with some Remarks on the Nature of the Spongiæ Marine. 389 
the bright and powerful rays of the sun, and while they were daily increasing 
in their green tints, to be covered with a great number of bubbles; thinking, 
however, that they were only air-bubbles escaping from some insect, or some 
molluscous animal or other, which so frequently lurks within the canals of the 
Sponge, I carefully chose two or three fresh specimens that were entirely free 
from any parasitical animal; I placed them in a dish of water, which was 
frequently changed, and submitted them to the direct action of the light of 
the sun, by allowing the brightest rays to enter through a giass window and 
to fall directly upon them; every day bubbles of gas continued to be disen- 
gaged from these Sponges, but only in any considerable quantity when ex- 
posed to the most luminous sunshine. Because plants in the solar light usu- 
ally evolve oxygen, which is principally derived from the decomposition of 
carbonic acid gas. I therefore naturally conceived that these bubbles of gas 
were most likely mere bubbles of oxygen*, which are so commonly given out 
by all plants when put under water and subjected to the same influence of 
light, and that they might thus prove, in some measure, confirmatory of my 
opinion as to the vegetable nature of this Sponge. 
But I must make a more particular statement respecting the currents of 
water that have been noticed flowing into and issuing out from the interior of 
the Spongilla, and which are analogous to those described by Professor Grant 
and other authors, as constantly occurring in the Spongiæ Marine, and 
esteemed by them the strongest evidences of their supposed animality. These 
currents I have repeatedly observed in most living masses of the River Sponge, 
. and have proved their actual presence in various ways. My attention was 
* Since plants absorb carbonic acid gas, and exhale oxygen: but, on the contrary, animals absorb 
oxygen and give out carbonic acid gas; if these bubbles on the application of a proper test were proved 
to be oxygen gas, this would fairly be conclusive of the Spongilla fluviatilis being a plant. Though, on the 
other hand, if these bubbles be chemically found nitrogen (azote), this would not decide the present 
species to be an animal, because it has been lately shown (by Dr. Daubeny in the Phil. Trans. for 
1836, and still more recently by Mr. Rigg in the Phil. Trans. for 1838,) that even plants, in certain 
stages and under certain circumstances, not unfrequently evolvé nitrogen. And if in such case we 
can determine by means of chemical tests, that this Sponge actually decomposes carbonic acid gas 
under the influence of solar light, we should then have arrived at a safe conclusion respecting its vege- 
tability. Surely similar chemical experiments might be instituted to demonstrate the true nature of 
the Marine Sponges. During the time I was making the experiments detailed in the text I had no 
opportunity of proving these questions. 
