1897 - 98 .] Dr Masterman on Ar chimeric Segmentation. 271 
mate animals may be shown to possess a greater or less number of 
these features, such that they will naturally fall together under the 
one major division, conveniently termed the Archi-coelomata. These 
features may be morphological or ontogenetic. 
(C.) Thirdly , evidence may be derived from morphology and 
from ontogeny of the metamerically segmented groups that a 
secondary segmentation has been phyletically superposed upon the 
primary. Thus, these animals may be shown to pass through 
ontogenetic stage's which closely resemble the archi-coelomate type 
(and, in consequence, some of the Archi-coelomata ), and to possess 
in their morphology more or less vestigial traces of the archimeric 
segments. 
(A.) In the classification of the Triploblastica the only sound 
basis of phyletic value upon which to rest is the condition of the 
mesoderm or third layer, and bound up with this is the coelome 
and its segmentation. It is impossible to enter here into the whole 
discussion of the theories with respect to the primary origin of the 
coelome, but I would adhere to that originally propounded by Mr 
Sedgwick* as the most in harmony with the facts of morphology, 
ontogeny, and physiology. 
Its essential feature is “ that the somites of segmented animals 
are derived from gut pouches, which are homologous with the 
pouches of Coelenterata .” 
Hot only are there a great number of morphological and onto- 
logical facts which point to the truth of this hypothesis, but it is 
confirmed by physiological considerations. Thus, the pouches of 
the Adinozoa present special differentiations of the primitive 
endoderm, the walls of which perform the functions of the mus- 
cular and reproductive systems, and the cavities of which contain 
a nutritive circulatory fluid. In the lowest Coelomata the coelome 
performs precisely the same series of functions, and only differs 
essentially from the ‘ pouch ’ of the Coelenterata by its loss of 
organic continuity with the endodermic gut-wall, whilst in the 
history of the coelome in the higher Coelomata each function 
becomes differentiated and confined to a special area of the organ, 
the morphological expression of which is the eventual division of 
* Quart. Journ. Micros. Science, 1886 . 
