THE SCHOOL OF THE SHORE 37 



shore animals. So these have to be on the 

 qui vive; they must feed while they can, and 

 take as much as they can. No doubt they can 

 get a good living, but they cannot get it easily. 

 One of the most important lessons that the 

 inhabitants of the shore have to learn is to be 

 always on the alert, and to make the most of 

 their chances. 



Let us take some particular cases of food- 

 getting. Encrusting the rocks in many places 

 there is the Crumb-of-Bread Sponge (Hali- 

 chondria panicea) with large exhalant aper- 

 tures where the water is swept out, and minute 

 pin-prick holes all over the surface by which 

 the water is swept in. After their early youth is 

 past, sponges are fixed animals, and one natu- 

 rally thinks of them as easy-going. But they 

 have to work hard for their living. They ob- 

 tain their food from microscopic creatures and 

 nutritive particles in the water, and in order to 

 get enough they have to pass large quantities 

 of water through their body every day. If an 

 animal's body be compared to a city, and the 

 tissues to streets, and the cells composing the 

 tissues to houses and workshops, and the jos- 

 tling particles of living matter inside the cells 

 to the people themselves, we would compare a 



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