THE SHORES OF BRITAIN. 65 
nalis) is a native of the Mediterranean, but is much 
more familiar to us than our native species, of which 
there are many. The appearance which it presents 
is that of an irregularly-shaped mass, more or less 
rounded, composed of a brown woolly substance, 
perforated by innumerable pores in all directions, 
and having in addition, wide canals communicating 
with each other, and terminating in round holes or 
mouths on the surface. But if we take a small por- 
tion of the substance, and place it under a common 
magnifying lens, we shall see that it is composed of 
shining, horny, nearly-transparent fibres, which, by 
uniting with each other at all angles and distances, 
form a loose and very irregular network, Now, 
when in a living state, every fibre was enclosed in a 
coating of thin, clear jelly, which formed the living 
animal, the horny fibres constituting, as I have inti- 
mated above, only the skeleton. Imbedded in the 
substance of many species, some British ones, for 
example, are found spicule, or needle-like crystals, 
of pure flint, varying much in shape in various kinds, 
while other species have similar crystals of lime. 
Where these occur in considerable numbers, the 
Sponge does not possess elasticity: it may be 
crushed, but it will not regain its original form. 
It is a singular fact, that Sponges of these threc 
different kinds are sometimes found growing close 
to each other, and all alike nourished by the same 
simple fluid, pure sea-water; yet they elaborate 
therefrom products so different as horn, flint, and 
lime. The animal nature of Sponges is not easily 
to be detected: no indication of sensation has ever 
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