vin BRACHIOPODA 187 



The body of the animal occupies a relatively small part of the space 

 contained in the interior of the shell, and lies towards the posterior end. 

 The rest of the space is lined by a pair of folds of the body- wall, the 

 mantle-folds (Fig. 99, d.m. v.m. ), the edges of which are beset with 

 minute setse. In the space (mantle-cavi(y) lined by these mantle- 

 folds lies a lophophore (lph.\ usually of complicated form, fringed with 

 long ciliated tentacles, and supported in many cases by a delicate, 

 sometimes simple, sometimes complicated, shelly process of the dorsal 

 valve, the shelly loop (Fig. 98, s.l.}. The mouth (Fig. 99, mth.) 

 situated in the middle of the anterior body- wall within the lophophore, 

 leads into a V-shaped digestive canal (st, , int. ) which may or may not 

 terminate in an anal aperture. A heart is present in the form of a 

 contractile sac, and there is a feebly developed vascular system. The 

 central part of the nervous system is in the form of a nerve ring, with 

 ganglia, which surrounds the oesophagus. There is a pair of large 

 funnel-shaped nephridia (nph.) which act also as reproductive ducts, 

 leading from the ccelome to the mantle-cavity. The sexes are some- 

 times separate, sometimes united. 



The Brachiopoda are all marine. They are widely distributed 

 geographically, and live at various depths from between tide-marks to 

 2,900 fathoms. At the present day the class includes only about 20 

 genera and 100 species, but in former geological periods the 

 Brachiopoda were much more numerous, 106 genera being known from 

 the palaeozoic rocks. 



