140 INFUSORIA.—PORIFERA OR SPONGES. . 
the mouth, becomes most conspicuous. The alimentary par- 
ticles introduced into the mouth commonly have to pass 
down a short canal before they enter the general cavity of 
the body ; and within this cavity a number of minute par- 
ticles are commonly aggregated into a sort of little pellet, as 
may be well seen when Infusoria are fed with carmine or 
indigo. One after another of these pellets being thus intro- 
duced into the interior, which is occupied by a soft sarcode, 
each seems to push onwards its predecessors ; and a kind of © 
circulation is thus occasioned in the contents of the cavity. — 
The pellets that first entered make their way out after a time — 
(their nutritive materials havihg been yielded up), generally — 
by a distinct anal orifice, sometimes, however, by any part of - 
the surface indifferently, and sometimes by the mouth. 
135. The multiplication of Infusoria ordinarily takes place 
by spontaneous fission, precisely after the manner of the : 
multiplication of ordinary cells (§ 33). This process, under 
favourable circumstances, may be performed with such 
rapidity, that, according to the computation of Ehrenberg, no 
fewer than 268 millions might be produced in a month by 
the repeated subdivision of a single Paramecium. Sometimes, 
instead of undergoing subdivision into two equal parts, the 
Animalcule puts forth a bud, which gradually increases, and 
then detaches itself from the parent stock. Whether any- 
thing equivalent to the sexual generation of higher animals 
occurs among Infusoria, is yet uncertain; but recent re-— 
searches afford a probability in the affirmative. 
136. In the tribe of Porrrera, or Sponges, we seem to © 
have the connecting link between Protozoa and Zoophytes. ~ 
For their animality does not lie so much in the individual — 
particles, as in those aggregations which begin to shadow — 
forth that distinction into organs which is carried out more — 
completely among Zoophytes: and there is a large section of 
the last-named group, in which the polypes are connected — 
together, not by’ a regular stony or horny stem, but by a— 
sponge-like mass ; while the extension of the fabric is provided — 
for by the budding out of this spongy portion of it, the 
orifices of whose canals after a time become furnished with — 
polype-mouths. The true Sponge (fig. 81) consists of a fleshy — 
substance, composed of an aggregation of particles of sarcode, 
supported upon a skeleton which usually consists of a net- 
