THE YOUNG SCIENTIST. 
93 
will be necessary to add fresh to it. By 
adding salt water, it is evident the whole 
would soon become too salt to sustain life. 
The Hydra. 
THIS fresh water polype is not exactly a 
microscopic or a very minute object. 
It is plainly visible to the naked eye when 
extended, and its beauty will amply pay an 
examination. Hydras may be found at- 
tached to leaves or sticks, during warm 
weather, in almost any pond. In the aquar- 
ium they attach themselves to the side 
turned towards the light, or to Anacharis 
alsinastrum and to the VaUsneria spiralis. 
The hydra can assume a variety of forms; 
it may appear vermiform (or like a worm), 
it may attenuate its body and its tentacles 
so much as to become almost invisible, or 
it may contract suddenly and show only as 
a speck of jelly. The tentacles are pre- 
hensile organs round a central cavity, and 
are also used for locomotion. These thread- 
like organs wind themselves round their 
prey like a lasso, and completely paralyze 
it. By gradual curving, the object is 
brought to the mouth and engulphed there, 
while any refuse or any undigested parts 
are rejected from the same opening. Like 
all the polypes and other low organizations, 
hydras can rapidly reproduce any part of 
their body; they may be cut into pieces or 
divided into halves, and each piece will go 
on performing the functions of the animal, 
and will be complete again in a very few 
hours. One hydra may swallow another 
without injury to either, or it may be 
turned inside out without any detriment to 
its vital functions. Their usual size, when 
full grown, is about three-quarters of an 
inch, and the red hydra [hydra rubra) is 
more easily seen in an aquarium than the 
other kinds. These singular beings are 
very voracious. They can swallow a worm, 
a cypris, or a eyclops, of twice their own 
size. Hydras multiply very rapidly; a por- 
tion of their body swells or buds out. and 
a young one presently appears, remains at- 
tached to the mother for several days, when | 
it floats off; or they throw out eggs at one 
time, and living young ones at another. 
Hydras appear to be long lived; I have a 
hydra in my aquarium which I collected 
ten years ago near Greenwood. * 
The Aquarium. 
rpHE secret of success in Aquarium man- 
agement is to sustain the proper bal- 
ance between animal and vegetable life, 
thereby securing purity of the water with- 
out change, with consequent life and health 
to the inhabitants; the aquarium must in 
fact be a miniature pond. Oxygen is ani- 
mal life, this plants give off. Carbonic acid 
is death to animals, this plants absorb — but 
plants need sunlight to perform their office 
to perfection. 
i Eighty-eight years ago. Sir John Dalyell 
began aquarium keeping. Three times a 
week his servants trudged two miles to the 
sea-shore to fill a four-gallon jar with sea 
water. Sixty- one years this routine was 
kept up. His specimens were of the orders 
lower than fishes, his receptacles cylindrical 
jars holding one animal each. The water 
was changed daily — oftener if the slightest 
sediment was to be seen. 
In 1842, the first true aquarium principles 
were recorded. Dr. Ward, recognizing the 
mutual dependence of the animal and vege- 
table kingdoms, arranged them together in 
water. Mrs. Annie Thynne followed with 
the same ideas some four years later. 
The marine aquarium presents attrac- 
tions the fresh water cannot afford — but the 
fresh water affords attractions sufficient to 
interest the novice. The care of the latter is 
less. In case of accident it can be sooner re- 
newed or replaced . It is easier to begin with 
and is more easily kept in order. The fresh 
water should precede the marine aquarium. 
The brooks, creeks and ponds will afford 
the furnishing, whether of animal or vege- 
table life, and the study of the life beneath 
the waters, if one may obtain their own 
specimens, will add new interest to it alL 
The aquarium to the young will afford les- 
sons that cannot be gained from books; les- 
sons upon Nature's immutable laws. Arti- 
ficial law may often be evaded with impun- 
ity, but natural laws never. The penalty in- 
evitably follows, and that penalty is olten- 
est death. The study of the aquarium will 
favor the development of thought and in- 
quiry, and the observations will become 
almost personal experience. — Ex. 
