170 ARCHICELOMATA. 
can be recognised as in other drchicalomata. Thus there is an un- 
paired epistomial cavity, a pair of lopbophoral cavities and a large 
trunk-cavity partially divided up by a ventral mesentery and certain 
bands. The trunk-cavity opens by paired tubes or zepfhridia into the 
mantle-cavity. Its walls also form the muscles and the gonads. The 
muscles are numerous and well developed, mainly for moving the 
shells and peduncle. The gonads and the trunk-cavity spread into 
the mantle. 
There seems a somewhat indefinite blood-system with a contractile 
heart situated dorsal to the stomach. The nervous: system is a ring 
round the cesophagus with dorsal brain and ventral subcesophageal 
ganglia, which latter are the larger. Numerous nerves supply the parts 
of the body. 
The sexes are separate and the development is larval. Most 
brachiopod larvee are pelagic and have three segments. 
PHYLUM ARCHICCQZLOMATA. 
The five preceding types represent the five most impor- 
tant divisions of this diverse phylum. The phylum includes 
the most primitive and simplest representatives of the truly 
coelomate animals. They are usually described as wn- 
segmented, but there can be discerned in them, with more or 
less clearness, a primitive avchimeric segmentation into three 
parts. The first is anterior to the mouth or pre-oral, and 
the other two are post-oral. They may be called the 
protomere, mesomere, and opisthomere. They are probably 
represented in the segmented worms by the fvostomium, the 
peristomium, and the rest of the body respectively ; hence 
these differ from the Archicwlomata in having the opistho- 
mere divided into a great number of segments or mefa- 
meres, oy metamerically segmented. 
In addition the Archicelomata usually have a nervous 
system, often in continuity with the ectoderm, a dorsal 
brain, an cesophageal nerve-ring and usually a ventral pair 
of ganglia. The ccelom retains its primitive relationships 
and any of the segments may have ciliated tubes to the 
exterior. The vascular system, if present at all, is a series 
of heemoccelic sinuses and the archimeric heart is, if present, 
dorsal to the alimentary canal. 
All have more or less primitive methods of feeding; they 
are mostly pelagic, sedentary or burrowing, and are modified 
accordingly. 
