SPONGES q 
GROUPS OF SPONGES. 
Sponges may be divided quite well into two groups, namely, Horny and 
Spiculigenous species; but it must be understood that, although in the major- 
ity of species it is quite easy to decide to which group any given sponge be- 
longs, there are species, the position of which is not so easy to determine, for 
they are so unequally made up of spicules and horny matter, that they occupy 
a position directly between the two groups, and form, in a measure, connecting 
links between them; that is, some contain very few spicules, others more . 
HORNY SPONGES. 
As the sponges comprising this group are the most important, we will 
first consider them, and a description of their general str ucture will answer for 
that of nearly all sponges. 
Let it first be understood that which is known as the sponge of com- 
merce, is simply the skeleton of the living sponge. A sponge when living is 
so different from this skeleton, that it is quite difficult for an expert to rec- 
ognize a living sponge when he sees it for the first time, even though he may 
have been perfectly familiar with the dried specimen or skeleton. 
When living, sponges are covered with what is known as sarcode or 
sponge «flesh, a flesh, however, which when compared with that of other 
animals, is inan exceedingly rudimentary condition. This flesh not only cov- 
ers the outer portion of the sponge, but also lines all of the internal orifices. 
It is of different degrees of firmness, but is usually quite soft, and is covered. 
where exposed, with a kind of skin which is of a somewhat denser structure 
than the flesh that lies within it. There are, in fact, especially in young spong- 
es, three layers ; outer, ( ectoderm ): middle, ( mesoderm ) ; inner, ( entoderm. 
Both skin and flesh are often quite brightly colored, some species being brill- 
iant orange, yellow or scarlet, but many species are black or brown externally. 
Those which are brightly colored change their hues quite quickly upon being 
