HUXLEY, ON CLASSIFICATION. 77 
Dr. Bowerbank’s discovery of the existence of vibratile cilia in the 
genus Grantia, but it is only quite recently that the precise nature of 
the arrangement of the apparatus which gives rise to these currents 
in ordinary sponges, has been made out by Lieberkiihn and by 
Carter. The canals which enter the deep substance of the sponge 
become dilated mto spheroidal chambers, lined with sponge particles 
(Fig. 3, A, ec, C), each of which is provided with a vibratile cilium ; 
and as all these cilia work in one direction—towards the crater—they 
sweep the water out in that direction, and its place is taken by fresh 
water, which flows in through the small apertures, and through the 
superficial chamber. The currents of water carry along such matters 
as are suspended in them, and these are appropriated by the sponge 
particles lining the passages, in just the same way as any one of the 
Rhiziopoda appropriates the particles of food it!finds in the water 
about itself. So that we must not compare this system of apertures 
and canals toso many mouths and intestines; but the sponge repre- 
sents a kind of subaqueous city, where the people are arranged about 
the streets and roads, in such a manner, that each can easily 
appropriate his food from the water as it passes along. 
In the sponges two reproductive processes are known to occur: the 
one of them asexual, corresponding with the encysting process of the 
Gregarinida; and the other, truly sexual, and answering to the 
congress of the male and female elements in the higher animals. In 
the common fresh-water Spongilla, towards the autumn, the deeper 
layer of the sponge becomes full of exceedingly small bodies, some- 
times called “seeds” or “ gemmules,” which are spheroidal, and have, 
at one point, an opening. Every one of these bags—in the walls of 
which are arranged a great number of very singular spicula, each 
resembling two toothed wheels joined by an axle—is, in point of fact, 
a mass of sponge particles which has set itself apart—gone into 
winter quarters, so to speak—and becoming quite quiescent, encysts 
itself and remains still. The whole Spongilla lies down, and the 
seeds, inclosed in their case, remain uninjured through the winter. 
When the spring arrives, the encysted masses within the “seed,” 
stimulated by the altered temperature of the water, creep out of their 
nests, and straightway grow up into Spongilla like that from which 
they proceeded. 
But there is, in addition, a true sexual process, which goes on 
during the summer months. Individual sponge particles become 
quiescent, and take on the character of ova; while, in other parts, 
particular sponge particles fill with granules, the latter eventually 
becoming converted into spermatozoa. 
These sacs burst and some of the spermatozoa, coming into contact 
with the ova, impregnate them. The ova develop and grow into 
