III. ANTHOZOA: HEXACORALLA 



231 



more tentacles and only eight septa (fig. 202), but which exhibits a condition 

 through which the young actinians pas,s; on the other hand, in the Zoantharia, 

 Ceriantliia;, and Antipatharia the rule of six has undergone extensive modifi- 

 cation. 



Sub Order I. ACTINARIA (Malacoderma). The sea-anemones are mostly 

 solitary, witliout sl^elelon; with numerous septa and tentacles. 1 hey occur in 



Fig. 203. — Astrangia dance*; five polyps 

 in various stages of expansion. 



Fig. 204. — Cainria arabica (after 

 Klunzinger). 



all seas from tide marks to the greatest depth. A few are free, but most are 

 sessile. Metridium* Bunodes* Sagartia* Bicidium* (parasitic on Cyoiiea), 

 Halcampa*. Zoanthe^e have two kinds of alternating mesenteries, individuals 

 of the colonies usually incrusted with foreign matter. EpiztaHthus''' lives symbi- 

 otically with hermit crabs (fig. 114). 



Sub Order II. ANTIPATHARIA. Six 

 pairs of septa and six (Antipathcs) or twenty- 

 four (Gerardia) simple tentacles; colony with 

 a black horny axis and no calcareous skele- 

 ton. Simulate the Gorgonids. 



Sub Order III. MADREPORARIA 

 (Scleroderma). This group, the richest in 

 species of any, is characterized by the great 

 development of the skeleton. Theca, septa, 

 and usually columella are present, and fre- 

 quently costae as well. Solitary forms are 

 few. Usually they form colonies, frer^uently 

 of thousands of individuals, bound together 

 by a ccenosarc extending over the surface of 

 the coral. A colony arises from a single 

 animal by continued fission or budding. 

 When the division is not complete the ani- 

 mals may form long series with numerous 

 mouths but with the other parts united, the 

 result being that the surface of the coral is 

 marked by long mnding grooves — incompletely separated theca — with sclero- 

 septa, as in the brain corals (fig. 204). The fossil Tetracoralla (p. 230) are 

 now regarded as modified Hexacorallans. (i) The Aporosa, a compact skel- 

 eton, the gastral canals running outside of the skeleton. Some, like Sderophylla 

 (fig. 198), are solitary. Others, like Oculina,* branch, and still others form 



Fig. 



—Ffiviii cavernosa (after 

 Klunzinger). 



