and Functions of the Sponge . 337 
small degree of irritability, by which they gradually contract 
the dimensions of the animal when it is irritated, and thus force 
out the water from its canalsf{2?eo&. auf N. II. 1819, p. 33.) ; 
although, in his experiments, he could not excite them to the 
slightest perceptible motion ; and in most of the known species 
these fibres are composed of minute siliceous tubes, which scratch 
glass and resist the action of the blowpipe. Lamarck, reason- 
ing from mere analogy, maintains, that every species of sponge 
possesses distinct polypi, closely resembling those of alcyonia, 
projecting from its surface ; and that these two genera of zoo- 
phytes differ only in the greater or less density of their gelati- 
nous matter ( An. sans Vert. t. ii. p. 348-9.) ; although his coun- 
tryman Jussieu, nearly a century ago, by desire of the French 
Academy, examined with the microscope the Spongia ramosa , 
fresh from the rocks on the coast of France, and reported, that 
he could discover no kind of polypi in that animal ( Mem. de 
V Ac , 1742) ; and the accuracy of Jussieu’s observations has been 
confirmed on a great variety of sponges, by every succeeding 
observer, as by Cavolini, Lamouroux, Schweigger, & c. It was 
scarcely consistent in Cavolini to consider the gelatinous matter 
as the muscular system of this animal ( Abhand. uber Pfianz-tlu 
SprengeVs edit p. 124-6.), after he had repeatedly tried in vain 
to excite it to contract. One naturalist, well acquainted with the 
characters and habits of these animals, infers from analogy, that 
they possess nerves ( Phil, of Zool. vol. i. p. 45.) ; while another, 
who has likewise studied them in the living and dried state, main- 
tains, that they are animals which possess no organ whatever, either 
for growth or generation ( Lamouroux Hist des Polyp, p. 14). 
From observing the canals of the sponge constantly empty, or 
filled only with water, Lichtenstein was led to believe this sub- 
stance to be merely a dead mass of the empty tubes of alcyonia, 
remaining after the decayed polypi had been washed out ( Shriv. 
af Nat Set Kiob. 1794). Blumenbach, and some other natu- 
ralists, apparently not aware of the close similarity of the fib- 
rous axis of the sponge to that of some zoophytes, already known 
to possess polypi, and its dissimilarity to that of any known 
plant, and obviously not acquainted with the rapid currents and 
feculent discharges from its orifices, described by Ellis, Schweig- 
ger, Bell, &c. still regard the sponge as a plant, and consequent- 
