AKD PHYSIOLOGY OP THE SPONGIAD^. 
799 
continuously as in the vegetable creation, but at intervals, and modified in the degree of 
its force by the instincts and necessities of the animal. And it may be readily seen that 
the faculty of inhalation is exercised in two distinct modes ; one exceedingly vigorous, 
but of comparatively short duration, the other very gentle and persistent. In the exertion 
of the first mode of inhalation, that is during the feeding period, a vast number of pores 
are opened, and if the water be charged with a small portion of finely triturated indigo 
or carmine, the molecules of pigment are seen at some distance from the dermal mem- 
brane, at first slowly approaching it and gradually increasing their pace, until at last 
they seem to rush hastily into the open pores in every direction. In the meanwhile the 
oscula are widely open, and pouring out with considerable force each its stream of the 
excurrent fluid ; and if the reflexion of one of the horizontal portions of a window-frame 
be brought immediately over an excurrent stream, it will frequently be seen that the 
surface of the water is considerably elevated by its action, even although the osculum be 
half or three-fourths of an inch beneath its surface, and this vigorous action will some- 
times be continued for several hours, and then either gently subside or abruptly termi- 
nate. Occasionally a cessation of the action may be observed in some of the oscula 
while in others it is proceeding in its full vigour, and sometimes it will be suddenly 
renewed for a brief period in those in which it had apparently ceased. These vacillations 
in the performance of its functions are always indicative of an approaching cessation of 
its vigorous action. AVhen the vivid expulsion of the water has ceased, the aspect of the 
oscula undergoes a considerable change; some of the smaller ones gradually close 
entirely, while in the larger ones their diameter is reduced to half or one-third of what 
it was while in full action. Simultaneously with the decline in the force of the excur- 
rent action the greater portion of the pores are closed, a few only, dispersed over the 
surface of the sponge, remaining open to enable the gentle inhalation of the fluid to be 
continued, which is necessary for the aeration of the breathing surfaces of the sponge. 
The breathing state of inhalation appears to be very persistent, and I have rarely failed 
in detecting it when I have let a drop of water charged with molecules of indigo quietly 
sink through the clear fluid immediately above an open osculum. These alternations of 
repose and action are not dependent on mere mechanical causes, and sponges in a state 
of quiescence may be readily stimulated to vigorous action by placing them in fresh cool 
sea-water, and especially if it be poured somewhat roughly into the pan and agitated 
briskly for a short period ; and this will take place even in specimens that have very 
recently been in powerful action. 
No general law seems to guide the animal in the choice of its periods of action and 
repose, and no two sponges appear to coincide entirely in the time or mode of their 
actions. In fact, each appears to follow the promptings of its own instinct in the choice 
of its periods of feeding and repose. 
In the littoral sponges there is a third condition of the animal, and that is during 
its exposure to the atmosphere in the intervals between high and low water, and in some 
sponges the pores and oscula are both completely closed. But this condition does not 
