438 POLYZOA ; BOWERBANKIA. 



these appear to be caused by minute follicles or sacs opening 

 from its cavity, in which bile is secreted for assisting the digestive 

 process. A fluid is poured out from them which tinges the 

 whole stomach, as well as its contents, with the characteristic 

 hue of that secretion ; and we may therefore regard these fol- 

 licles as constituting the simplest form of hepatic gland or liver 

 which we meet with in the animal kingdom. It is easy for the 

 comparative anatomist to trace the gradual concentration of these 

 scattered elements, up to the consolidated form in which they pre- 

 sent themselves in the highest grades of organisation (ANIM. 

 PHYSIOL. 356). From the upper part of the stomach, and by 

 the side of the entrance from the gizzard, arises the intestine ; 

 the orifice of which is surrounded by vibrating cilia. This 

 passes up as a straight tube by the side of the oesophagus, and 

 terminates by a distinct orifice outside the circle of tentacula. 



1072. The whole of this complex digestive apparatus floats 

 freely in the general cavity formed by the integument of the 

 animal ; the space between being occupied by a clear fluid, and 

 by the muscles which change the place of the animal in its cell. 

 The cell is formed by the outside of this integument ; of which 

 the lower part is so consolidated by horny matter, as to be nearly 

 unyielding ; whilst the upper third remains flexible. This 

 flexible part consists of two portions ; the lower half being a 

 simple continuation of the rest of the cell ; whilst the upper 

 consists of a row of delicate bristle-shaped processes, or setce, 

 which are arranged parallel with each other round the walls of 

 the cell, and are prevented from separating beyond a certain 

 distance, by a membrane of excessive tenuity, which surrounds 

 and connects the whole. This mode of termination is very 

 common in the cells of the Polyzoa ; and it is evidently a pro- 

 vision for allowing the freest possible motion of the upper part 

 of the body in its expanded state, to which it affords at the same 

 time support and protection ; whilst it completely defends it 

 when retracted or withdrawn into the cell, as will presently 

 appear. 



1073. For the purpose of retraction, two distinct sets of 

 muscles are provided; one acting upon the animal, and the 



