1897 - 98 .] Dr Masterman on Archimeric Segmentation. 275 
in the two radii at right angles to them. If we assume the former 
presence of an ectodermal involution (coelenterate stomodseum), as 
in the Adiniaria , then the fusion of the walls in two radii would 
leave the ectoderm as a pair of ectodermal funnels, the metazoan 
stomodseum and proctodseum, connected with the functions of 
ingestion and egestion respectively. In ontogeny, the stomodseum 
and proctodseum would naturally arise as separate invaginations. 
Thus a secondary axis, from mouth to anus, is believed by many 
morphologists to have been acquired in the history of the Metazoa 
( Heteraxonia ), and accepting this as a theory with a great amount 
of probability, we may inquire as to the further fate of the four 
coelomic pouches. 
The acquirement of the secondary axis of symmetry implies a 
rearrangement of the organs bilaterally about this axis, more 
especially as the differentiation of an ingestive aperture implies a 
locomotion in that direction. There are, as far as I can see, only 
two alternatives with regard to the arrangement of the four 
ccelomic pouches in relation to the new axis of symmetry. Either 
the axis will correspond to the septa between the ccelomic pouches 
and the latter will become symmetrically arranged as a pair of 
anterior and a pair of posterior pouches, or the axis will corre- 
spond to the centre of a pouch, and they will become arranged 
so that one will be pre-oral, two as paired lateral, and another 
post-anal. 
Of these two alternatives there can be little doubt that the latter 
is the correct one. So far as I know, all the Actinozoa except the 
Antijpatharia have the terminal mesenteries arranged in pairs on 
other side of the main axis, and in the medusae ( e.g ., Aurelia , 
Lucernaria , etc.), in which the mouth shows a tetramerous sym- 
metry, it is in the axes intersecting the gastro-vascular pouches 
(per-radial). Thus we have some justification for supposing that 
upon the assumption of bilateral symmetry about an oro-anal axis 
the four primitive coelomic pouches became arranged so that one 
was pre-oral, two were lateral, right and left, and one was post- 
anal. In correlation to this, the coelomate ancestor will present a 
body of three segments, one pre-oral and two post-oral. Organs 
arising in these segments will at this stage be symmetrical about 
the long axis, and others which, like the coelomic pouches, had 
