HALF AN HOUR WITH SEA-MATS AND SQUIRTS. 143 



We have already referred to the comparatively 

 highly endowed character of these minute creatures. 

 When microscopically examined, each individual is 

 seen to be enclosed in a double walled bag or sac, 

 of which the outer one is the horny case to be seen 

 in dried specimens. The inner wall is mem- 

 branaceous, and has two openings, one of which is 

 the mouth and the other the anus. The former is 

 surrounded by cilia or tentacles, by means of which 

 the animal not only breathes, as with gills, but also 

 produces currents in the sea-water, which bring to 

 it its food. Both the mouth and tentacles can be 

 wholly or partially withdrawn into the aperture, by 

 means of a muscle provided for the purpose. When 

 obtained alive, a colony like that of the Flustra 

 hispida (Fig. 71), after it has been plunged into 

 fresh sea-water, seems to interpose a pale, thin 

 cloud betwixt its irregular surface and the spectator's 

 eye. Let the vessel in which it has been placed, 

 however, receive a shock, and this cloud is instan- 

 taneously dispelled. The cloudy appearance has 

 been produced by the ciliated animals, which had 

 been drawn out of their cells to revel in the fresh 

 sea-water, and the misty shade by the movement 

 of their cilise. So timid and highly endowed with 

 caution are these minute animals, that they sink 

 into concealment on the apprehension of danger, 

 and show by their reappearance their relief from 

 alarm. Some naturalists are of opinion that this 

 extreme sensitiveness partakes much of the charac- 



