SPICXTLA FROM SPONGES. 189 



The mineral portion, as before stated, consists of spicula composed 

 either of silica or carbonate of lime ; the first kind is the most common 

 and likewise most variable in shape, and presents every gradation in 

 form, from the acuate or needle-shaped to that of a star. The calca- 

 reous spicula, on the contrary, are more simple in their form, being 

 principally acicular, but not unfrequently branched or even tri or quad- 

 riradiate ; the two kinds, the siliceous 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, 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 crystalline, 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 Plate IV. No. 3. The spicula occupy 

 certain definite situations in sponges ; some are peculiar to the crust, 

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

 the fibrous network of the skeleton, and others belong exclusively to 

 the gemmules. Thus, for instance, in Pachymatisma Johnatonia, ac- 

 cording 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 flesh ai-e 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 homy matter as to form, with a few short and 

 slightly-curved spicula, the flbrous 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 tubular 

 fibres j but in all existing species, with one or two exceptions, they are 

 solid. These tubular fibres are very commonly filled with portions of 

 iron, which accounts for the colour of many of the remains in flint. 



