoderms. 

 To do this, for example, we need only to assume that the outer layer of the shell 

 of Limulus has been reduced to a thin cuticular layer and is cut off entirely from 

 the underlying trabeculag. The skin and the skeletal parts derived from it would 

 then appear to be formed of two layers, a continuous outer layer, and an inner 

 one composed of more or less isolated fragments formed by local ingrowths from 

 the outer layer. 



The two kinds of exoskeleton, epidermal and subdermal, would then be 

 present at the same time, as indeed they are in Limulus. But in Limulus the sub- 

 dermal skeleton is in its initial stages, and the epidermal is the more voluminous. 

 In primitive vertebrates the epidermal skeleton is about to disappear, being repre- 

 sented solely by the enamel layer, and possibly the dentine, while the subdermal 

 layer has attained its maximum development. The distinction between the 

 epidermal skeleton of the arachnids, and the dermal one in vertebrates is, therefore, 

 only one of degree, not of kind. 



In the embryonic development of dermal bones in vertebrates, the arachnid 

 process of separating bony trabecules from the inner surface of the ectoderm is 

 apparently greatly abbreviated, but indications of it have been observed in the 

 development of certain cranial bones. These observations, therefore, need not 

 be looked upon with suspicion, or, if accepted, taken as evidence of the untrust- 

 worthiness of the germ layer theories. They should be regarded rather as the 

 naturally to be expected embryological indication of the derivation of the verte- 

 brate dermal skeleton from the epidermal armor of arachnid ancestors. 



IX. The dermal denticles are the oldest parts of the dermal skeleton of verte- 

 brates, and they naturally still retain the clearest indications of their derivation 

 from an arthropod exoskeleton. 



The chiten of arachnids, and the enamel and dentine of the dermal bones of 

 primitive vertebrates have essentially the same structure and mode of growth, as 

 shown by their pronounced laminae and the minute parallel canals that run at 



