14 



BRANCH PORIFERA 



Symbiosis. — Examples of symbiosis are found among them, 

 as that of the sponge and the cralx The sponge attached to the 

 crab is carried about by it and given better opportunity of ob- 

 taining food and oxygen, while the crab, in turn, is concealed 

 from its enemies by the sponge. In the fresh-water sponge, a 

 green alga sometimes grows, giving the green color to the mass. 



\'arious small marine forms are found in the sponges, giving 

 good examples of commensalism. Sponges are never parasitic. 



Fig. 7. — Spongers at work. The " sponge hook " is a three-toothed 

 curved hook attached to a pole, the length of which \aries with the depth 

 of the water. The sponge-glass is a common water-pail with the botlom 

 knocked out and a ])a!ie of window glass put in its place. It is used for 

 seeing below the surface where the water is disturbed by ripples. (Cobb, 

 in Circular 535, U. S. F. C, 1902.) 



Use. — They are of use as food for other animals, and their 

 skeletons form a very useful article of commerce. 



The sponges of shallow wattT are obtained by men in boats, 

 with a dredge or it long-handled hook or rako (Fig. 7) ; those of 

 the deeper waters, by divers. They are then exposed to the 

 air for a time and then heaped up in water again in tanks 



