1897-98.] Dr Masterman on Archimeric Segmentation. 291 
organs, and the metameres show, as already remarked, the archi- 
coelomate features. 
There is no indication at any stage of true metameric segmen- 
tation. 
In certain of the larvae, with well-developed protomere, the 
archi-coelomate central nervous system is present in this organ. 
The mesoderm arises typically by enterocoelic invagination, and, 
according to present knowledge, a single protocoelic pore (water 
Fig. 5. — Diagrammatic hori- 
zontal section through larval 
Cribrella. 
Fig. 7. — Diagrammatic horizontal 
section through Actinotrocha. 
pore) persists, whilst the left mesomere does not open to the ex- 
terior, hut secondarily into the protomere (stone canal). 
The free larval forms will be referred to later. 
Phoronis. 
This essentially sedentary and tubicolous group has been the 
subject of my recent work.* Its free larval stage, Actinotrocha , 
exhibits (fig. 7) the typical archimeric arrangement of the coelome 
in perfection, besides having a prototroch, mesotroch, and meta- 
troch. In all the other features of the Ar chi-coelomata it is typical, 
except that the metacoeles do not, in the larval stage, open to the 
exterior. 
* “ On the Diplochorda,” Q. J. M. 8., August 1897. 
