PROTOZOA AND SPONGES 421 



we cannot observe them without becoming convinced that 

 it is a process of filling and emptying; that the bladder 

 gradually fills with a fluid which is either secreted by its 

 walls or percolates into it from the surrounding tissue; 

 which fluid, when full, the bladder discharges by a sud- 

 den contraction of its outline. But whither the fluid goes 

 it is difficult to determine ; I have never been able, in this 

 or in any other instance of its occurrence though this 

 contractile bladder is characteristic of the extensive classes 

 Infusoria and JRotifera to see any issue of fluid from the 

 body at the moment of contraction, and therefore conclude 

 that it is discharged into the body, perhaps back again 

 into the tissues whence it was taken up, and whence it is 

 about to be collected again. Hence, it is probably the 

 first obscure rudiment of a circulation; the fluids im- 

 pregnated with the products of digestion being thus 

 collected and then diffused throughout the soft and 

 yielding tissues. 



The smaller bladder- like spaces that you see in consid- 

 erable numbers in the substance of the animal are collec- 

 tions of fluid contained in excavations of that substance, 

 which are called vacuoles, differing from vesicles, inasmuch 

 as they seem to have no proper wall or enclosing mem- 

 brane, but to be merely casual separations of the common 

 substance, such as would be made by drops of water 

 in oil. These vacuoles appear to be connected with the 

 digestive function; for very many of them are not clear, 

 but are occupied with granules more or less opaque, 

 and of exceedingly various dimensions. That these 

 collections of granules are food you will see by this 

 experiment. 



I mingle a little carmine with the water, just enough 



