56 THE OPEN BOOK OF NATURE 
tubes through which water finds its way. Food is 
extracted from the circulating water. A network 
of little flinty spikes, called spicules, makes a support 
to the bodies of many sponges ; in others it consists 
of horny fibres. The spicules are sometimes com- 
posed. of carbonate of lime. 
The C@LENTERATA form the third subdivision of 
the Animal Kingdom. This word is derived from 
the Greek koilos=hollow, and enteron= intestine. 
The Coelenterata have internal cavities which serve 
as stomachs, and these cavities terminate in a mouth, 
which is fed by means of moving arms or tentacles. 
The tentacles catch the prey and put it into the 
mouth, and the mouth passes it on to the stomach, 
where it is digested. | 
The Graptolites (Greek, grapho = I write) are known 
only in fossil forms; they are found in Cambrian, 
Ordovician, and Silurian rocks, and these creatures 
seem to have become extinct at the end of the 
Silurian Period. The accompanying illustration will 
give you an idea of the appearance of these fossils. 
They are found among shales ; many of them have 
been changed into iron pyrites, which has a glitter- 
ing, goldlike appearance, and they look like gilded 
designs on a black background. The graptolites 
were little colonies of small animals joined together 
one after the other, each animal living in a tiny 
horny cup, and all the cups of the colony were con- 
nected by a horny skeleton. They get their name 
from a fancied likeness of some of them to a quill- 
pen.* If you live in a district where graptolites are 
to be found, I hope you will secure some specimens ; 
they need careful looking for, and you must not 
