524 RADIATES: POLYPS. 



A. capitata, Ag., of the coast of North and South Caro- 

 lina, is six inches high in some cases ; lives in the mud. 



The Genus Mctridium has the column very contractile 

 and changeable in form ; often much elongated. 



The " Sea-Anemone," or Fringed Actinia, M. margina- 

 tuni, Milne-Ed w., is the most common of all the polyps 

 on the northeast coast of North America. The larger 

 specimens are about four inches high, and three inches 

 across the disk, in expansion. Figs. 515-517. 



The Sub-Order of Antipathacea comprises polyps which 

 have from six to twenty-four simple tentacles, are con- 

 nected, and secrete a solid axis. It contains two fam- 

 ilies, -- Antipathidae and Gerardidae. 



The Sub-Order Zoanthacea contains compound, fixed 

 polyps, which have the tentacles simple, short, and at the 

 edge of the disk. Families : Zoanthidae and Bergidae. 



SUB-SECTION III. 



THE ORDER OF MADREPORARIA. 



THIS Order embraces polyps which are simple or corn- 

 pound, with a broadly expanded form, simple conical ten- 

 tacles, and whose dermal tissues and usually the radiating 

 lamellae deposit solid coral. They abound in the warm 

 seas, to which they are mainly confined. There are four 

 sub-orders, Madreporacea, Astraeacea, Fungacea, and 

 Stauracea. 



The Sub-Order of Madreporacea contains those which 

 have the tentacles in definite numbers, twelve or more, 

 well developed, and encircling the narrow disk. The coral 

 is porous, dermal, and septal. The polyps are mainly 

 compound by budding, and the growth chiefly vertical. 

 There are four families, Eupsammidae, Figs. 523, 524, 

 and 526, Gemmiporidae, Poritidae, and Madreporidae. 



PORITID/E, OR PORITES FAMILY. - - This Family is char- 

 acterized by shallow cells, often hardly traceable within 

 the coral. 



