610 THE WONDERS OF GEOLOGY. Lect. VI. 



Flustra is an aggregation of an immense number of individual 

 polypes attached to a calcareous skeleton ; each of which is 

 doubtless susceptible of pain and pleasure independently of 

 the whole ; for we have a living proof in the Siamese 

 twins,* that even in our own species there may be an 

 united organization with separate nervous systems, and 

 individual sensations ; and as it is certain that each Polype 

 enjoys distinct volition, it is most probable that the sensa- 

 tions of each individual are independent of the general 

 mass. There is, however, a common sensibility pervading 

 the structure that binds together the community of zoo- 

 phytes, and by which certain actions are performed irre- 

 spectively of the individual polypes. Thus the compound 

 zoophytes termed Pennatulte, or Sea-pens, upon the slightest 

 touch withdraw themselves into the wet sand, and disappear : 

 and the arborescent Vorticellce^ upon the microscope being 

 agitated, instantly shrink down into a globular mass, and 

 all appearance of the elegant animalcules a moment before 

 so active, vanishes. In certain species of Flustra, F. 

 avicularia for example, each cell is protected by a bivalve 

 operculum, much resembling in form the beaks and head of 

 a vulture ; and these appendages open and shut apparently 

 without the control of the polype that occupies the cell ; 

 their functions seem to be related to the horny axis that 

 connects the group of independent living animalcules of 

 which the entire compound zoophyte is composed, and not 

 to the polypes themselves. % 



14. The Corals or Polyparia.§ — In the Flustra, then, 



* See the Philosophical Transactions for 1830, p. 177. 



f See " Thoughts on Animalcules," p. 49. 



% The reader will find some highly interesting observations on this 

 phenomenon in Mr. Darwin's " Journal of a Voyage round the World," 

 chap. ix. p. 200. 



§ Polyparia. The axis, framework, or skeleton of these groups of 

 polypes is termed polyparium, or polypidom (polype-habitation) ; and 

 those of a stony hardness are familiarly known as Corals : these 

 names therefore refer to the durable skeleton of the zoophytes,, and not 



