THE SPONGES 7 
by means of cells, or so interlocked that they form a rigid 
support to which the fleshy substance is bound and through 
which the numerous canals penetrate. 
In a relatively few species only does the skeleton con- 
sist of horn, though there are many in which horn and flint 
exist together. In the former event, if the skeleton be 
elastic and of sufficient size, it becomes valuable to others 
than the naturalist, for the familiar sponges of commerce 
are the horny skeletons of forms usually taken in the West 
Indies or in the Mediterranean Sea. In these localities the 
animals are pulled off by divers, or with hooks, and are then 
spread out in shallow water where the protoplasmic sub- 
stance rapidly decays. The remaining skeleton, thoroughly 
washed and dried, is ready for the markets of the civilized 
world. 
Examining a bit of such a “sponge” under a magnify- 
ing glass, it will be seen that the skeleton is not composed 
of various pieces, but of one continuous mass of branching 
fibers, which interlace and unite in apparently the greatest 
confusion; yet in the living animal these were perfectly 
adapted to the position of the canals and the general needs 
of the animal. 
Besides being a scaffold-work to which the fleshy portions 
of the body are fastened, the skeleton serves also for pro- 
tection. In some species, needle-like spicules as fast as 
they are formed are partly pushed out over the entire sur- 
face of the body, giving the appearance of a spiny cactus; 
or in other cases they are arranged in tufts about the canals, 
effectually preventing the entrance of any marauder. 
Thus perfectly protected, the sponges have but few natural 
enemies, and hence it is that in favorable localities they 
grow in great profusion. 
28. Race histories and life histories —We have now traced 
living things from their simplest beginnings, where they 
exist as single cells, and have seen that in bygone times 
similar forms have united into simple colonies, and these 
