272 Proceedings of Royal Society of Edinburgh. [sess. 
the primitive coelome into discontinuous elements, each with its own 
distinct function. 
It is necessary to emphasise these points with regard to the 
theory here adopted, because there are other theories of the coelome 
which, by an appeal to a judiciously-selected series of morpholo- 
gical facts, have been made to bear a superficial semblance to 
probability, although violating the elementary principles of physio- 
logical differentiation. 
I allude in particular to the suggestion that the coelome is to be 
derived from the gonads of the Pseudocoela. If once it be granted 
that a mass of reproductive cells have, in the course of phyletic 
history, lost their sexual specialisation and given rise, in the same 
individual, to muscles, connective and other mesodermic tissues of 
the differentiated coelome, there is surely no impediment to assum- 
ing that the Coelenterata have been derived from the Coelomata , or 
any one phylum from any other that may suit the individual 
fancy. 
Intimately bound up with Mr Sedgwick’s theory of the coelome 
is that of the blastopore. To quote the author’s words — “it 
necessarily follows (from the consideration of the Peripatus 
embryo) that the mouth and anus of the Triploblastica are 
derived from the gastroea mouth, i.e ., coelenterate mouth.” * I 
need not quote the arguments which were adduced for this theory 
by its author; but, since it was suggested, further research has 
only afforded additional evidence in its favour, f 
This mode of derivation of the triploblastic mouth and anus 
assumes the existence of a radial coelenterate ancestor of the 
Coelomata. In other words, an organism in which the parts were 
arranged in radial symmetry, about an axis passing through the 
mouth to the aboral pole. We, therefore, go no further than the 
theory to suppose that the gut-pouches were also radial, which 
implies that their number must have been at least three. Their 
possible number appears to lie between three, four, and five. The 
higher numbers, which are multiples of either of these, must have 
been derived from a prior stage with the lowest factor, whereas a 
radial symmetry of seven, eleven, or thirteen units has no pre- 
* A. Sedgwick, loc. cit., p. 67. 
t Cf. development of Serpula , Peripcitus, and Mollusca. 
