258 



CCELENTERATA. 



the sarcosepta, and the theca inside and at some distance from the col- 

 umn, the outer surface of which secretes only the inconstant epitheca 

 (fig. 209). From the above it would appear that the sclerosepta correspond 

 in number to the sarcosepta, but this is not always the case. Thus the 

 Helioporidae, which on the grounds of the skeleton were regarded as Hex- 

 acoralla, are shown by the soft parts to be undoubted Octocoralla. 



By means of their skeletons the Anthozoa produce large accumulations 

 of carbonate of lime, the well-known coral reefs, on the bottom of the sea. 

 These are formed by many species, the Madreporaria playing the most 

 important role. When the reef reaches the surface it produces an island, 

 the most noteworthy form being the atoll, a ring-like structure with a 

 central lagoon. The origin of these atolls, as well as that of fringing and 

 barrier reefs, was for a long time explained by Darwin's and Dana's theory 

 of coral reefs. Later investigations, notably those of Mr. Agassiz, afford 

 another explanation. 



Order I. Tetracoralla (Rugosa). 



Extinct forms from the paleozoic rocks with the parts arranged 

 in fours (fig. 211). The present tendency is to regard them as modi- 

 fied Hexacoralla. 



Order II. Octocoralla (Alcyonaria). 



These forms, which have eight single septa, are externally re- 

 cognizable by their feathered tentacles, eight in number (fig. 206). 



B 



FIG. 211. 



FIG. 212. 



FIG. 211. Diagram of septa in a tetracorallan. (Orig.) 



FIG. 212. Three stages in development of Renilla reniformis. (After Wilson.) 



A, cleavage of egg ; B, planula ; C, development of oesophagus ; ec, ectoderm ; 



en, entoderm; r/i, mesogkea ; o, oesophagus. 



They occur in all seas from near the shore to great depths. In 

 development there is a planula (fig. 212) in which the oesophagus 

 arises as a solid ingrowth which becomes perforated later. The 

 eight septa arise simultaneously. Usually colonies are formed by 

 budding and a polymorphism may occur, some individuals which 

 have reduced septa and lack tentacles, taking in water for the 

 colony. Many are phosphorescent. 



