PEOTOZOA AND SPONGES. 433 



interesting. That great bladder undergoes changes 

 besides those gradual alterations of place which are 

 dependent on the general form. It slowly but mani- 

 festly inci'eases in size up to a certain extent, when 

 it rather suddenly diminishes to a point, and im- 

 mediately begins to iill again, as slowly as before. 

 These alterations go on with some regularity, and 

 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 sur- 

 rounding tissue ; which fluid, when full, the bladder 

 discharges by a sudden contraction of its outline. 

 But whither the fluid goes it is difiicult 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 

 SoUfera — 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 

 impregnated with the products of digestion being thus 

 collected and then difi'used throughout the soft and 

 yielding tissues. 



The smaller bladder-like spaces that you see in 

 considerable numbers in the substance of the animal, 

 are collections 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 inclosing membrane, but to be merely 

 casual separations of 'the common substance, such as 



