176 



THE ANIMAL KINGDOM 



Figure 9.4. Photograph of the skeleton of the glass sponge, Eiiplectella. The hex- 

 axons are fused to form intersecting girders. (Courtesy of the American Museum of 

 Natural History.) 



are so placed in the deep ocean currents that the water flows through 

 them continually. 



Hexactinellids are especially coinmon in the deej) water off Japan, 

 where large numbers of Venus's flower basket may be gathered. Several 

 species of shrimps live within the large spongocoel, entering through 

 the sieve plate while young and unable to leave after they have grown. 

 For some mysterious reason they are almost always found in pairs, one 

 of each sex. A glass sponge with its imprisoned pair of shriinps is used 

 as a wedding gift in Japan, and symbolizes a marriage lasting until 

 death. 



Demospongia. The Demospongia include a family of fresh-water 

 sponges and a large variety of marine forms found at all depths. The 

 spicules have one, three or four rays. Spongin fibers are a protein secre- 

 tion of the ameboid cells which form an anastomosing network in the 

 body wall. They are resistant to digestion and decay, resembling hair and 

 silk in these respects. All the members of this class have the leuconoid 

 body plan. 



Those that lack siliceous spicules have a soft, pliable skeleton. The 



