302 Proceedings of Royal Society of Edinburgh. [sess. 
In the other groups of segmented animals, namely, the Annelida 
and Arthropoda , it is possible to make out a case of the same 
nature. The Arthropoda are greatly modified, especially in regard 
to their mesodermic organs; but as they are very commonly 
regarded as being derived from a segmented £ worm ’ of some 
kind, it will be sufficient for our 
present purpose to take the Archi - 
annelida and Polychceta as the 
lowest truly segmented worms. 
In the morphology of these there 
is a constantly recurring pre-oral 
£ segment,’ the prostomium, derived 
from the pre-oral lobe of the larva, 
intimately associated with which 
is the supra-cesophageal nerve- 
ganglion. This segment is un- 
paired and may bear special appen- 
dages, such as palpi and antennae 
(or tentacles). It is lined by a 
mesodermic wall, and in certain 
forms ( e.g ., Magelona) it comes to 
very closely resemble the epistome 
of some of the Ar chi-coelomata {e.g., 
Rhabdopleura). Its homology with 
the protomere of the archimerically 
segmented forms is supported by 
its structure and development (fig. 
22 , 1 ). 
The segment immediately follow- 
ing the prostomium, the peri- 
stomium or buccal segment (fig. 
22, 2), differs in many respects 
from the segments following after. It contains the post-oral 
nerve-ring and the sub-oesophageal nerve-ganglion, has a definite 
coelomic space, and, in many cases, has certain processes (tentacular 
cirri) which recall to mind the post-oral tentacles of the Archi- 
cortomata. This is especially the case in certain of the sedentary 
or tubicolous forms, such as Magelona. In the Archi- annelida 
m. 
- TEL. 
Fig. 22. — Horizontal section 
through typical Annelid, show- 
ing the arrangement of the 
mesoderm. 
