BRYOZOA. 149 



to exercise a certain caution before emerging from their cells. One 

 or more of the tentacles have been seen to be protruded and turned 

 over the side of the cell, as if to ascertain the presence or absence of 

 an enemy,* 



I must now proceed to describe these tentacula (c, c), which are 

 the means whereby the Bryozoa obtain their food. Tlicy differ 

 considerably from the corresponding tentacula in the Hydrozoa and 

 Anthozoa, in being stiffer and provided with vibratile cilia. These 

 cilia are arranged on opposite sides of tlie tentacle, along which sides 

 they occasion, by their active vibration, opposite currents of the 

 surrounding water : they vibrate, not in the plane of the arms, but at 

 right angles with it, and bend obliquely in the form of a hook.j In 

 some species a few fine hair-like processes, which are motionless, 

 project from the back of the tentacula. The action of the tentacular 

 cilia appears to be under the control of the animal, and they are 

 sometimes seen completely at rest. The arms are tubular throughout, 

 and have an aperture at each extremity. The ring upon which they 

 are set forms a projecting edge around the mouth. The particles of 

 food are carried down the inner surface of each arm, and the mouth 

 and pharynx expand to receive such as are appropriate, as if by an 

 act of selection. The rejected particles pass out between the bases of 

 the tentacula, or are driven off by the centrifugal currents. 



In the fresh-water Alci/oneUce and Cristatelloi the mouth is pro- 

 vided with a tongue- shaped process covered with vibratile cilia. 

 Nordmann describes each of the eight ciliated tentacles in the Tendra 

 zostericola as being traversed by two longitudinal canals.J 



The pharynx in all Bryozoa is less dilatable than is the mouth 

 of the Hydra or Actinia. The constriction of the pharynx, by 

 which the food is driven into the oesophagus, is a very well-marked 

 action. The parietes of the gullet consist of three layers of which 

 tlie two outer ones seem to be muscular, and are thick in Tendra : 

 the inner layer is a thin epithelium over which a network of de- 

 licate canals in a polygonal pattern is spread. The cardiac orifice 

 (^jig. 71, ^) seems to project into the oesophagus upon a valvular pro- 

 minence ; it opens into a small globular cavity (/<), which has the 

 construction of a gizzard : the interior of this cavity is lined by a 

 strong epithelium, the cells of which project into the cavity like 

 pointed teeth, and the food is subject to comminution in this cavity. 

 "With the gizzard is associated, as in birds, a distinct glandular com- 

 partment of the stomach {i) ; but this is situated between the gizzard 

 and intestine, not between the gizzard and oesophagus : its walls are 



* XXXV. p. 414. t XXXV. p. 411. % CXX. p. 187. 



L 3 



