CILIOBRACHIATE POLYPES ; BOWERBANKIA. 529 



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 seta, 

 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 Ciliobrachiate Polypes; and it is 

 evidently a provision 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 com- 

 pletely defends it when retracted or withdrawn into the cell, as 

 will presently appear. 



1101. For the purpose of retraction, two distinct sets of 

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

 other upon the cell. The first set arises from the bottom of the 

 cell, and is attached to different parts of the digestive tube. 

 The second set arises from the upper part of the firm portion 

 of the cell, and is attached to the flexible continuation of it. 

 When the animal wishes to withdraw itself, the tentacula are 

 first closed together into a straight line ; and immediately the 

 whole digestive tube begins to descend, its different parts being 

 folded upon one another, and drawn back within the integument, 

 which is inverted over them so as to form a close sheath around 

 the tentacula. When the animal is completely retracted, the 

 oesophagus is bent upon itself like the letter S ; and the tentacula 

 lie straight in its axis, inclosed in the inverted tube of integument, 

 their extremities being on a level with the top of the unyield- 

 ing portion of the cell. By the time that the end of the arms 

 are on a level with the base of the setce, the retraction of the 

 latter commences. This is performed exactly in the same 

 manner as that of the tentacula ; and the flexible portion of the 

 cell is inverted over them, so as to form a complete envelope. 



