














304 SPONGES. 
structure and its general form from the sponges of commerce. — 
The latter, whose irregular swelling outlines are so fami 
iar to us, are of foreign origin, the better kinds coming 
from the more eastern shores of the Mediterranean, the 
coarser and larger kinds from the Bahamas. The commer 
cial value of these is based upon the horny nature aul — 
closely interwoven texture of their internal skeleton. 4 
A sponge is, typically, a gelatinous mass, in which is im- 
bedded numerous little spikes and plates, of a horny, calci- 
reous, or siliceous substance ; or hair-like threads of various — 
. forms, which are so thickly disposed and closely knit te — 
gether by animal matter, that they form a sort of open-work — 
frame supporting the looser tissues. q 
n the common sponge this frame-work is wholly composed 
of horny hairs, which are so densely packed and elastic that ; 
they immediately resume their original shape after being d 
compressed. The gelatinous matter is in all cases cleaned — 
out after the sponge is torn up from its rocky bed, and thon j 
which we utilize are only the horny skeletons of the living 
animals. So loosely constructed and fragile, however, %® — 
the large branching species of our own coast, that a 
specimen may be crushed to powder in the hand. 
The exterior of our beach specimens have a furry look, 
to the projecting points of the spiculæ, which a 
through the outer skin. Scattered holes of consider d 
size reveal portions of the interior, and between them arè 
innumerable smaller pores. These larger apertures Con ; 
with distinct channels which ramify through the mass. ™ 7 
directions, and, when surrounded by their native ele ae 
expel continuous jets of water. In fact the whole 1s pei 
apparatus for absorbing and ejecting sea-water, well dest a 
ing its old name of sea-lungs. 
The surrounding liquid is taken in through the pe 
pores of the outer side, and, passing through the ene? 
interstices of the structure, is finally collected in 
channels and thrown out again, together with quan 
due 
ities of 
