8 



ORGANS OF SUPPORT, 



pear to be only fragments of triradiate spicula. The tri- 

 radiate spicula chiefly bound the pores, canals, and orifices, 

 while the curved ends of the clavate spicula hang over the 

 exterior entrance of the pores to protect them. The cal- 

 careous spicula do not appear to occur in any of these animals 

 along with silicious forms, and the true horny tubular filaments 

 appear also to occur alone in the more tropical species, with- 

 out either silicious or calcareous spicula. The calcareous 

 forms of these animals appear to be much more rare, and 

 generally much smaller than the silicious or the horny species. 

 In the horny species of poriphera the skeleton consists 

 of thin elastic tubular translucent filaments united together 

 and distributed around the pores, canals, and vents. 

 These horny, tough, flexible threads have a close analogy in 

 their mode of distribution through the whole interior of the 

 body to the tough connecting matterof the spicula in the earthy 

 species, and they give form and support to the whole fabric. 

 Sometimes the internal canal which extends through these 

 tubular horny filaments is filled with an opaque matter which 

 gives a greater friability to the threads ; but most frequently 

 they contain only a transparent colourless fluid, as we see in 

 the fibres of the common 

 officinal sponge, which is 

 a poripherous animal be- 

 longing to this horny 

 group. The skeleton of 

 all the poripherous ani- 

 mals is so soft and flexible 

 in the living state, that 

 none of the lengthened 

 forms appear to be capable 

 of growing in an upright 

 position from their base 

 of attachment. They hang 

 down from the under sur- 

 face of submarine bodies, 

 as represented in these 

 figures. A specimen of 

 the common officinal 

 sponge with a horny fi- 

 brous skeleton, is repre- 



FIG. IV. 



