104 



CORALS 



the great variety they exhibit in the form and texture of 

 the skeletal structures they produce, it is necessary to 

 relate in a few words some of the anatomical characters 

 which distinguish them. 



As with many other corals, the Alcyonaria are colonial 

 in habit ; a large number of animal organisms of the form 

 known as Polyps, in organic connexion with one another 

 by a system of nutritive canals, constitute the structure 

 which is known as the Alcyonarian. In most of the Alc^'O- 

 naria all the polyps of a colony have a similar anatomical 

 structure, showing when fully expanded eight pinnate 



Fig. 44. — Diagram to illustrate the structure of a dimorphic Alcyonarian. 

 Aut., autozoid ; C, endodermal canals; il//., mesenteric filaments; St., siphono- 

 zooids ; St., stomodaeum. 



tentacles (Fig. 47, A.) and a general octoradiate symmetry 

 of their organs, but in some genera, which are usuallv more 

 spongy in texture than the others, there are in addition to 

 the normal polyps or autozooids a large number of polvps 

 which are arrested in development and never produce anv 

 tentacles. The latter are called the siphonozooids, and 

 their primary function appears to be to create and maintain 

 by means of ciliary action a flow of water through the canal 

 system (Fig. 44). 



Another character of the Alcyonaria, with verv few 

 exceptions, is the power of forming calcareous spicules. 

 These spicules, varying greatly in size, shape, and distribu- 



