270 
Proceedings of Royal Society of Edinburgh. [sess. 
On the Theory of Archimeric Segmentation and its 
bearing upon the Phyletic Classification of the 
Coelomata. By A. T. Masterman, D.Sc. 
(Read June 6, 1898.) 
In a recent paper embodying the results of my work upon 
Actinotrocha * I was led to suggest a theory of the segmentation 
of coelomate animals, the essential features of which were as 
follows : — 
1. There can he demonstrated, in the morphology of the Coelo - 
mata , two distinct types of segmentation — (a) a primitive or archi- 
meric type, having in its constitution certain evidences of a radial 
origin, and ( b ) a secondary or metameric segmentation, superposed 
upon the former and hearing evidence of a bilateral origin. 
2. A certain number of the Coelomata retain, under varied dis- 
guise, 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 Ar chi-coelomata is proposed. 
3. In the truly segmented animals, such as the Annelida , Arthro- 
jpoda , 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 Coelomata as a group. In 
this way may be obtained a schematic outline of an organism pos- 
sessing each system of organs in its most archaic condition. 
(E.) Secondly , taking this as a central type of the organisation 
of the Ar clii-coelomata, several of the lower divisions of the coelo- 
* Quart. Jour ii„ Micros. Science , Aug. 1897. 
