GEEGARINIDA, RH120P0DA, SPONGIDA, AND [NPUSORIA. 15 



ters 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. 



In the sponges two reproductive processes are known to 

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

 cysting process of the Gregarinicla ; 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 " gem- 

 mules," which are spheroidal, and have, at one point, an open- 

 ing. Every one of these bags — in the walls of which are ar- 

 ranged 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 dies 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 w r ater, 

 creep out of their nests, and straightway grow up into Spongillse 

 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 ciliated germs (D, fig. 3), which make their way out, 

 and, after swimming about for a while, settle themselves down 

 and grow up into Spongillse. 



