32 SPONGID^. 



tures. In size, the Sponges far exceed all other 

 Protozoa ; aggregate masses of these animals being 

 sometimes met with, chiefly on the shores of tro- 

 pical seas, which cover surfaces of many yards in 

 extent. 



3. Skeleton. — The skeleton or framework of 

 the Sponge, which is best seen in dried specimens, 

 is usually composed of a number of horny fibres, 

 anastomosing one with another in such a manner as 

 to form an irregular though intricate network. In 

 some sponges, e. g. Gh^antia, this fibrous network 

 is altogether wanting. In the Sponges of com- 

 merce it attains an unwonted degree of softness 

 and elasticity, which qualities, combined with the 

 comparative paucity of their spicula, give to these 

 substances their chief value in a commercial 

 point of \dew. In certain tropical Sponges, on the 

 other hand, it is found to be entirely made up of 

 siliceous particles ; these, however, still presenting 

 that characteristic reticulated arrangement which 

 may be regarded as essentially distinguishing the 

 ' fibre ' from all the other structures which together 

 make up the body of the Sponge. 



4. Aquiferous System. — With the porous 

 aspect of ordinary Sponges most persons are 

 familiar. On more careful examination it will be 

 seen that these pores are of two kinds: i. the 

 larger pores, which are comparatively few in num- 

 ber and frequently elevated on slight prominences ; 

 and 2. the smaller pores, which are much more 

 numerous, crowding the entire surface of the 

 Sponge so as to occupy the interspaces between 

 the larger apertures. The latter are properly de- 



