SPONGES 41 



higher sponges is much more comphcated. In the higher 

 sponges there is no general body cavity, but the whole 

 sponge mass is full of large tubes, each with its osculum. 

 Moreover, there are innumerable inhalent pores leading into 

 special pockets, or chambers in which the flagellated cells 

 are situated. Recall that the flagellated cells in the very 

 simple sponge are situated in the general body cavity. 

 The chambers containing the cells with flagella communi- 

 cate, by small canals, with the large tubes. Thus there is 

 provision for the same circulation of water as in the simple 

 sponges. Sponges are of various dolors. They may be red, 

 orange, blue, purple, green, and gray. 



Fresh-water sponges. — Most of the sponges are found 

 in the sea, but a few live in fresh water. The fresh-water 

 species are very widely distributed in lakes, ponds, rivers, 

 etc., hence they may be found in many locahties. Repro- 

 duction is brought about by a budding process. A spherical 

 mass of cells, surrounded by a hard siliceous case and known 

 as a gemmule, is formed within the body of the parent 

 sponge. In the autumn this bud, or gemmule, is set free in 

 the water by the decaying of that part of the body in which 

 the gemmule lies. The gemmule, after being set free, settles 

 to the bottom of the water, where it passes the winter. With 

 the change in temperature in the spring, the cells within 

 the case escape through the natural hilum, or orifice of the 

 gemmule, begin growing, and soon develop into a mature 

 sponge. The fresh-water sponges belong to the family 

 Spongillidse. 



Skeletons of sponges. — Some sponges have no skeleton. 

 In those sponges in which a skeleton exists., it is formed by 

 the cells of the middle layer and may consist of different 

 substances. In some, the skeleton is formed from spicules 



