and Functions of the S pong e. 119 
pable of moving by changing the shape of their bodies, as the 
ova of the madrepore, gorgonia, and sertularia are said to do, 
but are seen to swim about, by the rapid vibration of the ciliae on 
their surface, while the shape of their body remains perfectly 
unchanged. The pores of the Spongia panicea are frequently 
obliterated in dried specimens, by the hardening of the gelati- 
nous matter into an opaque membrane over the surface ; and I 
have frequently produced the same appearance in other flat spe- 
cies, by drying them before their gelatinous matter had been 
sufficiently extracted by boiling water. This artificial covering 
resembles the gelatinous mantle of medusae drying and harden- 
ing in the summer’s sun, which we have seen strewed over many 
of the shores of Europe, and is probably the same with the com- 
pact crust spoken of by many naturalists as covering different 
species of sponge. 
In most sponges, the currents through the pores, canals, and 
fecal orifices intermit, as we have seen above, without inconve- 
nience, during every recess of the tide ; for no fecal orifice that 
is above water is ever observed to pour forth a stream, even 
though the rest of the animal be entirely under the surface ; and 
it is curious to observe, on the sides of pools, one-half of the ani- 
mal under the surface, carrying on a circulation of water con- 
stant through life, while the other half above the water of the 
pool is subject to frequent and long intermissions. 
A fecal orifice, raised only half above the surface of the water, 
produces a current which has a powerful effect on the particles 
floating near it. When a portion of sponge is confined in the 
same basin of water for about two days, the currents appear to 
have entirely ceased ; but, on plunging it again into water newly 
taken from the sea, they are renewed in about two minutes, and 
continue nearly with their original force ; but I have seldom 
kept sponges alive, in their adult state, for more than a week. 
I have frequently caused the ova to fix themselves on watch- 
glasses, and have reared them for a month. As far as I have 
been able to observe, the animal never intermits spontaneously 
the currents, and renews them again in the same water. In 
ceasing, they are observed to die away gradually ; and no burn- 
ing, or tearing of any part of the animal, causes them to intermit, 
though it hastens the period of their total cessation. A ther- 
mometer placed in the water, and another plunged to an equaj 
