STRUCTURE OF THE SPONGE. 51 



replete with minute crystallized spicula. The 

 spicula are usually simple needle-like points, but 

 sometimes they are three or four pointed. 



Multitudes of spicula are placed longitudinally 

 around the internal canals, of which they form the 

 walls ; they may be obtained by washing a sponge, 

 of which the animal matter is decomposing, or by 

 fusing it before the blow- pipe ; they are mostly 

 found to consist of silica or flint, and minute as 

 they are, are capable of scratching glass. 



A small portion of silica has been detected in the 

 ashes of the common sponge, as one of the consti- 

 tuents in the composition of its elastic fibres. It 

 appears, moreover, that the proportion of silica 

 increases according to the firmness of the fibres of 

 sponge, and that where these are elastic, animal 

 matter predominates. The forms of the spicula are 

 constant in every species, and consequently become 

 tests in their identification. In some species, the 

 spicula are calcareous ; and whether calcareous or 

 silicious, they assume the forms which the crystals 

 of lime and silica present under ordinary circum- 

 stances. 



Though every species of sponge has its charac- 

 teristic figure, still no two individuals of the same 

 species agree in external form, or in the number 

 and precise directions of their large canals. Among 

 the higher animals, every species resembles the 

 rest of its species in the form of the limbs and 

 teeth ; in the length and figure of the ears, tail, 



