437 



some imperishable substance like that covering the lateral eyes of 

 L i m u 1 u s and other arthropods generally. 



Now when it is remembered that in almost every one of these 

 points L i m u 1 u s differs from all other arthropods, and Cephalaspis 

 from all other vertebrates, we will have some idea of the remarkable 

 resemblance existing between them. 



It may be urged that the bones of the Cephalaspidians are 

 covered by a layer of ectoderm that is absent in Limulus. There 

 is not one iota of evidence that such a layer exists in the Cephal- 

 aspidae, unless we beg the whole question and start on the assump- 

 tion that we are dealing with highly organized fishes. But if there 

 were such a layer, its presence could be easily accounted for by 

 supposing that in Limulus the surface ectoderm became continuous 

 over the chitenous network. The latter, with its entangled sense 

 organs and glands, would then appear to rise from what good 

 orthodox biologists would call, the "mesodermic layer" of the skin. 

 There is no conclusive evidence to show that dermal bones in verte- 

 brates may not have originated from the ectoderm in this manner. 

 Indeed, two audacious spirits have recently dared to suggest that 

 such classically "mesodermic" structures as cranial cartilages of verte- 

 brates arose from ingrowths of the ectoderm ! These are welcome 

 excursions from the senseless dominion of the germ layer dogma. 

 They harmonize with the view just presented and also with the state- 

 ments I made some five years ago to the effect that the nervilemma 

 of the brain, optic ganglia and nerves of Acilius were, at a com- 

 paratively late embryonic period, derived from the ectoderm. See 

 "Origin of the Vertebrates from Arachnids", p. 366, and especially 

 "Eyes of Acilius", p. 145. 



The dermal bones vary considerably in extent in different in- 

 dividuals. They are not visible in specimens less than eight inches 

 long and probably do not make their appearance till the animal is 

 full grown. Even when the spongy bone is largely developed, the 

 characteristic lacunae in the chitenous bars may be absent. This 

 shows, I believe, that the lacunae are not fully developed till long 

 after maturity is reached, for it was in the very oldest individuals, 

 with much scarred and worn armor, that I found them best developed. 



It is of great interest and importance to observe that Limulus 

 has, so to speak, solved the problem as to how an arthropod might 

 escape from the bondage of a cuticular exoskeleton, which must be 

 shed periodically with all its well-known accompanying dangers. In 

 Limulus the chitenous network is never cast off. Indeed, Limulus 



