1897-98.] Dr Masterman on Ar chimeric Segmentation. 305 
phyletic history, and in both its early stages (monocytic) and in 
its latest (muscles, etc.) its walls are discontinuous, and its enclosed 
space is confluent with the blastocoele (or haemocoele). 
Just as the history of Echiuridce indicates that they may be 
metamerically segmented forms, which have secondarily lost their 
metamerism, so that of the Rotifera points to the conclusion that 
they may be descended from archimerically segmented forms, and 
have lost the archimeric segmentation of their mesoderm. 
The recognition of this archimeric segmentation in larval anne- 
lids could be followed up more fully through the ‘collar’ of 
Psygmobranchus, Spirorbis, and 
Pileolaria , which evidently belongs 
to the mesomere, and through the 
peculiar prostomial organs found in 
so many species (fig. 23). 
As regards the nervous system of 
the Annelida , it is evident that the 
supra-oesophageal ganglion belongs 
to the protomeric segment, and the 
sub-oesophageal to the mesomere. Fig. 23.— Larva of Spirorbis (after 
. t • Fewkes), showing archimeric 
As the mesomenc ring disappears m f , . & 
° rr segmentation. 
the Euchorda, the supra-oesophageal 
ganglion is the only part of the annelid nervous system which 
is represented in that of the Eu-chorda. 
In the higher Annelida , the Arthropoda , and the Eu-chorda, 
there is the same tendency to a reduction of the first two archi- 
meric segments, and a gradual migration forwards of the meta- 
meric segments, and at the same time these tend to become 
grouped into regions in which the segments themselves become 
more or less unrecognisable. Thus, the archimeric, or primitive 
pre-oral, head disappears altogether, and its place is taken by a 
metameric secondary head, and in the same way the archimeric 
thorax or mesomere becomes replaced by a metameric thorax. 
It will thus be seen that if the trochophore be held to be the 
larvalised representative of the archi-coelomate type, then the 
Rotifera, Entoprocta, and some smaller groups, such as Echino- 
deres, can claim to be placed in the Archi-coelomata, and the same 
