850 THE SPOXGIDA OR PORIFERA. 



already explained, on the surface of the sponge : the former, by 

 innumerable minute pores^ which the animal has the power of 

 opening and closing ; and the latter, by larger apertures or osaila, 

 which also can be increased or diminished. 



The ciliattd ox fiagellate, collared cells, lining the canal system, 

 form one of the essential elements — Saville Kent insists the essen- 

 tial element — of Sponge Structure ; and it is this flagellate lining 

 that constitutes the endoderm of Haeckel and others, who group 

 the Sponges with the Corals and Zoophytes {Coelenteratd), The 

 tenacious, hyaline, and glairy sarcode, in which amcebiform cells 

 are distributed, and the skeleton formed, is the mesoderm of the 

 Haeckel school, and cytoblastema of James Clark, the amcebiform 

 cells (according to the latter authority) being termed the cell- 

 eleJ7ie7its, and by others cytoblasts or cytodes. The outer or super- 

 ficial stratum of the sarcode, or cytoblastema — in some instances 

 very distinct from the rest of this substance — is the investi?ig 

 viembra?ie of James Clark, the dermal me?nbrane of Bowerbank, 

 and the ectoderm of Haeckel. These different terms imply very 

 divergent views, but it, is sufficient to state here that the entire 

 sarcode, superficial or deep, of a sponge possesses remarkable 

 vital and contractile powers, pores being opened at various points 

 and closed so as to be unnoticeable, lacerations being readily 

 repaired, and fragments or separations from the main body being 

 capable of change of form and slow movement. 



The skeletal structures of Sponges form a basis for their 

 classification; thus there are (i) the Calcareous Sponges, the spicules 

 of which consist of Carbonate of Lime ; (2) the Siliceous Sponges, 

 in which the spicules are of Silica or flinty matter; (3) the Horny 

 or Keratose Sponges, the skeletons of which consist of a tough 

 horny substance ; and (4) the Gelatinous Sponges, which possess 

 neither spicules, horny supports, or skeletal structures of any kind. 



The " Sponges " used for household purposes are the horny 

 skeletons of Spongia officinalis. 



Halichondria panicea and H. encrustans (Figs, i, 2, 3, Plate 

 XIV.) possess skeletons of siliceous spicules, joined together with 

 horny substance. 



Sycandra {GraJitid) compressa and ciliata (Figs. 4, 5, Plate 

 XIV.) have skeletons made up of calcareous spicules. 



