On the Theory of Archimeric Segmentation and its bearing upon the Phyletic Classification of the Cœlomata

Abstract

In a recent paper embodying the results of my work upon Actinotrocha, I was led to suggest a theory of the segmentation of cœlomate animals, the essential features of which were as follows:—1. There can be demonstrated, in the morphology of the Cœlomata, two distinct types of segmentation—(a) a primitive or archimeric type, having in its constitution certain evidences of a radial origin, and (b) a secondary or metameric segmentation, superposed upon the former and bearing evidence of a bilateral origin.2. A certain number of the Cœlomata retain, under varied disguise, the archimeric segmentation throughout life, together with a number of other primitive characters in common. These groups, being genetically allied, may be conveniently classified in one division, for which the name Archi-cœlomata is proposed.3. In the truly segmented animals, such as the Annelida, Arthropoda, and Eu-chorda, the bilateral or metameric segmentation ⟨of the posterior archimeric segment⟩ tends to completely replace the archimeric. In at least two of these groups there has been an independent evolution of metameric segmentation.In attempting to bring forward facts in support of this theory, one may approach the subject from three stand-points.(A.) Firstly, an attempt may be made, by general reasoning, to construct a hypothetical ancestor of the Cœlomata as a group. In this way may be obtained a schematic outline of an organism possessing each system of organs in its most archaic condition.