HOW SPONGES GROW, 
17 
openings for the exit of waste matters. Among these large 
openings are multitudes of minute openings wiiich serve as 
mouths. These mouths lead by branching canals into little 
pockets or chambers which are lined with digestive, ciliated 
Fig. 15. — Development of a sponge (Sly con ciliatum). A, B, morula seen in sec- 
tion; c, segmentation cavity; C, blastula stage; D, gastrula about to be- 
come stationary, with a few spicules; J57, sponge become stationary, with 
spicules. Highly magnified. 
cells; the sponge, then, has myriads of mouths and stom- 
achs (Fig. 14). 
Sponges develop, like' all the 
higher animals, from true eggs. 
The egg, after fertilization, 
begins to grow, and divides into 
two, four, eight, sixteen, and 
more spheres, until it looks like a 
mulberry, which seen in section 
is as in Fig. 15, A, B, This is the 
segmentation stage or vio?^ula. 
The cells farther multiply, and 
arrange themselves into a single fig. i6.-ciiiated embryo .t bias- 
layer, when the embryo is called X^^. '"a^^gn^^^^ 
a blastula. Some of the cells are 
ciliated, and as a blastula the embryo leaves the parent 
sponge and $wims about in the sea (Fig. 16 and Fig. 15, C). 
