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 malpighian vessels, whereas the merostomes (Limulus) and all unquestionable Crustacea are devoid of these vessels." He suggests that the arachnids also form an ancient type, and that the common origin of the merostomes and the arachnids must be sought for far down in the pedigree of the arthropods. "The agreement between scorpions and eurypterids would then be derived from causes quite independent of close relationship, and perhaps from a convergence in some branches of the two stems which are formed by the merostomes and the arachnids."

Opposed to this, Lankester and his followers hold that the presence of gills and the absence of malpighian tubules are features associated with aquatic life, and hence of no critical classificatory value, and Laurie and Claus assert that the morphological value of the gills has been greatly overestimated. Laurie notes that the branchiae of eurypterids, like those of Limulus, are constructed on a type unknown amongst the Crustacea, and further, that structures such as these, which are the product of a physiological necessity, are not of much value in indicating close relationship. He cites as arguments against the crustacean relationship "the segmentation of the body and position of the genital aperture—which does not agree with that of any known crustacean, the absence of anything representing the first antennae, the chelate structure of the one pair of preoral appendages and the fact that there is no trace of the typical crustacean biramous structure of the appendages." Admitting the relationship of the eurypterids to the arachnids, Laurie thinks that "the eurypterids must be considered as intermediate between Crustacea and Arachnida, in the sense that they are among the most primitive arachnids, and therefore nearer the junction point of the two terms." Agreeing with the other authors as to the close relationship of the eurypterids with Limulus, Laurie sees a point of great morphological importance "in the fact that Limulus has a pair of platelike appendages on the second abdominal segment," and takes this to indicate that "Limulus branched off from the eurypterid stem before the genital operculum was so highly specialized as it is in the eurypterids, and, consequently, before the appendage of the second