398 



THE MICROSCOPE. 



being principally acicular, but not unfrequently branched 

 or even tri- or quad-radiate ; the two kinds, the sili- 

 ceous and calcareous, according to Dr. Johnston, not 

 having hitherto been detected co- existent in any native 

 sponges. 



The spicula exhibit a more or less distinct trace of a 

 central cavity or canal, the extremities of which are closed, 

 or hermetically sealed ; in their natural situation they are 

 invested by an animal membrane, sarcode, which is not 

 confined to their external surface : but in many of the 

 large kinds, as pointed out by Mr. Bowerbank, its presence 

 may be detected in their central cavity, by exposing them 

 for a short time to a red heat, when the animal matter 

 will become carbonised, and appear as a black line in their 

 interior. 



Many authors have described the spicula as being crys- 

 talline, and of an angular figure, and have considered them 

 analogous to the raphides in plants; but it requires no 

 great magnifying power to prove that they are always 

 round, and, according to their size, are made up of one or 

 more concentric layers, as shown in fig. 212, No. 2. The 

 spicula occupy certain definite situations in sponges ; 

 some are peculiar to the crust, others to the sarcode, 

 others to the margins of the large canals, others to the 

 fibrous network of the skeleton, and others belong exclu- 

 sively to the gemmules. Thus, for instance, in Pachyirta- 

 tisma Johnstonia, according to Mr. Bowerbank, the spicules 

 of the crust are simple, minute, and fusiform, having their 

 surfaces irregularly tuberculated, and their terminations 

 very obtuse ; whilst those of the sarcode are of a stellate 

 form, the rays varying in number from three to ten or 

 twelve. 



Silica, however, may be found in one or more species of 

 sponge of the genus Dysidea, not only in the form of 

 spicula, but as grains of sand of irregular shape and size, 

 evidently of extraneous origin, but so firmly surrounded 

 by horny matter as to form, with a few short and slightly- 

 curved spicula, the fibrous skeleton of the animal. In these 

 sponges the spicula are of large size, and are disposed in 

 lines parallel with the masses of sand. 



Most of the sponges of the earlier geological periods had 



