XXVI INTRODUCTION. 



dage i8 very conspicuous in the common Bicellaria ciliata ; 

 I have also noticed it in Buyula. There is nothing in the 

 structure to indicate that it differs physiologically from 

 the rest of the sac. It may possibly have a merely mecha- 

 nical function in promoting and assisting the constant 

 flux and reflux of the food within the digestive cavity. 



Specialization reaches its height in the digestive system 

 of such forms as Bowerbankia and Bugula, and its mini- 

 mum probably in that of Eucratea and Valkeria (Wood- 

 cut, fig. 33, p. 550) ; and between the two extremes there 

 lies a rather wide range of structural difference. In the 

 latter we have a perfectly simple condition of the various 

 parts, and the process of digestion is effected, so to speak, 

 by a small number of agents. 



The details of the digestive process may be studied to 

 the greatest advantage in many of the Polyzoa, the walls 

 of the cell being often so transparent as to permit the 

 most thorough investigation of all that is passing within. 

 In the pharyngeal chamber (where this is differentiated) 

 the particles of food gather and are sifted ; when a certain 

 quantity has accumulated, it is forcibly ejected into the 

 oesophagus, and hurried downwards by the contractions 

 of the tube into the stomach. Lodged in the true diges- 

 tive sac, it comes under the influence of the strong peri- 

 staltic action of its muscular walls, and is driven to and 

 fro by its incessant sweep with a kind of rhythmical regu- 

 larity throughout the length of the cavity. While this 

 mechanical treatment is in progress, the biliary glands 

 pour in their secretion and the food takes on its rich brown 

 colour. At the same time the indigestible elements are 

 gradually eliminated and gather near the pyloric orifice, 

 where they are kept rotating for a while by the action of 



