12& TOILERS IN THE SEA. 



it back again by an outflowing stream, which find 

 an exit in the oscula, or cloacal openings of the 

 sponge. These latter are either dispersed or clus- 

 tered together, according to the species, and are 

 opened or closed at will. This we may term the cir- 

 culatory system of the sponge structure, by means of 

 which water charged with nutriment is inhaled, cir- 

 culates, becomes exhausted, and is ultimately expelled, 

 with the suspended rejectamenta. Dr. Bowerbank 

 remarks that "the power of inhalation appears to 

 be exerted in perfect accordance with the similar 

 vital functions in the higher classes of animals, not 

 involuntarily and 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 membrane, 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 



