16 INTRODUCTION TO CLASSIFICATION. 



cilia in the water-passages of the sponge, but it is only quite 

 recently that the precise nature of the arrangement of the appa- 

 ratus which gives rise to these currents, has been made out. 

 The canals which enter the deep substance of the sponge become 

 dilated into spheroidal chambers, lined with sponge particles 

 (Fig. 4, A, e, 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 Rhizopoda 

 appropriates the particles of food it finds in the water about 

 itself. So that we must not compare this system of apertures 

 and canals to so many mouths and intestines ; but the sponge 

 represents 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. 



Two reproductive processes are known to occur in the 

 sponges : the one of them, asexual, corresponding with the en- 

 cysting 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, sometimes 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 resem- 

 bling 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 SpongiUa 

 dies down, and the seeds, inclosed in their case, remain un- 

 injured 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 straight- 



