MOLLUSCOIDEA — BRYOZOA 175 



the opening of the chitinous cup. In this expanded condition, 

 the position of feeding, the mouth is seen to occupy the ante- 

 rior end of the introvert ; it is surrounded by a circular ridge, 

 the lophophore, bearing about 14 tentacles. The tentacles sur- 

 rounding the mouth are densely covered with cilia on their 

 inner surfaces and the vibration of these cilia drive currents of 

 water towards the mouth. The particles of food in these 

 water-currents, as well as those caught by the prehensile tenta- 

 cles, pass through the mouth into a wide pharynx, thence 

 through a short oesophagus into the stomach. It is here that 

 the digestive canal makes its U-shaped bend, for the intestine 

 rises from the forward end of the stomach and extends in an 

 anterior direction, near and parallel to the oesophagus, to its 

 termination in the anus, which is near the mouth, but outside 

 the circle of tentacles. 



There are no blood vessels and no excretory organs. The 

 digested food passes through the digestive canal into the pouch- 

 like body cavity (coelome), which contains a fluid in w^hich are 

 colorless corpuscles, the leucocytes. The movement of this 

 fluid is effected largely by the movements of the animal. The 

 collection of the nitrogenous waste matter is probably carried 

 on largely by these leucocytes. 



The tentacles are the chief organs of respiration; they are 

 hollow, forming narrow prolongations of the coelome and prob- 

 ably effect a transfer of the fresh oxygen of the sea water for 

 the effete gases of the body cavity. 



No nervous system has been traced in Bugula. In some 

 other genera it takes the form of a small, rounded ganglion be- 

 tween the mouth and the anus which gives off nerves to various 

 parts of the body. Aside from the tentacles, which may be 

 organs of touch, there are no organs of special sense in any of 

 the Bryozoa, except possibly the epistome of the Phylactolae- 

 mata ; this is a fold of the lophophore which closes the mouth 

 and may in a manner taste what passes into the mouth. 



Each zooecium is lined internally by a cellular substance, 



