68 
Proceedings of Royal Society of Edinburgh. [sess. 
ectoderm forms a more or less permanent and connected skeletal 
structure ; endoderm simple and glandular, mostly ciliated — that in 
dorsal collar region forms in most a vacuolated skeletal area, the 
notochord ; and in some are paired endodermal openings to the ex- 
terior in the trunk region, gill-slits ; there are five ccelomic pouches 
usually secondarily filled with connective-tissue, and the trunk 
cavities containing the generative organs ; there may or may not be 
a vascular system and well-differentiated muscular layers ; a meso- 
dermal chondroid tissue, hypertrophied at various parts. Dioecious 
or hermaphrodite, solitary, sedentary, colonial, or gregarious. 
It is undeniable that Plioronis shows close affinities with the 
Brachiopoda and the Gephyrea] and, on the other hand, Balano- 
glossus shows equally remarkable affinities with Echinodermata. I 
believe these resemblances will all eventually find expression in the 
constitution of one large primitive group which we may provisionally 
term the Trimetamera , emphasising their fundamental feature of 
having the mesoderm divided up into a pre-oral and two post-oral 
coelomic pouches, all, primitively, opening to the exterior by 
ciliated pores, the earliest condition of nephro-gonaducts. The pre- 
oral segment is specialised for locomotion and for sense functions, 
and degenerates upon the assumption of a sedentary life. The 1st 
post-oral segment is mainly connected with locomotion and ingestion 
of food, and is usually tentacular, whilst the trunk segment, or 3rd 
post-oral, is concerned with digestion and reproduction. 
From this type, by segmentation of the trunk, are derived the 
polymetamerous forms ( Chcetopoda , Annelida , and Chordata). 
The Trimetamera I believe to he the most primitive Coelomata 
derived directly from the Coelenterata , and they persist in varied 
forms to the present day in the Hemichordata, Echinodermata , 
Brachiopoda, Chcetognatha, unarmed Gephyrea , and possibly the 
Mollusca. 
The division of the body into three segments has been made 
permanent in these forms, because of a peculiar physiological and 
morphological fitness in this number, as seen by a secondary acquire- 
ment by polymetamerous forms of a prosoma, mesosoma, and meta- 
soma, or head, thorax, and abdomen, which can he exactly compared 
(physiologically) to the three segments of the Trimetamera. The 
following table will express the likely relationships : — 
