THE SPONGES. 25 



hand, low animals are simple. Thus differentiation of or- 

 gans involves the division of physiological labor. 



For example, an Amoeba has but a single organ, a nu- 

 cleus; but still it moves. A more varied amount of work 

 is done by the inf usorian with its cilia, its incipient mouth, 

 and its contractile vesicle. Sponges with their distinct 

 cells, mouths, digestive chambers, eggs, and spicules do a 

 still greater amount of varied work. The jelly-fish, with its 

 nervous system, muscles, stomach and eyes, each perform- 

 ing its separate duties, affords an instance of still greater 

 differentiation of parts, and consequent division of labor; 

 while in the lobster, butterfly, or fish or bird, the subdivi- 

 sion of the parts of the body and of the work done by each 

 separate organ is still greater and more wonderful. 



The sponges are found in all parts of the sea, in shallow 

 or deep water, and in very different circumstances; some 

 grow in mud; others, like the colossal Neptune's cup, stand 

 boldly up in the water on the coral-reef, and others grow in 

 flat patches over the surface of stones and shells. Still 

 others bore tunnels into shells and lime-rock, where, safe 

 from harm, they do much good in causing the shells and 

 stones to break up into gravel and sand, thus making a soft 

 bed for worms and other creatures to live in. Sponge-life 

 is spread throughout the ocean, in its abysses as well as its 

 shallows, in fresh-water lakes and in sluggish streams, and 

 everywhere it touches that of many other creatures. Yet, 

 owing to their tough, flinty skeleton, few sponges are eaten 

 by fishes and other large animals; but when they die the 

 slime is devoured by the one-celled animals. 



