140 THE YOUNG SCIENTIST. 
Tlie Actinozoa (anemones, etc.) and the 
Hydrozoa (polyps) constitute the Ccelen- 
TERA of Erey and Leuckart. "In all 
these animals," says Professor Huxley, 
"the substance of the body is differen- 
tiated into those histological elements 
Avhicli have been termed cells, and the 
latter are previously disposed in two 
layers, one external aiid one internal, con- 
stituting the ectoderm and endoderm. 
Among animals which possess this his- 
tological structure the Coelenterata stand 
alone in having an alimentary canal, 
Avhich is open at its inner end and com- 
municates freely by this aperture witJi the 
general cavity of the body," and " all (un- 
less the Ctenoi)hora should prove a partial 
exception to the rule) are provided with 
Tery remarkable organs of offence or 
defence, called thread-cells or nemato- 
<3ysts." In describing the Polyps we 
have given illustrations of these weapons. 
The remaining classes, which have been 
roughly associated as Protozoa, must evi- 
dently be rearranged. Sponges, Ehizo- 
pods (Amoeba3, etc.), and Gregarines, 
have strong resemblances, but recent re- 
searches may place the former higher. 
The Infusoria comprehend creatures too 
various to remain under one head, and 
very many of them too highly organized 
to be called "x^rotozoons," or first life- 
forms. 
Those who wish to pursue this subject 
further may consult Professor Huxley's 
'Elements of Comparative Anatomy,' 
from which the preceding quotations have 
been taken. 
A system of classification founded upon 
anatomical and developmental considera- 
tions frequently differ considerably from 
one w^e might arrive at if all the creatures 
were arranged according to their faculties 
and the extent and accuracy of their re- 
lations to the external world. Such a 
classification would not in any way super- 
sede the former, but it would prove very 
instructive and offer many valuable sug- 
gestions. Sonje years since, Professor 
Owen ]n'oposed to divide the Yertebrates 
according to the perfection of their brains, 
but other anatomists did not find his di- 
visions sufficiently coincident with facts. 
Yery little has been done towards an 
exact science of human phrenology. The 
difficulties remain pretty much as they 
were many years ago, and our compara- 
tive phrenology, if we may use such a 
terra, is in a very imperfect state. Wlien 
we come to the lower animals we do not 
know what peculiarities of the brain of 
an ant make it the recipient of a higher. 
Instinct, or give its possessor greater 
<?apacities for dealing with new and un- 
expected difficulties than are possessed 
by most other insects, and if any reader , 
has a marine aquarium, and will make a 
few experiments in taming prawns, and ' 
watching their iDroceedings, he will dis- 
cover symptoms of intelligence beyond 
\vliat the structure of the creature would 
have led him to expect. 
Anisnals usually possess someone lead- 
ing characteristic to which their general 
structure is subordinated. Man stands 
alone in having the whole of his organiza- 
tion conformed to the demands of a think- 
ing, ruling brain. To pass at once to the 
other extreme, we observe in the lower 
infusoria a restless locomotion, probably 
subservient to respiration, but utterly 
[ inconsistent with a well developed life 
of relation, or with manifestations of 
thought. The life of an animalcule may 
be summed up as a brief and restricted, 
but vigorous organic energy, and if the 
amount of change which a single creature 
can make in the external world, is incon- 
ceivably small, the labors of the entire 
race alter the conditions of a prodigious 
amount of matter. Microscopic vege- 
table life is an important agent in purify- 
ing water from the taint of decomposing 
organisms. By evolving oxygen it brings 
putrescent particles under the influence 
of a specific combustion, which, though 
slow, is as effectual as that which a fur- 
nace could accomi)lish. In this way min- 
ute moulds burn up decaying wood. 
Microscopic animal life helps the re- 
generative process, and, together with the 
minute vegetable life, restores to the or- 
ganic system myriads of tons of matter, 
which death and decay would have handed 
over to the inorganic world. In a very 
small pond or tank the quantity of this 
kind of work is soon appreciable, and if 
we reflect on the amazing amount of 
water all over the globe, including seas 
and oceans, which swarm with infusoria, 
the total effect produced in a single year 
must seem considerable, even when com- 
pared with that portion of the earth's 
crust that is subject to alteration from all 
other causes put together. If we add 
to the labor of the Infusoria those of 
other creatures whose organization can 
only be discovered by the microscope, and 
take in the foraminifera, polyps, polyzoa, 
etc.. Ave shall have to record still larger 
obligations to minute forms of living 
things. The coral polyp builds reefs that 
constitute the chief characteristic of cer- 
tain regions in the Pacific ; foraminifera 
are forming or helping to form strata of 
considerable extent, wdiile diatoms are 
making deposits many feet in thickness, 
composed of myriads of their silicious 
shells, or adding their contributions of 
silex, very large in the aggregate, to all 
sedimentary rocks. Testimony of this 
kind of work is found by the navigator 
who examines the ice in Arctic seas, and it 
comes up with soundings from the ocean 
depths. 
I On the surface of the earth the amount 
