INTRODUCTION. 



XXV 



fig. xi./, /). The swimming-bell (the float and natatory 

 organ, and the most striking feature of the structure) is 

 partially closed below by a filmy membrane or veil, in the 

 centre of which there is a circular aperture (Woodcut, 

 fig. xi. e). Its margin is furnished with a larger or smaller 

 number of tentacles, and in many cases with a series of 

 small sacs, in which one or more refractile spherules are 

 enclosed. The latter are in all probability simple organs 

 of sense ; but of what nature, we are not in a position to 

 determine (Woodcut, fig. xii.) . Their function has been 



Fig. xii. 



,(. The t-ireular c.uml. />. The ocellus, c. The sac or litlioi-jsi. c'. The 

 spherule of lime. 



