18 SPONGES 
oxidize the flesh, they also gather food from the water which they press against 
the walls of the cell into.which it is absorbed, while the vitiated and depleted 
water flows on and out into the large excurrent tube, while the sphincter mus- 
cle, which guards the entrance of the small tubes, prevents its return into 
them. 
Sponges propagate in two ways, by ovules and by buds. In winter, ov- 
ules are formed in sponges, which in spring are thrown out of the exeurrent 
openings. These may be quite easily seen in some of our fresh water species. 
These give rise to small, ciliated animals which swim about freely at first, but 
which soon become attached, and from them arise sponges. 
Buddmg may be illustrated by my studies of the Tube Sponge. There 
it takes place, normally, at or near the base of some cylinder. The origin of a 
bud is first indicated by the thickening of the skin of the sarcode on the surface 
at some particular point, usually in some depression. The thickening of this 
appears to check the growth of the sarcod* in that direction, as far as the thick- 
ened skin extends, and consequently the growth of the accompanying skeleton, 
but there is still an outward growth around the area formed by the thick- 
ened deposit. ‘Thus a tube is begun, which at first grows wider (see Fig. 13. 
B, 0, where a life-sized sponge is given \ , but after assuming the length of 
three inches or more, continues about the same diameter (see Figs. 6,7, and 
11). Often, however, especially should sand gather in the bottom of the tube, 
an extra membrane or skin is thrown across the tube, accompanied by sarcods, 
and followed by the production of horny fiber, and the narrow, basal portion 
of the tube is cut off, thus it becomes about the same diameter for its entire 
jength. 
