101 
Functions of the Sponge. 
pagated an important error, which has pervaded the works of 
zoologists for half a century, and greatly retarded the progress 
of this interesting branch of comparative anatomy. 
Before examining the structure of the sponge, I had fre- 
quently seen, with the assistance of a microscope, the currents 
established in sea-water, by the rapid vibration of the ciliated 
tentacula of many Polypi, particularly of the Sertularia and 
Alcyonia , and likewise by the ciliated circular margins of such 
as possessed no tentacula. And, as every writer since the time 
of Marsigli agreed in considering the round apertures on the 
surface of the sponge as the passages through which nourish- 
ment is conveyed to the animal, I naturally began by a careful 
examination of these canals, in a variety of sponges. But, in- 
stead of finding any ciliated margins, or distinct polypi, within 
these large tubes, or any apparatus sufficient to create a current 
into them, I found them, particularly in the Spongia panicea , 
where they are wide and distinct, lined throughout their whole 
winding and anastomosing course, with a smooth, soft, glistening 
membrane. This transparent colourless membrane was very 
evident at the angles of separation between the branches of the 
internal canals ; for at these places few of the horny fibres shot 
into it, and it could there be raised by the point of a needle ; but, 
throughout the rest of the tube, it appeared stretched from fibre 
to fibre, and so firmly connected with the axis or skeleton of the 
animal, that it could not possibly contract so as to empty the 
whole of the internal canals, without a general contraction of the 
entire sponge. But as I knew already that the animal never 
contracted its body, nor could be forced to do so by the strong- 
est irritants, I found it impossible to explain the power of suction 
ascribed to these canals by any theory ; and, in this dilemma, I 
had recourse to the microscope, well assured, that if currents 
really passed to and fro through the round apertures, they 
might be seen by the same means which had so often detected 
the currents of much smaller zoophytes. 
In the month of November last, I therefore put a small 
branch of the Spo?igia coalita , with some sea-water, into a watch- 
glass, under the microscope, and, on reflecting the light of a 
candle up through the fluid, I soon perceived that there was 
some intestine motion in the opaque particles floating through 
the water. On moving the watch-glass, so as to bring one 
