m 
and Functions of the Sponge. 
paratively rare, and remains only about an hour exposed to the 
atmosphere, is, at Prestonpans Bay, one of the most abundant 
and hardy species, hanging in thousands from the roofs of the 
most exposed caves, and remaining uncovered for three hours, 
during moderate tides. Although most of the species are thus 
periodically exposed to the air, this is no way necessary to 
their existence ; for the same species which grow nearest to the 
shore, are likewise inhabitants of the deepest water, as is seen by 
their being cast ashore attached to stones, shells and fuci, after 
storms, and by their frequent appearance in dredges employed 
in deep-water ; and we frequently find specimens of the Spongia 
papillaris , and Spongia tomentosa or urens , lining the sides of 
limpid pools during the retreat of the tide, where one-half of the 
animal above the surface of the water is subjected to long and 
regular visitations of the atmosphere, while the other half be- 
neath the surface of the pool is never exposed to its influence 
from birth till death. 
In all these sponges, we can not only perceive distinctly the 
currents rushing from the fecal orifices ; but, with a little atten- 
tion, we can likewise perceive, with the naked eye, the pores on 
their surface, by which the water enters into the internal canals ; 
and, in some of the species where the pores are large, we can see, 
without the assistance of a glass, particles of matter drawn into 
the pores. I have not met with any kind of living sponge, in 
which the pores and fecal orifices were not visible, although one 
might be led to suppose, from the statements of Schweigger, and 
other naturalists, that some species of this animal want these 
openings, and are entirely covered with a gelatinous crust, 
through which water soaks by a kind of infiltration. When we 
place a branch of the Spongia coalita , in a watch-glass, with sea- 
water under the microscope, and look attentively along the side 
of the branch, at a distance from any fecal orifice, we see the 
small particles suspended in the water, beneath the surface, rush 
with an increasing velocity towards every part of the smooth sur- 
face of the branch ; the smaller particles pass in and disappear, 
while the larger are arrested, and cling to the side of the sponge, 
where, in the natural abode of the animal, they would remain, 
till washed away by the ceaseless motions of the sea. A thin 
portion cut from the surface of the coalita , and viewed through 
the reflecting microscope, is seen to be every where pierced with 
