. Animal physiology. Physiology, Comparative; Physiology, Comparative. 114 STRUCTURE AND ACTIONS OF SPONGE. the living state, this net-work is covered with a gelatinous flesh, so soft that it drains away when the sponge is taken out of the water. The mass is traversed, however, by a great number of canals; which may be said to commence in the small pores upon its surface, and which discharge themselves into the wide canals that terminate in the large orifices or vents, which usually project more or less from the surface of the sponge. Through this system of canals, there is continually taking
. Animal physiology. Physiology, Comparative; Physiology, Comparative. 114 STRUCTURE AND ACTIONS OF SPONGE. the living state, this net-work is covered with a gelatinous flesh, so soft that it drains away when the sponge is taken out of the water. The mass is traversed, however, by a great number of canals; which may be said to commence in the small pores upon its surface, and which discharge themselves into the wide canals that terminate in the large orifices or vents, which usually project more or less from the surface of the sponge. Through this system of canals, there is continually taking place, during the living state of the animal, (if animal it may be called,) a circulation of water, which is drawn in from without, through the minute pores, then passes into the large canals, and is ejected in a constant stream from the vents. Of the immediate cause of this movement, no satis- factory account has yet been given; no cilia can be seen, with the most powerful microscopes, to line the canals; and it is certain that no movement of the sponge itself has anything to do with it. But the purpose of this extraordinary circulation, is evidently to convey to the animal the nutriment which it requires, and to carry off the matter which it has to reject. No distinct indications of sensation, or of power of voluntary motion, have been seen in the sponge; so that it has no definite claim to a place in the animal kingdom, beyond that which it derives from its analogies of structure, and from the movement of the germs by which it is reproduced. These, like the germs of the polypes, swim freely about for some time before fixing themselves, by the action of the cilia with which they are covered. 136. No allusion has yet been made to the very interesting and important class which includes those Animalcules that do not belong to the class Rotifera. It seems impossible to arrange them under any of the great general divisions of the animal king- dom ; for, whilst they appear to corre
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