276 Proceedings of Royal Society of Edinburgh. [sess. 
tlieir origin prior to the formation of the new axis, will be gradu- 
ally moulded into bilateral form. 
These three segments may be conveniently designated the pro- 
tomere, mesomere, and metamere, and their coelomic cavities, proto- 
coele, mesocoeles, and metaccele respectively, and although the first 
and last are primarily unpaired, yet the animal being now bilateral 
there will be a constant tendency for them to assume a paired or, 
at least, bilateral condition.* Thus the primitive opening of the 
protoccele to the exterior may become paired, as in the ‘ proboscis- 
pores,’ or even the whole protoccele as in Sagitta, and the metacoele 
probably became very early separated into two by the backward 
progression of the alimentary canal until the anus was terminal. 
Primitively, each of the archicoeles will open to the exterior by a 
monocytic egestive opening, in connection with which an ecto- 
dermal excretory canal will be secondarily acquired, thus forming 
primitive nephridia.f On the assumption of bilateral symmetry the 
protocoelic and metacoelic nephridia will become secondarily paired, 
and as the gonads become confined to the metacoeles (as pointed 
out below), the metacoelic nephridia alone will continue to function 
as gonaducts. These three pairs of ar chi- nephridia are to be found 
surviving in such varied organs as proboscis-pores, collar-pores, 
nephridia, and even as oviducts. Those forms which retain the 
mesocoelic pores (collar-pores of Balanoglossus, and Cephalodiscus , 
and stone canal of Echinodermata ), as a rule, have the metacoelic 
pores completely metamorphosed into oviducts, whereas those forms 
which lose their collar-pores ( Brachiopoda , Phoronis ), as a rule, 
retain the metacoelic nephridia with gonaducal functions. 
Let us now inquire into the condition of the coelomic pouches in 
this Stage III.* The protocoele is pre-oral, and is therefore in the 
most disadvantageous position for the direct supply of nutrition 
from the alimentary canal, and the metacoeles, enveloping the gut 
for their whole length at a part where digestion is effected, will 
be in the most advantageous position. Their walls will therefore 
be bathed with nutritive fluids supplied direct from the alimentary 
processes. The mesocoeles will in this respect, as in that of location, 
* For figures of Stage II., see Quart. Journ. Micros. Science , vol. xxxviii. 
p. 327, and of this Stage III. on p. 328. 
t See Zool. Anzeiger , Nos. 501, 502, and 503. 
