SPONGIA OFFICINALIS. 



which are embedded in it. The anastomosing filaments 

 which compose the skeleton of such sponges, when examined 

 under a microscope and highly magnified, appear to be tabu- 

 lar, as represented Plate I. Fig. 1, c. 



Many species, although exhibiting the same porous struc- 

 ture, have none of the elasticity of the officinal sponge, a cir- 

 cumstance which is due to the difference observable in the 

 composition of their skeletons or ramified framework. In 

 such the living crust forms within its substance, not only tena- 

 cious bands of animal matter, but great quantities of crys- 

 tallized spicula, sometimes of a calcareous, at others of a sili- 

 cious nature, which are united together by the tenacity of the 

 fibres with which they are surrounded. On destroying the 

 softer portions of these skeletons, either by the aid of a blow- 

 pipe or by the caustic acids or alkalies, the spicula remain, and 

 may readily be examined under a microscope ; they are then 

 seen to have determinate forms, which are generally in rela- 

 tion with the natural crystals of the earths of which they 

 consist ; and as the shape of the spicula is found to be simi- 

 lar in all sponges of the same species, and not unfrequently 

 peculiar to each, these minute particles become of use in the 

 identification of these bodies. 



Crystallized spicula of this description form a feature in the 

 structure of the sponge which is common to that of many 

 vegetables, resembling the formations called Raphides by 

 botanical writers. Some of the principal forms which they 

 exhibit are depicted Plate I. Fig. 1, a, 6, which likewise will 

 give the reader a general idea of the appearance of the sili- 

 cious and calcareous sponges, after the destruction of their 

 soft parts has been effected by the means above indicated. 

 The figures, Plate I. rf, e, /, and , exhibit detached spicula 

 of different forms, highly magnified. The most convenient 

 method of seeing them is simply to scrape off a few particles 

 from the incinerated sponge upon a piece of glass, which, 

 when placed under the microscope, may be examined with 

 ordinary powers. 



On placing a living sponge of small size in a watch-glass, 

 or small glass trough filled with sea-water, and watching it 

 attentively, something like a vital action becomes apparent. 

 The entire surface is seen to be perforated by innumerable 

 pores and apertures, some exceedingly minute, opening on 

 every part of its periphery ; others of larger dimensions, placed 



4 



