viii ROTIFERA, POLYZOA, AND BRACHIOPODA 187 



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

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

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

 tractile 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 (///) which act also as reproductive ducts, 

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

 times separate, sometimes united. 



The Brachiopoda are all marine. They are widely distributed geo- 

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

 twenty-nine hundred fathoms. At the present day the class includes 

 only about twenty genera and a hundred species, but in former geo- 

 logical periods the Brachiopoda were much more numerous, 106 genera 

 being known from the palaeozoic rocks, there being nearly two thou- 

 sand fossil species. 



Lingula pyramidata occurs in sand at or near low water from Chesa- 

 peake Bay to Florida. Our common northern species, Terebratulina 

 septentrionalis, lives north of Cape Cod, attached to rocks in from ten 

 to fifty fathoms. 



