SPONGES. 317 



smaii outgrowths occur, and these gradually become larger, 

 and finally an ostium is formed. From the eggs are formed 

 little free-swimming embryos, which later settle down and 

 grow into the adult. 



Sponges are largely marine, only a few forms, and these 

 of no economic importance, occurring in fresh water. The 

 sponges of commerce come from the Mediterranean, the 



FIG. 144. Sponge (Dactyocalyx). From Liitken. 



Red Sea, and Florida and the West Indies. They are 

 brought up by divers, or by hooks which are dragged over 

 the bottom. The fleshy portions are allowed to decay, 

 then the skeleton is washed, and the sponges are packed in 

 bundles for the market. There are different grades of elas- 

 ticity and fineness of fibre, and consequently different values. 

 The finest sponges come from the eastern part of the 

 Mediterranean. Sponges occur as fossils, especially in the 

 Cretaceous rocks. 



There are two great groups of sponges. In the first, 

 called CALCAEEA, the skeleton is composed of carbonate of 

 lime; in the second, SILICEA, there is sometimes a skeleton 

 consisting of silica (quartz), sometimes of horny fibres, 

 sometimes of both horny fibres and siliceous spicules; and 

 again, there are a few forms which have no skeleton. 



