SPONGES. ]^:5 



gression 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 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, uncovered. 

 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 direction backwards, or towards 

 the tail ; but when they are floating in the water, 

 the same action propels them forwards in the op- 

 posite direction, that is, with the broad ciliated 

 extremity foremost. They thus advance, without 

 appearing to have any definite object, by a slow 

 gliding motion, totally unlike the zig-zag course 

 of animalcules in search of prey. Yet they appear 

 to have a consciousness of impressions made upon 

 them ; for on meeting any obstacle, or striking 

 against each other, they retard a little the motion 

 of their cilia, wheel for a ^ew seconds round the 

 spot, and then, renewing the vibrations, proceed in 

 their former course. 



In about two or three days after these gemmules 

 have quitted the body of the parent, they are 

 observed to fix themselves on the sides or bottom of 

 the vessel in which they are contained ; and some 

 of them are found spread out, like a thin circular 

 membrane, on the surface of the water. In the 

 former case, they adhere firmly by their narrow 

 extremity, which is seen gradually to expand itself 

 laterally, so as to form a broad base of attachment. 



