1897 - 98 .] Dr Masterman on Ar chimeric Segmentation. 293 
dently acquired. These comprise the reduplication of the, gut (which 
is apparently dorsal in Phoronis , Ectoprocta, and Brachiopoda , 
but ventral in Entoprocta ), a reduction of the protomere to a vestige , 
the epistome, or to extinction , and the forward projection of the 
tentacular processes of the mescmeres to form a lophophore. It is not 
difficult to derive these characters as directly due to the environ- 
ment of a sedentary animal, by a consideration of types like the 
Cirripedia , Tunicata , S ipunculids, and the groups under consider- 
ation. 
Such being the case, the systems of classification which depend 
upon such characters as these are of the same nature as those in 
which all triploblastic forms of an elongated type are thrown 
together as Vermes. 
Thus all the community of structure in the Molluscoida, which 
is not directly traceable to a similarity of environment, is no more 
than that which exists in the other phyla of the Ar chi-coelomata 
cited here, and this fact, taken in conjunction with the consider- 
ation that there are very marked differences in structure in the 
groups, justifies us in assuming that the Brachiopoda , Ectoprocta, 
and Phoronida are separate phyla of the Ar chi-coelomata. 
The evidence with regard to the Entoprocta is somewhat of the 
same nature as that in the case of Rotifera and Sipunculids , to be 
referred to later. 
Ontogeny — Derivation of the Mesoderm and Archicoeles. 
In connection with the gastroea theory of Haeckel, there are 
shown to be in existence a great number of animals, which, ex- 
tremely diverse in habits and in structure, yet retain the diplo- 
blastic structure of the gastrula, as an underlying basis of their 
organisation. These animals, in their own ontogeny, do not by 
any means, without exception, show the typical mode of forma- 
tion of this diploblastic condition, the second layer being formed 
in different cases by unipolar or multipolar ingression, by delami- 
nation or by true embolic invagination. 
In the case in point it will not be necessary to the truth of the 
theory here put forward, that all the forms which have an archi- 
meric segmentation as the fundamental basis of their organisation, 
