536 MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 



relationships of the septa to the mesenteries and tentacles being established, Professor Haeckel's 

 classification of the Aiithozoa into Tetracoralla, Hexacoralla, and Octocoralla, applies with equal 

 force to the polyps as to the coralhim. 



It was early recognized that externally the polyps of corals verj^ closely resemble Actinian 

 polyps, and subsetiuent investigations along anatomical and histological lines hare but served to 

 emphasize the unity of structure. The question therefore natunilly suggests itself as to how far 

 the principles of classification adopted in the latter can l)e applied to the former. The earlier 

 subdivisions of the Actiniaria, as for example those adopted in Gosse's Actinologia Britaanica 

 (1860), and in Andres' Le Attinie (1883), rested wholly upon external characters. Mainly as 

 a result of the Actinological researches of the ))rothers O. and R. Hertwig (1879), the great 

 value of the arrangement of the mesenteries as an aid in classification was first realized. The 

 report by Prof. K. Hertwig (1882) on the " Challenger" Actiniaria, as well as subsequent contri- 

 butions b_v numerous workers, show how very widely and successfully anatomical characters may 

 be employed in determining phylogenetic relationships in the group. The main subdivisions of 

 the Actiniaria — Hexactinea\ Zoanthea?, Cerianthew — now rest most finnly upon the one character 

 of mesenterial development and adult arrangement, while other distinctive features found to be 

 associated with them pi-ove that the selection has ]\v no means an arbitrary significance." The 

 mode of development and adult arrangement of the mesenteries enable forms to l)e associated 

 which agree in more fundamental details than is possilile l)y any other selection, thus proving 

 that the mesenteries most nearly afford a basis for a true natural classification. 



In addition to the aid from the mesenteries, systematic chanicters of greater or less value 

 among the Actiniaria are afforded by the arrangement and form of the tentacles; the distribu- 

 tion and extent of development of the musculature, especially the presence or absence of columnar 

 ectodermal muscle fibers, and the natui-e of the sphincter nuiscle; the presence of vesicular or 

 other outgrowths of the column wall; free, fixed, colonial, or simple habit; presence or absence 

 of acontia, etc. 



We may now see how far these structural characters, so helpful among the Actiniaria, have 

 been or can be applied to the classification of the Madreporaria, or what others maj' be forth- 

 coming within the group itself. G. C. Bourne, in 1887, discussed the subject of the arrangement 

 of the Madreporaria in connection with his studies of the anatomj' of the soft parts of certain 

 species, and remarks (p. 12): 



"It has long lieen felt that a classification of Madreporarian polyps based on a study of the 

 corallum alone is unsatisfactory, and that any attempt to remodel the old classifications should 

 depend on a systematic study of the relations between the corallum and the polyp. Owing to the 

 difficulty of obtaining material, and of dealing with it when obtained, the mimber of forms 

 examined is as jM't small, and the results of recent researches have not advanced us very far 

 towards an improved classification." 



Later (p. 24), he writes: 



"I have treated the (juestions relating to the corallum at length, because every fresh form that 

 is examined convinces me that the expectations formed of founding a new classification of the 

 Madreporaria on the anatomy of the polyp are to meet with disuppointment. There is singularly 

 little variation in the forms hitherto examined." 



However, the same author (p. 29), regarding the presence or absence of radial symmetry and 

 of a " Randplatte" as of taxonomic importance, suggests the following arrangement as warranted 

 by the facts known at that time: 



1. Madreporaria with no directive mesenteries and a perfectly i-adial synnuetry, Lophohelia^ 

 Mussa, EuphyWui. 



2. Madreporaria with directive mesenteries and a combined ladial and bilateral symmetry, 

 Tubinaria, Rhodojmimmia, Fnngia, and many others. 



3. Madreporaria with reduced radial symmetrv and marked bilateral arrangement of parts, 

 Madrepora, PociUopara., Seriatopom. 



« The tendency in Actinological writings is now to regard eacli of these three divisions as ranking in importance 

 with the principal divisions of the Anthozoa — Alcyonaria, Antipatharia, etc. 



