ANTHOZOA ZOANTHARIA. 737 



developed transverse muscles 1 . The body is elongated and pointed 

 aborally where it is perforated by a pore. It is covered by a tough invest- 

 ment of mucus, discharged nematocysts, and foreign bodies derived from 

 the soil in which the animal lives immersed. The tentacles are in two 

 circles, one marginal and long, the other circumoral and shorter. The 

 former have a series of slit-like pores on the oral aspect. There is a sub- 

 ectodermic layer of plaited longitudinal muscles, not present in any other 

 Actiniaria, which thins away in the dorsal median line, denoted externally 

 by a furrow. The animal is hermaphrodite. Sterile and fertile mesenter- 

 ies are said to alternate one with another. 



So far as concerns the histological structure of Actiniaria, it agrees 

 more or less closely with that of Tealia, described on pp. 241-42. The 

 ectoderm sometimes secretes a cuticle, e. g. in Phellia among Hexactiniae^ 

 and in Zoanthus. The consistency and degree of development of the 

 mesoglaea is liable to variation ; it is homogeneous, with or without cells, 

 or fibrillate. The muscle cells either lie evenly on the surface of the 

 mesoglaea, or the latter is thrown into supporting folds or plaits, or may 

 even completely inclose the cells. The endoderm cells of Cereanthus bear 

 many cilia, not a single flagellum, as in Tealia. 



The Antipatharia are degenerate, but the different genera in various 

 degrees. The ectoderm of the base secretes an organic or horny lamellate 

 skeleton. GepJiyra Dohrnii is the least modified. Its zooids are either 

 isolated or in small groups united by their bases, and fixed by their basal 

 plates to the stems of the Axiferan I sis which they embrace. The tentacles 

 number about eighty and are disposed in circles. The mesenteries are 

 numerous, and bear each a mesenterial filament. Gerardia forms colonies 

 growing upon the stems of Gorgonia, on rocks, shells, &c. There are 

 twenty-four tentacles, a large and a small alternating, and twenty-four 

 mesenteries all complete. The zooids are united by a reticulum of vessels 

 which communicate with the interseptal spaces. Antipathes is also colonial. 

 The basal skeleton is erect, and carries rows of lateral branches, sometimes 

 branched in turn. It is tubular, and the tube crossed from place to place 

 by partitions ; it is covered with small spines, and is inclosed in an epithelial 

 ectodermic (?) sheath. The zooids are disposed in a line on the branches, 

 usually on the aspect looking upwards, i. e. towards the apex of the stem. 

 They have six short tentacles. The zooid is elongated in the axial plane 



1 In large specimens the mesenterial filaments undergo a change close to the lower border of 

 the stomodaeum. They are drawn out into a number of simple or branched ' mesenterial threads.' 

 These structures commence as small papillate projections of the mesentery, bordered of course by the 

 mesenterial filament. The papillae grow out, become filiform, and may branch. Hence a cross 

 section of such a thread shows as it were two mesenterial filaments back to back. An acontium 

 differs from a mesenterial thread (i) in being attached to the surface, not the edge, of a mesentery ; 

 (2) in being simple and filiform, and single in cross section ; and (3) in containing very large 

 numbers of nematocysts. 



3B 



