SKELETONS OF SPONGES. 



187 



some of them are partially formed before the birth of the polyps 

 which have to become their inhabitants. 



The skeletons of some of the Campanularia, consist of tubular 

 granular matter, encased in a horny translucent substance, which is 

 enlarged at the end of each branch into cells for the domiciliation of 

 the polyps. The outline is waved from the cells with a jointed appear- 

 ance, which allows of a certain elasticity to the cells. 



Alcyonium digitatum are branched, and have tubes; some are curved, 

 others straight, and many take the form of a cross. The dark-red 

 margin of Gorgonia petechialis is a mass of large, red, flattened, angular 

 spicula ; while inside they are small and in bundles, forming a net- 

 work around the pores or canals, thin and slightly-curved, with a small 

 swelling in the centre. 



fig. 103. 



1. Transverse section of a branch ofMyriapore. 2. A section of the stem of Virgu- 

 laria mirabilis. 3. A spiculum from the outer surface of a Sea-pen. 4. Spicula from 

 crust of his hippuris. 5. Spicula from Gorgonia elongata. 6. Spicula from Alcyo- 

 nium. 7. Spicula from Gorgonia umlraculum. 



In a New Zealand specimen of Alcyonium the spicula are calcareous, 

 of a brown colour, shaped like a cucumber, having all parts of their 

 outer surface studded with conical tubercles j some of them exceed the 



