80 - - YOOLOGY. 
sponges of commerce, and which may be regarded as the skeleton of the 
animal: sometimes the skeleton is formed of calcareous or siliceous needles, 
which are beautiful microscopic objects, admirably grouped and arranged. 
In a living sponge, the whole framework is covered and its cavities filled 
with a gelatinous or glairy substance, which is the really living part. 
A mature sponge is, in fact, composed of a vast number of creatures some- 
what resembling Ainlebe, ageregated together, and having their frame- 
work in common. A young sponge, as it comes forth fom the egg of 
its parent, has the faculty of moving about in the water by shooting out 
processes of its soft body, like the Rhizopoda. By-and-by, it attaches itself 
to some rock or other fixed substance, and becomes itself immovably 
fixed, ceasing to possess the power of locomotion. Its plant-like 
framework begins to grow, and the animal mass rapidly increases. 
Every one knows that the surface of a piece of sponge is perforated with a 
multitude of very small openings; amongst which are a number of larger 
ones, and that the inside is full of little cavities. In the live sponge there 
are currents of water continually flowing in at all the smaller openings, 
which are carried through the whole mass, conveying to every part of it . 
the particles of matter which’ serve for nourishment, and also the air for 
its respiration. These currents of water are collected into canals, and 
pass out by the larger openings, carrying with them all the fefuse, 
and many sponge buds, as they may be termed, which become detached 
from the parent, and go forth to found new ae as well as the eggs 
which are occasionally ‘produced. 
Infusoria.—The Infusoria are a class of Protozoa of somewhat higher 
organisation than the Rhizopods and Sponges. They have a mouth and a 
cullet, which, however, does not convey the food into a stomach, but into 
the general internal cavity of the body. Like the other Protozoa, they are 
composed of a gelatinous substance, but it is covered with a skin, and does 
not exhibit that variability of form so remarkable in the Rhizopoda., 
Many of them are also furnished with hair-like organs, called cilia,! the 
motion of which carries them with great rapidity through the fluid in 
which they live, and also creates currents to bring food to the mouth. 
The Infusoria often multiply by spontaneous division, and each half 
‘speedily acquires a mouth and cilia for itself. The multiplication of some 
Infusoria in this way is extremely rapid. One species has been observed 
to undergo division every twenty-four hours, from which would result 
16,384 individuals in a fortnight, and 268,435,456 in four weeks. It 
is from the fact of their occurring in all infusions of animal and vege- 
table substances which stand for a short time exposed to the air, that 
the Infusoria receive their name. There are very many different kinds, 
exhibiting great variety of form and structure. Their multitudes are so 
1¥From Latin cilium, an eyelash, 
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