14 NATURAL HISTORY. 



can trace in a rough way the general course of the canals of the living 

 animal. 



The boring sponge (Cliona) is a small form, which always attacks cal- 

 careous objects in the sea. It obtains a lodgment on some shell, and then 

 proceeds to excavate its galleries in all directions until the shell is com- 

 pletely riddled. In this way it destroys the shell, and plays an important 

 part in the economy of nature. How it bores is an unsolved question. 

 It had been thought that it was by means of the spicules, but it has 

 recently been shown that the young sponge begins to bore before any skel- 

 eton is formed. 



All sponges are not marine ; a few live in fresh water, several species 

 being found in the United States. While the marine species reproduce 

 their kind by means of eggs, the fresh-water sponges have, besides, another 

 method of perpetuating the species. In the colder climates the frosts of 

 winter kill the fresh-water sponges, while in the warmer zones the heat of 

 summer may dry up the ponds in which they live. By the second method 

 of reproduction both of these dangers are avoided. At certain times por- 

 tions of the sponge tissue become enclosed in hardened capsules, and thus 

 are able to withstand drought or cold. When winter comes, the sponge dies 

 down, leaving its skeleton standing, and entangled among the fibres are 

 these reproductive buds. With the return of spring the buds burst, and 

 rapidly grow into a new sponge, frequently utilizing the last year's frame- 

 work as a scaffolding for the new growth. 



Fresh-water sponges sometimes become a nuisance, for they especially 

 affect the sluggish water of the storage basins of water-works, and by 

 their death in the fall so pollute the water that it is offensive both to 

 taste and smell. Several of our eastern cities have been badly troubled in 

 this way. 



The silicisponges (those with a silicious skeleton) embrace the so-called 

 glass-sponges, the beauty of which is well known, as they have now 

 become quite common, not only in museums, but even as ornaments in 

 houses. First among them stands the Venus's flower-basket (Euplectella) , 

 in which the delicate spicules form a most beautiful reticulating network. 

 The first specimens brought to European markets caused no little discus- 

 sion, but soon their true nature appeared. They are found in the seas 

 around the Philippine Islands. Another interesting form is the glass- 

 rope sponge (Ilyalonema), in which a portion of the spicules take the 

 form of a loosely coiled rope. This rope is merely an anchor which 

 extends down into the mud, holding the sponge in place. The last form 

 of glass-sponge which we need mention is the Holtenia figured, which needs 

 no further description. 



