380 EVENINGS AT THE MICROSCOPE. 



intermission, and without the slightest recognisable rule 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 

 bodyatthe 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 



