SPONGES. ] 57 



from the parent sponge. When thus set at 

 liberty, they do not sink by their gravity to the 

 bottom of the water, as would have happened 

 had they been devoid of life ; but they continue 

 to swim, by their own spontaneous motions, for 

 two or three days after their separation from the 

 parent. In their progression through the fluid 

 they are observed always to carry their rounded 

 broad extremity forwards. On examining this 

 part with the microscope, we find that it is 

 covered with short filaments, or cilia, which are 

 in constant and rapid vibration. These cilia 

 are spread over about two thirds of the surface 

 of the body, leaving the narrower portion, which 

 has a whiter and more pellucid appearance, un- 

 covered. They are very minute transparent 

 filaments, broadest at their base, and tapering 

 to invisible points at their extremities: they 

 strike the water by a rapid succession of in- 

 flexions, apparently made without any regular 

 order, but conspiring to give an impulse in a 

 particular direction. When the body is attached 

 by its tail, or narrow end, to some fixed object, 

 the motion of the cilia on the fore part of the body 

 determines a current of fluid to pass in a direc- 

 tion backwards, or towards the tail; but when 

 they are floating in the water, the same action 

 propels them forwards in the opposite direction, 

 that is, with the broad ciliated extremity fore- 



