434 THE SYSTEMATIC POSITION OF THE TEILOBITES. [Aug. 1 894, 



of the trilobites with the arachnoid series. He advanced reasons for 

 accepting the Author's homology of the median cephalic pore of the 

 trilobites with the aperture of the dorsal gland of Apus, and for 

 believing that in the latter we are dealing with an organ early 

 differentiated in the crustacean series, but now for the most part 

 lost — the ' dorsal organ ' of embryologists being its vestigial homo- 

 logue. He believed that the facts and arguments brought forward 

 by the Author of the paper proved the trilobites to be Crustacea, 

 and fully justified their association with Apus as an early offshoot 

 on the crustacean line. He considered that in demonstrating the 

 progressive fusion of head-segments among the trilobita the Author 

 had shown those animals to have so far undergone a parallelism of 

 modification to all other great groups of arthropods. If, as he 

 believed, the degree of this fusion" was the surest guide to the 

 position of any one member in an arthropod series, that being the 

 higher in proportion as the fusion is numerically the greater, the 

 places customarily assigned to the Scorpionidae and the Araneidaa by 

 the advocates of the ZwnwZus-an-arachnid theory must be trans- 

 posed — the scorpions becoming the culminating members of the 

 arachnoid series. Judged from this standpoint, the superficial re- 

 semblances between Limulus and Scorpio appeared to him closely 

 akin to those between, say, the flying squirrels and Galeopithecus, 

 or between the Rana jerboa and Bnfo jerboa of Borneo, and sug- 

 gestive of isomorphism by convergent modification. To definitely 

 assert that IAmulus is an arachnid appeared to him on a par with 

 saying that the ' flying lemur ' is a squirrel, and the Bufo jerboa a 

 frog. 



Mr. Malcolm Laurie also spoke. 



The Author, in reply, said that none of the objections dealt with 

 points of any morphological importance. The head-shield in Apus 

 developed by backward prolongation into a carapace, and in the 

 trilobites gave rise to the pleura? by segmental repetition, as any 

 prominent cuticular structure might be repeated. It was enough 

 that the antennae in both were inserted at the sides of the labrum, 

 and that the trunk-limbs were of the same type, with ' endopodite,' 

 ' exopodite,' and gills, and, what was still more important, with 

 broad lines of insertion. That the trilobites might be myriapods 

 could not have been seriously suggested. The subject was neces- 

 sarily speculative, and the value of a speculation depended upon the 

 evidence in its favour ; in the present case, all the available evidence 

 tended to establish the affinities proposed. 



