SPONGES. 21 



(47.) Many species, although exhibiting the same porous structure, 

 have none of the elasticity of the officinal sponge, a circumstance to be 

 attributed to the difference observable in the composition of their 

 skeleton or ramified framework. In such, the living investment forms 

 within its substance not only tenacious bands of animal matter, but 

 great quantities of crystallized spicula, sometimes of a calcareous, at 

 others of a silicious nature, 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 blowpipe 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, generally in 

 relation with the natural crystals of the earths of which they consist ; 

 and as the shape of the spicula is found to be similar 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. 



(48.) 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 rapTiides by botanical writers. Some 

 of the principal forms they exhibit* are depicted in fig. 9, a, b, d, 

 e, /, g, which likewise will give the reader a general idea of the appear- 

 ance of the silicious and calcareous sponges after the destruction of 

 their soft parts has been effected by the means above indicated. The 

 figures d, e, f, and g likewise represent detached spicula of different 

 shapes 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 ex- 

 amined with ordinary powers. 



(49.) 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 f. The entire surface 

 is seen to be perforated by innumerable pores and apertures, some ex- 

 ceedingly minute, opening on every part of its periphery; others of 

 larger dimensions, placed at intervals, and generally elevated upon 

 prominent portions of the sponge. Through the smaller orifices the 

 surrounding water is continually sucked as it were into the interior of 

 the spongy mass, and it as constantly flows out in continuous streams 

 through the larger openings. The annexed diagram, fig. 10, A, will 

 give the reader an idea of the most usual direction of the streams. The 

 entering fluid rushes in at the countless pores distributed over the gene- 

 ral surface of the sponge, but in its progress through the canals in the 

 interior becomes directed into more capacious channels, communicating 

 with the prominent larger orifices, through which it is ultimately ejected 

 in equable and ceaseless currents. Organized particles, such as neces- 



* Savigny (Jules Cesar), Zoologie d'Egypte : gr. fol. Paris, 1809. 

 t Dr. Grant, in the New Edinburgh Philosophical Journal, 1827. 



