I70 ARCHICCELOMATA. 



can be recognised as in other Ai'chicxloinala. Thus there is an un 

 paired epistomial cavity, a pair of lophophoral cavities and a largi 

 trunk-cavity partially divided up by a ventral, mesentery and certaii 

 bands. The trunk-cavity opens by paired tubes or nephridia into th< 

 mantle-cavity. Its walls also form the muscles and the gonads. Th( 

 muscles are numerous and well developed, mainly for moving th( 

 shells and peduncle. The gonads and the trunk-cavity spread int( 

 the mantle. 



There seems a somewhat indefinite blood-system with a contractile 

 hear/ situated dorsal to the stomach. The nervous system is a rin^ 

 round the oesophagus with dorsal brain and ventral suboesophagea 

 ganglia, which latter are the larger. Numerous nerves supply the part; 

 of the body. 



The sexes are separate and the development is larval. Mos 

 brachiopod larvae are pelagic and have three segments. 



PHYLUM ARCHICGELOMATA. 



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 un- 

 segmented, but there can be discerned in them, with moreoi 

 less clearness, a primitive archimeric 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, inesomere, and opisthomere. They are probably 

 represented in the segmented worms by the prostomium, the 

 peristommm, and the rest of the body respectively ; hence 

 these differ from the Archiccelomata in having the opistho- 

 mere divided into a great number of segments or meta- 

 meres ^ or metamerically segmented. 



In addition the Archicoelomata usually have a nervous 

 system, often in continuity with the ectoderm, a dorsal 

 brain, an oesophageal nerve-ring and usually a ventral pair 

 of ganglia. The coelom 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 haemocoelic 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. 



