100 VIGNETTES FROM INVISIBLE LIFE. 



slipper- shaped ciliated infusorians. In this stage 

 they are seen swimming freely about within the 

 membranous sac of the parent ; and here again, as 

 with the statoblasts, one is at a loss to understand 

 how they are to get out. The sac is closed organi- 

 cally round the two ends of the alimentary tube, 

 and any rupture of it would, by letting out the 

 internal fluid, be fatal to the creature's existence. 

 At any rate, an important purpose is served by 

 confining them for any length of time within it ; 

 for here their delicate structure is beautifully pro- 

 tected and nourished, and we see them grow and 

 pass from stage to stage of development until the 

 outline of the several adult organs are visible and 

 all seems ready for their full expansion. Yet here 

 they remain ; and I cannot find that any one has 

 ever seen them escape except by means of artificial 

 rupture of the sac. I have found them free in open 

 water, and, bottling some up once, thought I had 

 found a new species of infusoria. The microscope, 

 however, soon undeceived me, by revealing the im- 

 mature and incipient structure of a Polyzoon. It is 

 probable that they only escape or are born with the 

 dissolution of the parent, as is the case with the 



