ii8 Infrodiu'fion to Ajiimal JMorpJiology. 



remain free (Antipathes), or the unbranched axis may be 

 spirally twisted (Cirrhopathes). About thirty species are 

 known, mostly from deep Palaearctic sea bottoms. 



Sclerodermic corals only exist in clear sea water, 

 unmixed with freshwater, where the winter temperature 

 does not sink below 66° F., and usually between the 

 limits of 1-30 fathoms. 



Three forms of reef exist: — i. Barrier reefs, gird- 

 ling continents or islands, at a distance from land. 



2. Fringing reefs, surrounding islands, only separated 

 therefrom by shallow channels, and sloping seaward. 



3. Atolls, or circular coral islands, with a lagoon in 

 the centre. The first and last forms grow on a 

 gradually subsiding, the second on a gradually rising, 

 sea bottom. 



CHAPTER XIX. 



ALCYONARIA. 



Sub-class 2. Alcyonaria [Edivards, Octactinia, Elirt ti- 

 ler g). — ^Actinozoa, with eight short, broad, pinnately- 

 fringed tentacles, and mesenteries in multiples of 

 four; usually with spicules, but no continuous en- 

 deronic skeleton. When the outer skeleton is conti- 

 nuous, it is epithecal, and there may be centrally a 

 sclerobasic axis or a tubular system. The eight septa 

 are not in pairs, nor do they bear longitudinal bands 

 of muscle. The following orders are contained : — 



I. Alcyonacese — Sclerobasis none; ectoderm leath- 

 ery, with no epitheca, but with calcareous spicules. The 



