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Book Chapter

Part III.—Homology and Phylogeny: Homology

January 01, 1939


Since the stenopodous, uniramous, ventral appendages (the first chelicerate) place the Aglaspida unqualifiedly among the Arachnida and since their general body form and aquatic habit place them within the Merostomata, comparisons are made with bona fide representatives of that subclass, and thereby inferentially with the succeeding order among the Arachnida, the Scorpionida. In addition, comparisons are made with the Trilobita, in spite of their great dissimilarity to the Aglaspida, only because of the very prevalent tendency of past and present systematists to link Merostomata in general and the Aglaspida in particular with those Crustacea. With respect to such pseudoarachnids as Sidneyia, Emeraldella, Habelia, Molaria, Amiella, and Roddyia, comparisons would be irrelevant in view of the completely crustacean nature particularly of those that preserve the ventral appendages.


The fact that both trilobites and aglaspids have similar jointed limbs repeated on the cephalothorax (cephalon of the trilobites) and on the free segments is, in the opinion of the writer, merely a consequence of their both being primitive expressions of their lines. Because they are both primitive arthropods, it does not necessarily follow that they are closely related to each other. The Trilobita are crustacean in consequence of their antenniform first appendages and of the biramous nature of the others; the Aglaspida are as definitely arachnid on the basis of their cheliciform first appendages and of the uniramous nature of the remainder. Disregarding the ventral characters, many observers continue to be impressed with the general similarity in form . . .

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GSA Special Papers

Cambrian Merostomata

Gilbert O. Raasch
Gilbert O. Raasch
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Geological Society of America
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January 01, 1939




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