908 Marvels of the Universe 
developing the fibrous skeleton, and yet others form a kind of lining [to 
the body substance. At certain seasons this “‘ body” produces egg-cells, 
which ultimately develop into little oval embryos, with a convex base 
covered with tiny threads, or “ cilia,” which by their lashing movement 
drive the larval sponge freely about the water. 
The inner part of a sponge consists of yellow solid and interlacing 
filaments or threads, which have already been in part described. In 
some species these threads are extremely fine. For instance, in the best 
Turkey Sponge they are about the nine-hundredth part of an inch in 
diameter. They are very tough and will resist the action of practically 
any acid; but they, nevertheless, are not sufficiently firm to balance 
and support many sponge growths, and are, therefore, assisted in their 
work by numberless minute flint needles, which are known as spicules. 
Very marvellous is the diversity of form to be found in the spicules. 
They are shown as they appear under the microscope on page go4, but 
though the examples are numerous, they form by no means an 
They are to 
be found scattered through the bulk of the sponge, sometimes as single 
exhaustive collection of these strange architectural pins. 
Photo by] [W. Plomer Young. 
particles ; in other cases, hooked one into the other to form a complete 
flinty framework. 
the composition and structure of the skeleton, and are again sub- 
co) 
As a general rule, sponges are classified according to ORO Ge Chase Leaae 
of Sponges and of limited 
growth. 
divided according to the various types (single- 
rayed—four-rayed, etc.) of their spicules. 
But no mention has yet been made of the 
internal economy of the sponges, for this is 
not the least marvellous part of their structure. 
Let us take a simple sponge, such as the 
Sycon shown in the upper illustration on this 
page. You will notice that at the upper end 
is an open mouth much like the mouth of a 
well. This is known as the oscule, and if you 
were to examine the sides of this sponge you 
would find a great number of little pores. It 
is by reason of these characteristic pores that 
the whole group of sponges has been given a 
collective Latin name, which signifies pore- 
bearers. Careful examination will show the use 
of these pores; they will be seen to allow the 
passage of minute streams of water, which 
pass through diverse channels and passages 
until they meet in the central hollow and pass 
out in one continuous stream through the 
oscule. But the water that passes away has 
been changed; it has been deprived of the 
minute particles of matter which were held 
A TREE SPONGE. 
This wonderful Sponge is to be found on the banks of the 
Amazon. 
It attaches itself to branches of trees which are sub- 
merged during the floods. and is so constructed that it can 
survive the period of drought when it is quite out of reach of 
the water. 
in it, and it has also exchanged its oxygen for 
carbonic acid gas. How, then, has this change 
Let us cut a _ horizontal 
find 
section 
neatly- 
taken place. 
of the sponge, and we will 
