1897 - 98 .] Dr Masterman on Archimeric Segmentation. 303 
the peristomium is simple, but well-defined and prominent 
(fig. 22, 2). 
Thus, morphologically, the peristomium differs markedly from 
the hind-segments, and ontogeny shows a still greater difference. 
This * segment ’ I would homologise with the mesomere of the 
Arclii-coelomata , whilst all the segments posterior to it are second- 
arily formed from a bilateral segmentation of the metamere or last 
segment (fig. 22, 3). 
In the young of Nerine and other species, the peristomium is 
found to hear long bristle-like provisional setae, which are also 
found in adult fossil forms. The suggestion has been made that 
they are survivals of organs belonging to an unsegmented ancestral 
stage.* For ‘unsegmented, 5 we may say archimerically segmented , 
and they may then be at once compared with the mesomeric setae 
of the Brachiopod larvae. On this view, the three segments of the 
latter are morphologically equivalent to the prostomium, peri- 
stomium, and the rest of the body of the Annelid. Thus, both the 
Brachiopoda and Chcetognatha, as also Balynoglossus, may, in a 
sense, be described as ‘ worms 5 with three segments only, but 
these three are not equivalent to the first three of the Annelid. 
We must rather regard the Annelid as consisting of three archi- 
meric segments, in which the third is hypertrophied and seg- 
mented. 
In the ontogeny of the most primitive Annelida ( e.g ., Poly - 
gordius ), the larval trochosphere has a pre-oral, a post-oral, and a 
peri-anal band of cilia, which we may regard as the prototroch, 
mesotroch, and metatroch respectively. 
The first two have nerve-rings in connection with them, and the 
post-oral nerve-ring apparently forms the peri-oesopliageal ring of 
the adult (fig. 15). 
The trochosphere is thus to a large extent divided into one 
pre-oral segment, the pre-oral lobe, and two post-oral (fig. 15, I., 
II., III.). 
Internally, the cavities of the first two appear to be continuous, 
because the mesoderm in these segments is in the monocytic 
(mesenchymatous) condition. It is only later that the definite 
coelomic lining of the pre-oral lobe or prostomium and that of the 
* Agassiz, A, Annals Lyc. Nat. Hist., JSew York , vol. viii. 
