COLLECTION OF POLYPI. 535 



have only one species of the penicillus, the p. ca- 

 pltatus, from the American seas. 



Amongst the twenty- eight species of the genus 

 alcyonium, which are contained in five frames, is 

 the a. purpureum,) from the seas of New Holland. 



Sixty-five species of the genus spongia arc ar- 

 ranged in nineteen frames; they all differ in 

 their form and texture. The animals which in- 

 habit them are still unknown ; but it has been ob- 

 served, that, when in their element, the sponges 

 have a glutinous envelope or pulp which is irri- 

 table, and by comparing them with the alcyonia, 

 there is no doubt that they are formed by very 

 small, pellucid animals. When taken out of the 

 sea this glutinous pulp becomes brittle and soon 

 falls off; the main body of the sponge is still 

 flexible because it is composed of elastic fibres, 

 and the numerous holes or vacuities in its texture 

 admit and retain the water. We may have a 

 still better idea of the manner in which a sponge 

 is formed by comparing it, with the gorgonia, a 

 genus of the following section, of which we have 

 twenty-six species, occupying twelve frames. 



The gorgonia is composed of a horny axis co- 

 vered with a glutinous and calcareous paste, 

 which becomes crisp by desication : if we ima- 

 gine this axis reduced to the thinness of a horny 

 thread and ramified with the branches crossing 



