280 



CHAPTERS ON EVOLUTION. 



assumption that its personality, like that of the city, is compound, 

 and is not simple and single, like that of the element or unit. As we 

 shall hereafter see more plainly, the sponge must be judged, like 

 every other living being, not by its appearance or by what it simu- 

 lates, but by what it originated from. As an apparent collection of 

 organisms, it might well be regarded as a veritable colony. On 

 other grounds, the sponge might appear as rightfully entitled to be 

 considered as single and undivided an animal unit as a man. The 

 grounds on which these opinions are based need not now be speci- 

 fied, but the history of how a sponge grows, finds its appropriate 

 place at this stage of our inquiry. The most typical sponges, as" 

 already shown, grow each from an egg (Fig. 187, i), which passes 

 through characteristic stages of development (2, 3, 4), and finally 

 becomes a cup-shaped body (5), possessing a double wall (<:, </), the 

 cavity of the cup opening outwardly by a distinct mouth (m). Then 

 pores or openings (7, /) are formed in the wall of this cup, placing 



FIG. 188. JJfDRJE. 

 (Young hydrae are represented budding from the parent in each figure.) 



its interior in a new fashion in communication with the outside world 

 The outer wall of the cup, and the inner wall likewise, consist of cells ; 

 and those of the inner wall finally come to possess cilia, which, by 

 their constant motion, cause currents of water to flow into the inside 

 of the cup through the pores, and outwards by its mouth. 



The nourishment of a sponge is subserved by these water 

 currents, bringing food and oxygen to its living cells; and the 

 simple or cup-shaped sponges (Fig. 187, 7), of which many species 

 are known, exhibit a history resembling that of which the outlines 

 have just been sketched. The horny sponges, the skeletons of cer- 

 tain species of which we use in our domiciles, may and do develop 

 into organisms of a more complex character than the cup-sponges 

 present, and they may also originate from buds as well as from 

 eggs. The common green fresh-water Spongilla, found growing on 



