'28 STRUCTURE AND LlFE-HlSTORY OF A SPONGE. 
hairy appearanccj to which attention has already been directed. 
The spicules of the corona^ are partly colossal spicules similar 
to the foregoing, partly tri-radiate and quadri-radiate spicules, with 
three rays of the latter and two of the former embedded around 
the mouth, the remaining ray in each case projecting vertically 
upwards. 
This nearly completes our account of the structure of the 
sponge ; that which remains, the nature of the reproductive 
organs, will form a natural introduction to the account of its life- 
history. 
Certain amoeboid cells have already been described as inhabi- 
tants of the mesoderm, and the origin, or probable origin, of these 
cells has also been pointed out. What the subsequent history of 
them all may be we do not yet surely know, but some of them, at 
all events, continue for some time wandering through the meso- 
derm, and instead of contributing food to the rest of the sponge, 
obtain their nutriment from it, like parasites ; thus they grow big, 
at the same time they become lazy, and at length cease to move 
about at all ; then they assume a spherical form, and remain sta- 
tionary in a cavity of the mesoderm. If they now retain their 
simple cell-form, simply increasing in size, they are known as om. 
Some of them, however, undergo a structural change (at least, so 
we infer, for this is one of the stages in the history of the sponge 
which has not yet been directly observed), and as a result of this 
change, the nucleus of the cell disappears, and the cell itself 
becomes fibrillated, the fibres all radiating from the centre of the 
cell to the exterior. It is now called a sperm-ball. When it 
becomes mature, the fibres within are set free; each consists of 
a little highly-refractile conical head, with a long tail, which vibrat- 
ing rapidly, propels the spermatazoon^ as the structure is called, 
head foremost through the water. 
The spermatazoon is the male element, the ovum the female 
element of the sponge. Both occur in the mesoderm. It is a 
