SPONGES 



attracts it, so it drifts along near the surface, its 

 own power of propulsion being nothing in com- 

 parison with the force of winds and currents. In 

 the course of time it acquires a distaste for the 

 light, and thereupon heads downward, and sooner 

 or later bumps against the bottom. Here it 

 sticks, — the chance of striking a favorable spot 

 for growth being probably about one in a million. 

 The larger cells increase rapidly and form an outer 

 covering, the locomotor cells disappear within, and 

 thereafter are of use in making their own private 

 currents, whose eddies bear oxygen and particles 

 of food to what is now a sponge. 



No human being would hesitate if compelled to 

 choose between being a seal and a sea -weed. Yet 

 there are some advantages in being rooted to one 

 spot. The seal has to go after its meals and flee 

 from its enemies, while a sea-weed and a sponge 

 simply sit and let the currents bring food to them. 



Now and then a certain type of lucky spongelet 

 alights on the back of a hermit crab's shell, and 

 like a burr on a dog's coat, gets a free ride. In the 

 case of the sponge this is for life, for it speedily 

 dissolves the shell and forms an elastic and ever- 

 growing coat for the fortunate crab. From now 

 on, sponge and crab live amicably together. 



The relation of a sponge to itself is a most intri- 

 cate problem. It is so plant-like that if one be 

 chopped up into small pieces, each cutting, in the 

 course of seven or eight years, will form a bath 

 sponge of commercial size. My hundred-and- 



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