and Functions of the sponge. -w 125^ 



near the fecal orifices. There is an apparatus at the entrance 

 of the pores, of a nature very different from any of the parts al- 

 ready described, and which throws much light on the functions 

 of these openings. When we cut a thin layer from the surface 

 of the *S'. papillaris (fig. SI.), and look down through one of its 

 pores with the reflecting microscope, we perceive a very delicate 

 net- work of gelatinous threads (fig. 25, c) thrown over the en- 

 trance of the pore. This piece of structure is so fine as to be 

 perfectly invisible to the naked eye, and is always effaced in 

 dried specimens. It is present in every pore of the living ani- 

 mal, and consists of several broad filaments of a soft transpa- 

 rent, colourless, and perfectly homogeneous substance, which 

 pass directly inwards from the bounding fasciculi, (fig. 25, a b} 

 or gelatinous margins of the pores, to be connected with one or 

 more central meshes, formed of the same threads, and lying in 

 the same plane. This gelatinous net-work, consisting generally 

 of six or seven meshes, lies always beneath the defending fasci- 

 culi (fig. 24. b) in the species^ where these occur. And, while 

 it is admirably protected by the depending spicula of the pores, 

 as in the *S'. panicea, where these spicula spread over it like the 

 rays of a fan, it serves to guard still more completely the inte- 

 rior? of these passages from particles of sand or small floating 

 animalcules. By making deeper sections, we sometimes observe 

 one or more net- works of a simpler structure (fig. 26. c), but of 

 the same nature, lying beneath the first. None of the project- 

 ing granules, which line the whole internal surface of the canals, 

 and compose the parenchymatous matter, are seen on any part 

 of these net- works, and their position, regularity, and constant 

 appearance, sufficiently point out their function, and show, inde- 

 pendently of the surrounding frame-work, and the currents 

 passing constantly in, that the pores are not the open cells of 

 polypi, nor accidental perforations, made by worms or animal- 

 cules in a pulpy substance. When we examine carefully the 

 base of sessile species of sponge, we observe, that the part which 

 forms the connecting medium between their body and the rock 

 on which they spread, is a tough consistent gelatinous substance 

 (fig. 21. h), similar to that which lines the canals, and passes 

 over the surface between the pores ; it insinuates itself into all 

 the inequalities of the surface to which it is attached, and is the 



