32G A MONOGRAPH OF THE AUSTRALIAN SPONGES, 



The chemical composition which is met with in the Sponges, is 

 the carbonic acid salt ; Ca C 3> with very little organic substance, 

 which mainly forms the narrow and often invisible axial canals. 



The calcareous spicules, which sometimes attain a considerable 

 size, are produced as a secretion by cells and grow by apposition. 

 Ca C 3 and Ca S 4 are always in abundant solution in sea water. 

 These substances are taken up by the Sponge-cells in contact with 

 the water (Canal Epithelpe), and transmitted to Meso-dermal cells, 

 which produce the skeleton. 



b. Silicisjwngice. 



The skeleton of the silicious Sponges consists, of Si O., chiefly. 

 There is always an axial canal, composed of oi'ganic substance, and 

 it appears that there are also sometimes concentric layers of 

 siliceous acid and oi'ganic substance intermittent. The young 

 spicules contain more organic substance in proportion than the old 

 ones, and appear therefore softer and can easily be dissolved in 

 re-agents. 



Also, these spicules seem to originate always in cells, some- 

 tin tes (Hyalonema), they attain a length of 10 Cm. and a thickness 

 of 2-1 mm. They grow, as the concentric layers in their substance 

 indicates by apposition. The silica is procured from hydrate of 

 siliceous acid, Si H^ 3 or from soluble siliceous salt, which are 

 met with in small proportions in solution in the sea water. 

 Si Ka 3 for instance. 



c. Ceraosponyiw. 



The horny substance which forms the greater part of the 

 substance in the skeleton of these Sponges, and many of the 

 Monactinellse is a substance similar to hornchitin or silk, F. E. 

 Schulze calls it Spongiolin. Several chemical analysis have been 

 made of it. The percentage of nitrogen is very great. 



The horny fibres of the Sponges are the product of certain 

 gland cells Spongoblasts, described above, which secrete horny 

 substance. The fibres always consist of concentric layers and 

 grow by apposition only and not by apposition and intussusception. 

 (F. E. Schulze : Die Familie der Spongidee. Von Lendenfeld : 

 Das Hornfaseiwachsthum der Aplysinidpo.) 



