294 Proceedings of Royal Society of Edinburgh. [sess. 
should show an ontogeny in which the mesodermal layer or coelome 
is formed in the phyletic manner. At the same time, it is remark- 
able to find that most of the Archi-ccdomata agree in one parti- 
cular method of mesoderm formation. 
As early as 1876 Huxley suggested an alliance of the Enter o- 
pneusta , Echinodermata , Brachiopoda, and Chcetognatlia together 
under the title Enterocoela , to emphasise the development of the 
coelome in these groups by archenteric diverticula.* In Phoronis 
there is said to he a modified form of this method f and the develop- 
ment of Cephalodiscus is unknown. In the Mollusca the meso- 
derm is broken up and largely replaced by blood-spaces, and in 
many of them an archenteric method of mesoderm formation is 
pursued. 
From the preceding portion dealing with the suggested derivation 
of the mesoderm of the Ar chi-coelomata from archenteric diverti- 
cula, it is evident that for a correct repetition of phylogeny the 
archimeric segments or their mesodermic elements should arise as 
follows : — Four diverticula of the gut should arise in a horizontal 
plane — one pre-oral, two lateral, and one post-anal. The post-anal 
should then divide into two to form a pair of metacoeles. 
This method is closely followed by such a type as Balanoglos- 
sus, in the demersal larva of B. Kowalevski. 
The other Arclii-coelomata do not, as far as is known, derive their 
mesoderm in such a typical manner, hut if this method of formation 
he assumed to he the primitive and phyletic one, it is not difficult 
to derive the other methods of mesoderm-formation as modifications 
due to secondary ontogenetic processes. 
According to our theory, the axis of radial symmetry in the 
primitive coelomate corresponded with the central axis of the 
archenteron, and the four archimeric coelomic pouches arose sym- 
metrically to this axis and at the aboral end. 
On the assumption of bilateral symmetry, the main axis of the gut 
by an elongation in one direction came to lie in a horizontal plane, 
and hence at right angles to the archimeric axis of symmetry. 
In a truly phyletic ontogeny this change should he brought about 
by an elongation of the blastopore in the direction at right angles 
* Huxley, T. H., Journ. Linn. Soc., vol. xii. 
t Caldwell, W. H., Quart. Journ. Micro. Soc., 1885. 
