THE SPONGES 27 



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 



