43 



ment of the budding pleon of a Limulus at the stage figured by Packard (pi. v. fig. 27, 

 op. cit.) were so different in its vertical relation from that of the antecedent segment as 

 to support the assertion as to the limited locality of its attachment, the legitimate infer- 

 ence would be that it represented a corresponding part of a body-segment. If succes- 

 sively developed axial divisions of the body-segments are only to be regarded as such 

 when they happen to bear true appendages, many of the segments of the Mero- 

 stomes, besides the terminal one, must be i*elegated to the category of ' peculiar ' 

 median appendages — a view which would much obscure and complicate the problem of 

 determining the affinities of those primeval crustaceans on the basis of well-founded, 

 homologies. 



Concurring with my colleague, Mr. H. Woodward, in the views of the affinities which 

 are expressed by his extended application of Dana's term Merostomata, which thus be- 

 comes something more than a mere synonym of Gronovan's Xiphosura, I would remark, 

 in reference to the relations, in time, of the latter to Pterygotiis, Eurypteriis, and allied 

 extinct Silurian forms, that these manifest a more generalized character than do the 

 Xiphosures. One cannot say that they are persistent or arrested embryonal forms or 

 stages of development ; for we have seen that Limuhis, as soon as the germ-heaps are 

 aggregated into unity or shape, assumes its concentrated character. Both families, 

 together with the Trilobitidce, exemplify that lower condition of the Crustacea which has 

 been expressed by the term Eiitomostraca, in which, as Mr. Woodward has well remarked, 

 the older, long and slender forms are analogous, in shape as well as in geological rela- 

 tions, to the macrurous Malacostraca, and the short and broad forms to the Bra- 

 chyura. If we further indulged in suggesting that the Merostomata might be the 

 ancestors of Arachnida, we might also conjecture that the Myriopods have come out of 

 Trilobites ; but this, at present, is not science. A superficial resemblance to the latter, 

 as we have seen, is shown by the absence of the pleon in the earlier stages of the King- 

 crab ; but the very fact of the late appearance of this terminal division of the body, 

 after all the segments, with their appendages, of the antecedent division (' thoracetron ') 

 have been formed, is decisive against any real or representative resemblance of the 

 embryo Idmulus to the Trilobites — on the acceptance, at least, of the original and valuable 

 observations by Barrande *, of the successive and later appearance of the abdominal 

 (' thoracetral ') segments, in the space between the head (' cephaletron ') and pygidium 

 (' pleon '), in the embryos of Sao hirsutus,Agnostus nudus, and Trimicleus ornatus (fig. 12). 

 These developmental phenomena bear a significant analogy to those observed by New- 

 port t in the Julklce — the successive appearance, viz., of body-segments, in the space 

 (ib. b) anterior to the terminal or pygidial division (ib. c) ; such thoracetral segments 

 also appearing at successive moults, as in the Trilobites. 



These, with other facts noted in the anatomical sections of the present paper, such as 

 the fusion of the pair of cephaHc ganglia, the shortness and thickness of the ' crura " 



* Systeme Silurien du Centre de la Boheme, 4to, 18.52 ; section vii. pp. 257-276, " Metamorphoses ot mode d'exist- 

 ence des Trilobites." 



f " On the Organs of Reproduction and the Development of the Myriapoda,'' Phil. Trans, vol. cxxxi. 1841 ; and 

 Owen, 'Lectures on Invertebrata,' 8vo, 1855, p. 394. 



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