14 



PORIFERA. 



globules, which was the living 

 part of the sponge, secreting, as 

 it extended itself, the horny 

 fibres which are imbedded in it. 

 The anastomosing filaments which 

 compose the skeleton of such 

 sponges, when examined under a 

 microscope, and highly magnified, 

 appear to be tubular, as represented 

 in fig. 1. c. 



Many species, although exhibit- 

 ing the same porous structure, 

 have none of the elasticity of the 

 officinal sponge, a circumstance 

 which is due to the difference ob- 

 servable in the composition of their 

 skeletons or ramified frame-work. 

 In such the living crust forms 



within its substance not only tenacious bands of animal matter, 

 but great quantities of crystallized spicula, sometimes of a calca- 

 reous, at others of a silicious 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 re- 

 main, 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 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.* 



Crystallized spicula of this description form a feature in the 

 structure of the sponge which is common to that of many vege- 

 tables, resembling the formations called Raphides by botanical 

 writers. Some of the principal forms which they exhibit are de- 

 picted in fig. 1 a b, which likewise will give the reader a general 

 idea of the appearance 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 9 and g, exhibit 

 detached spicula of different forms highly magnified. The most 



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



