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on these animals; members of the different learned societies 
were to be seen poking about ditches and ponds in search 
of specimens and practising experiments daily. Carefully 
packed-np bottles were sent to distant friends, by post ; and 
even ambassadors were full of the all-engrossing theme. 
Hydrae are found only in fresh water, and generally in 
such as flows very slowly or is quite still. The best way to 
obtain specimens is to take a handful or two of weeds, such 
as duckweed, star- wort, water-crowfoot, &c., from any clear 
pond or ditch, and place them in a glass vessel of water. 
After waiting half an hour, it is probable that several indi- 
viduals may be seen in various attitudes, some hanging loosely 
down, others erecting themselves in graceful curves and 
throwing out their pendent tentacles many times longer than 
their bodies, others shooting up their arms right above their 
heads, others contracted so as to look like a mere dab of jelly ; 
others with anterior and posterior extremity attached to the 
side of the glass, others floating on the surface of the water, 
the posterior discs or tail-ends serving to keep them from 
sinking ; some of a beautiful grass-green colour, others light 
flesh, others white, others red. 
The body of the hydra is of a gelatinous nature, varying in 
shape according to the position it assumes : contracted, it is in 
some species a mere tubercle with short radiating papillae ; it be- 
comes a narrow cylinder when extended ; one end is expanded 
and forms an adherent disc, the other is furnished with a mouth 
surrounded by a number of exceedingly contractile arms or tenta- 
cles, which vary in number both according to the species and the 
ages of individuals. The hydra's body is composed of two mem- 
branes, technically termed ectoderm and endoderm, the former 
being external, the latter constituting the lining of the inside 
cavity. The tentacles are membranous tubes, being in fact 
nothing more than the tubular prolongations of the two mem- 
branes of the body. These arms, which are the animal's instru- 
ments for seizing its prey, are situated a little below the orifice of 
the mouth, which, when closed, is sometimes protruded like a 
snout above the tentacles. Both membranes present on the sur- 
face irregularly-rounded nodules, with a number of vacuoles. 
These nodules contain, especially in the tentacles, capsular bodies 
(thread-cells), in which may be seen, under a high power of 
the microscope, certain curious organs (see fig. 7), consisting 
of spines and filaments, supposed by some to have the power 
of stinging, each capsule containing one filament with its 
spinous appendages. These organs may readily be seen by 
crushing the hydra between two bits of glass. There are 
traces of the presence of muscular fibres in the tentacles; 
but whether this muscular apparatus is sufficient to effect the 
