380 EVENINGS AT THE MICKOSCOPE. 



intermission, and without the slightest recognisable rale or 

 order. The projections are obliterated or exaggerated ; 

 the sinuosities are smoothed, or deepened into gulfs, or 

 protruded into promontories ; firths form here, capes 

 there ; but not by starts, but evenly, and with sufficient 

 rapidity to be appreciable to the eye while under actual 

 observation ; though the alterations are more striking if 

 you take your eye off the object for a few seconds, and 

 then look again ; and still more so if you try to sketch 

 the outline. Individuals vary greatly in dimensions ; this 

 specimen is about one hundred and twentieth of an inch 

 in long diameter, but others I have seen not more than 

 one-tenth as large as this, and some twice as large. 



Disregarding now this peculiarity of change of form, 

 which has procured for it the name of the old sea-god that 

 was so difficult to bind, we will concentrate our attention 

 on some other points not less interesting. That great 

 bladder undergoes changes besides those gradual altera- 

 tions of place which are dependent on the general form. 

 It slowly but manifestly increases in size up to a certain 

 extent, when it rather suddenly diminishes to a point, and 

 immediately begins to fill again, as slowly ■ as before. 

 These alternations 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 surrounding tissue ; 

 which fluid, when full, the bladder discharges by a sudden 

 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 con- 

 tractile bladder is characteristic of the extensive classes 

 Infusoria and Rotifera — 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 from whence it was taken up, and from 



