STRUCTURE AND LIFE-HISTORY OF A SPONGE. 3 1 
in number, sixteen in each equatorial ring, and eight in each polar 
ring. The number of cells is further increased by division, the 
sac becomes more spherical in form, and the apical end of the 
cleavage cavity closes up. The next stage in the development is 
a differentation of the cells, the eight which surrounded the basal 
opening of the cleavage cavity, and which are larger than the rest, 
acquire a dark appearance, owing to the development of a great 
number of dark granules within them. The remaining cells are 
much clearer, and multiplying in number, are converted into elong- 
ated small prism-like cells, radially arranged. The basal end of 
the cleavage cavity now closes up, and the dark granular cells 
increase in number. The next stage is either abnormal, incon- 
stant, or in any case subsequently reversed; in it the layer of 
granular cells becomes flattened, depressed, and then pushed into 
the cleavage cavity, which is thereby diminished, or even almost 
obliterated, the granular cells applying themselves to the inner 
face of the prismatic layer. The form so produced is similar to 
the gastrula of other animals, but it is not permanent, the granular 
cells withdrawing themselves, and subsequently resuming their 
former position. 
“ We may, therefore, return to the larva, where we left it in the 
normal course of development. The embryo is now more or less 
egg-shaped ; the smaller end consists of the numerous small clear 
prismatic cells, which are now furnished with flagella, projecting 
from their outer ends ; the larger end consists of the fewer (32) 
larger, rounded, dark, granular cells, sixteen of which are arranged 
to form a girdle round the equator, next to the prismatic half. 
The cleavage cavity still exists as a more or less spherical space 
in the middle, bounded half by the pigmented ends of the prism- 
atic cells, half by the granular cells. By the movements of its 
flagella the larva now is liberated from its encapsuled cavity in the 
mesoderm, passes through the endodermal layer, into the radial 
tube, and so, borne along by the out-flowing currents of the circu- 
