Some oj the Loiver Metasoans 381 



sponge is transported about and in turn its spicules and construction give 

 protection to the crabs. 



Even though the sponges are sessile, they are well adapted for at- 

 taining adequate amounts of food. It has been estimated that a small 

 leuconoid sponge of fairly simple construction may pass 22.5 liters of 

 water through its body per day. This sponge was but 10 cm. high and 

 but 1 cm. in diameter. Obviously the amount of water passing through 

 a larger sponge would be immense. 



Economic Importance. — The skeletons of some sponges are util- 

 ized for many purposes such as bathing, washing, padding, and mop- 

 ping. These are the marine sponges whose skeletons are formed of 

 spongin. Before the sponge is used, the protoplasm is removed, leav- 

 ing the soft absorbent spongin skeleton. These sponges are most abun- 

 dant in the warmer waters of the Mediterranean and the Gulf of Mexico. 

 Unfortunately the fishing of these forms has often been indiscriminate, 

 and the industry has declined considerably. 



There are a few species of sponges which live on the shells of 

 oysters. These actually are able to dissolve the shell and consume the 

 oyster. 



The Classes of Sponges. — The three classes of sponges are dif- 

 ferentiated from one another on the basis of the material forming their 

 skeletons. The spicules forming the skeletons are variously shaped from 

 simple to many-branched (Fig. 124, A). The type of canal system found 

 is not necessarily characteristic of the class. 



The three classes are as follows : 



Class I. Calcarea. Skeleton formed of calcareous spicules which are one-, 

 three-, or four-rayed. These are entirely marine forms which are comparatively 

 small and inconspicuous. They show all types of canal formation. Example: 

 Leucosolenia, Scypha. 



Class II. Hexactinellida. Skeleton formed of siliceous spicules which are 

 ordinarily six-rayed. These are entirely marine forms which often occur at 

 great depths. The skeletal remains of these forms often appear like delicate 

 pieces of spun glass. They are attached at the base by a tuft of spicules, and 

 are all radially symmetrical. Example : Euplectella, the Venus's flower basket. 



Class III. Demospongiae. Skeleton formed of siliceous spicules, spongin 

 fibers, or both. The spicules, however, are not six-rayed. These are usually 

 larger sponges that may occur in great masses. Many are highly colored and 

 very bizarre in appearance. The canal system is always leuconoid. Example: 

 Spongia, the common bath sponge. 



