T. 0. Brown — Development of the Mesenteries. 535 



Akt. XXXIV. — The Development of the Mesenteries in the 

 Zooids of Anthozoa and its Bearing upon the Systematic 

 Position of the Rugosa ; by Thomas Clachar Brown. 



For more than half a century the systematic position of the 

 Paleozoic corals designated as Madreporaria rugosa by Milne- 

 Edwards and Haime, and Tetracoralla by Hgeckle has been 

 repeatedly discussed. Many writers have considered them to 

 be absolutely distinct from the modern stony corals and 

 actinians. Others have considered them as very closely related 

 to the modern forms; for example, Ogilvie* would derive the 

 various groups of Mesozoic and later Zoantharians directly 

 from as man} 7 different groups of Paleozoic forms, giving as 

 her chief reason for doing so the similarity in the microscopic 

 structure of the skeleton or corallite. Duerden, on the other 

 hand, approaching the subject from the developmental stand- 

 point and basing his conclusions upon the mode of addition of 

 new mesenteries in the zooid and the addition of new septa 

 in the corallite, holds the opinion that the rugose or tetrameral 

 corals are most closely allied to the Zoanthids among living 

 forms.f It has been suggested several times that the Tetraco- 

 ralla w T ere derivatives of hexameral forms, and that the 

 tetrameral plan was superimposed upon an original hexameral 

 form. \ 



The object of the present paper is to call attention to the 

 structure and development of a number of recent Anthozoa 

 and to point out the probable bearing of these facts upon the 

 interpretation of the corallites of Paleozoic species. 



It seems to have been taken for granted by many workers 

 on modern Anthozoa that a great unbridged gap separated the 

 Alcyonaria (Octocoralla, JELseckel) from the Zoantharia (Actini- 

 aria, Madreporaria, etc.). The Alcyonarian zooids have eight 

 mesenteries, never more, and eight tentacles which are branched 

 or pinnate. As a rule, when they build a skeleton it is 

 composed either of horny material, or of spicules of carbonate 

 of lime, or of a combination of these two. When a hard 

 skeleton is formed with calyces for the zooids, with septa-like 

 ridges within, these septa-like structures or pseudosepta bear 

 no relation to the intermesenterial spaces. 



The Zoantharian zooids, on the other hand, have six or more 

 mesenteries, and six or more simple unbranched tentacles. 



*M. M. Ogilvie, Phil. Trans. London, vol. clxxxvii, p. 83 ff, 1896. 



f J. E. Duerden, Ann. Mag. Nat. Hist., vol. ix, p. 381, 1902 ; vol. x, pp. 96, 

 382, 1902 ; vol. xi, p. 141, 1903 ; vol. xvii, p. 356, 1906 ; Biol. Bull., vol. vii, 

 1904 ; vol. ix, 1905. 



X R. Ludwig, Palseontographica, vols, x and xiv. L. F. de Pourtales, 111. 

 Cat. Mus. Comp. Zool., vol. iv, 1871. J. E. Duerden, loc. cit. 



Am. Jour. Sci.— Fourth Series, Vol. XXXIX, No. 233.— May, 1915. 

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