32 STRUCTURE AND LIFE-HISTORY OF A SPONGE. 
lation, out of the sponge into the surrounding water, where it spins 
about in a lively whirling kind of dance. As it grows, the little 
‘ blastula,’ as it may now be called, becomes less ovate and more 
spherical, and then commences to pass through one of the most 
important stages of its existence ; the flagellated layer begins to 
lose its spherical contour, becomes flattened, depressed, and is at 
length drawn quite within the hemisphere of granular cells ; it then 
applies itself to the inner face of this layer, entirely obliterates the 
cleavage cavity, and thus gives rise to the true gastrula form. It 
will be seen that the embryo is now somewhat bee-hive shaped, its 
wall is double, consisting of an outer ectodermic layer of granular 
cells, and an inner endodermic layer of prismatic cells, the cleavage 
cavity has disappeared, and a new cavity with a widely-open aper- 
ture below has arisen by invagination. This aperture is bounded 
by the row of sixteen granular cells, which previously formed the 
equatorial girdle of the embryo, they now grow radially inwards 
towards the centre of the aperture, and thus diminish its area, till 
it becomes reduced to a comparatively small opening; it is the 
larval mouth, as the cavity produced by invagination is the larval 
stomach. So far the larva has led an active, if not an industrious, 
existence, with the continual promise of better things ; it now prO" 
ceeds to a step, which, in the history of animal development, has 
generally proved fatal to further progress of importance, and has 
indeed often led to change in a backward direction. Settling 
down on some fixed object, such as a bit of stone, or the stem of 
a seaweed, it exchanges a free for a fixed and stationary existence. 
The granular cells surrounding the mouth grow inwards towards 
its 'centre, and completely obliterate it, at the same time they grow 
outwards over the surface of attachment in transparent, irregular, 
jagged, pseudopodia-like processes, which solder the young sponge 
securely to its seat. 
By the absorption of a part of their granules, the granular cells 
lose, to a great extent, their opacity, so that one can see the layer 
