2i 8 POLYZOA. PART m. 



SECTION IX. 



BRYOZOA, OR POLTZOA. 



A BRYOZOON is a microscopic polype, inclosed in an 

 open horny or calcareous sheath, out of which it can 

 protrude and draw in the anterior part of its body. It 

 is seldom or never seen alone, on account of its tendency 

 to propagate by budding. When the buds spring from 

 the sides of the sheath or cell, it is known as the Sea 

 Mat, or Flustra. The Flustra, which is common on our 

 coasts, spreads its hexagonal cells like a delicate net- 

 work over sea-weeds, shells and other marine substances. 

 Sometimes the polypes are so closely arranged on both 

 sides of a leaf that a square inch may contain 1,800. 

 In the calcareous genera, Eschara and Cellipora, the 

 cells have a lid movable by two muscles, so that the 

 polypes can close the orifice, and shut themselves in. 



In the greater number of the Polyzoa the polype 

 has a cylindrical form, a mouth at its anterior extre- 

 mity surrounded by an annular disk, which forms the 

 roof of the internal cavity containing the stomach and 

 the other digestive organs. The disk is furnished with 

 eight, ten or a greater number of tubular tentacles, 

 which surround the mouth, their tubes being continua- 

 tions of the internal cavity below. The mouth leads 

 into a funnel-shaped space, separated by a valve from 

 the gullet ; and the gullet ends in a capacious stomach. 

 Short vibratile cilia are arranged like a fringe on the 

 opposite sides of each tentacle, which form two currents 



