CLASS II. PORIFERA. 649 
when detached from the mass of its fellows, enjoys the faculty of motion by the extension of its 
substance in various directions. 
A glance at a piece of common sponge will show that its surface is everywhere perforated 
with an infinite number of minute holes, among which a 
considerable number of large openings are scattered. When 
a sponge is examined in a living state, a rapid stream of 
water may be observed issuing constantly from these larger 
orifices. This excurrent stream of water is rendered observ- 
able by the fact that it bears with it a number of minute 
particles from the interior of the sponge. This water is im- 
bibed through the minute pores distributed in such profusion 
over the entire surface of the sponge ; after passing through 
these, and traversing the cavities formed in CA^ery direction 
by the reticulated structure of the mass, it is collected into 
canals, by which it is finally conducted to the larger open- 
ings of the surface and then thrown out. 
The primary objects of this continued flow of water 
through the substance of the sponge appear to be two-fold ; 
first, the conveyance to the individual cells of which the 
living portion of the sponge consists, which maybe regarded 
as so many stationary animalcules, the minute particles of 
nutritive matter necessary for their support and that of the 
general mass ; and, secondly, the removal of fecal matter from the interior of the sponge. But 
nutrition and the removal of efliete materials are not the only purposes to which it is applied : 
respiration, which, judging from analogy, is as necessary to the sponges as to other animals, must 
be effected by the medium of this current ; and it also fulfills a very important part in the 
propagation of the species. 
This latter is effected in various ways. In some cases, little ciliated gemmules are produced in 
the gelatinous mass coating the fibers of the sponge ; and after a certain period, becoming detached 
from the parent, are borne out through the large orifices by the action of the current already de- 
scribed. After this exclusion they swim about for some time, presenting a pretty close resemblance 
to some of the infusorial animalcules. But this life of freedom is not of very long duration ; the 
little gemmule selects its place of attachment, fixes itself, and gradually becomes developed into 
a perfect sponge. 
In the genus Spongilla, a somewhat diflferent mode of reproduction occurs. Seed-like bodies 
are produced in the substance of the sponge, and always in the central or first-formed portion. 
These, in their earliest stages, consist of several cells, merely united together into a globular 
or ovoid mass, lying freely in cavities of the substance of the sponge. By degrees this mass 
of cells acquires a more definite form, and becomes enveloped in a capsule, on the surface of 
which, after a time, a finer crust of silicious spicula is developed. When the cells are pressed 
out of the capsule, under water, they soon swell out and burst, the germs contained in them 
being gradually diffused over the bottom. These are of very minute size, the largest not 
measuring more than the one three-thousandth part of an inch in diameter. In form they pre- 
sent some resemblance to the corpuscles of the blood. In a few days the germs are found to 
have collected into separate groups, each inclosed in a mucilaginous substance. From these 
germs active animalcules are produced, exactly resembling the cells of which the gelatinous sub- 
stance of the mature sponge is composed, and possessing the same power of locomotion by the 
extension of different parts of the body, yet even in a greater degree. 
As might be expected from the structure of these animals, they manifest but httle indication 
of any general sensibility, though it has been stated that a shock, by which the entire mass is 
simultaneously moved, will produce a very distinct effect upon it. 
Sponges groAV attached to almost every thing which may serve them as a point of support 
whether fixed or floating ; some cover rocks, shells, and other submarine objects with a close 
Vol. II.— 82 
THE LIVING SPONGE MAGNIFIED. 
