POLYZOA ; BOWERBANKIA. 439 



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 

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

 latter commences. This is performed exactly in the same man- 

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

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

 The protrusion of the body takes place by a contrary series 

 of processes. The bundle of seta first makes its appearance out 

 of the apex of the cell, and is followed by the flexible portion 

 on which it is set. The tentacula next pass up between the setae 

 and thrust them asunder ; while the integument is seen gradually 

 rolling onwards from around the tentacula. These latter con- 

 tinue to emerge, and the integument to be everted from around 

 them, until the base of the tentacula has risen above the top of 

 the expanded setae ; when the act of protrusion is completed, the 

 tentacula separate and expand, and the cilia commence vibrating. 

 These, actions are generally performed with an extraordinary 

 rapidity ; and it is only when they are executed with unusual 

 slowness, that the several movements concerned in them can be 

 distinguished. 



1074. Upon reviewing the complex organisation of this 

 minute creature, it must be admitted that its mechanical func- 

 tions are executed with a degree of perfection, which cannot but 

 excite our surprise and admiration ; and not less interesting is it 

 to observe the other vital operations exhibited by it, which the 



