MEMOIRS OF THE NATIONAL ACADEMY OK SCIENCES. 537 



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4. Madreporaria with a basal pseudotheca and no " Randplatte," Fldbelh 



None of these characters, however, can be regarded as of importance in the foundation of the 

 principal subdivisions of the group. Investigations on the Actiniaria, as well as those here given 

 on the Madreporaria, indicate the exact value to he assigned such details ;L s the presence or 

 absence of directive mesenteries, while questions of symmetry, unaccompanied by developmental 

 history, have very little significance. For example, the apical polyps of Madrepora exhibit 

 externally the most perfect radial symmetry, while the radial polyps from which they are derived 

 are markedly bilateral. The presence or absence of directives is a secondary, not primary, char- 

 acter, already shown to he dependent upon the mode of asexual reproduction of the species, and 

 has therefore no fundamental significance. The marked bilateral arrangement of the parts in 

 Madrepora, Pocillopora, and Seriatqpora are a retention of larval characteristics. 



The quotations from Bourne accurately represent the opinion of zoophytologists with regard 

 to the classificatory value of the Madreporarian polyp, and little progress along such lines has 

 since been made, while much attention has been concentrated on the skeleton. 



Undoubtedly the mesenteries are the organs of greatest taxonomic importance among the 

 Anthozoa; for the tentacles and most other outgrowths which may occur are arranged in strict 

 accordance with them, and in tin 1 Madreporaria the arrangement of the septa follows most 

 directly upon that of the mesenteries. From a truly morphological standpoint all other polypal 

 structures are of subordinate value. It may. therefore, be safely accepted that so far as any 

 classification among- the Madreporaria can be founded upon differences in the mesenteric system 

 it will he fundamental, and of course, the same remark applies to the septal svstem. as this is 

 determined by the former. 



Reviewing the arrangement of the mesenteries so far disclosed within the Madreporaria a 

 perfect uniformity occurs as far as the protocnemic stage, or stage with only six pairs of mesen- 

 teries. It seems doubtful, however, whether any species of living coral invariably retains this 

 primary condition in all its mature polyps. No such group of Actinians i.s now known since 

 Faurot (1895) discovered four or six pairs of rudimentary metacnemes in E/ltranhia. By far 

 the majority of the adult polyps of Porites and Madrepora never get beyond the protocnemic 

 stage, but occasionally such examples occur. According to Moseley and Fowler, the adult polyps 

 of Pocillqpora and Seriatopora have only six pairs of mesenteries, hut Verrill and Queleh 

 mention that occasionally twenty-four septa are present, which would imply the occurrence of 

 twelve pairs of mesenteries in the polyp. 



Tin' protocnemic stage being probably alike in all modern corals, it is clear that any diver- 

 gences in the mesenterial plan must be looked for in the subsequent development, that is, in the 

 metacnemic succession. 



Two altogether different types of metacnemic sequence and adult arrangement are now 

 known — theone in which the metacnemes appear in unilateral (isocnernic) pairs all round the polyp, 

 and in the adult present a cyclic disposition, represented by the majority of corals; and the other 

 with a bilateral origin and arrangement of the mesenteries throughout, as yet definitely ascertained 

 only for the genera Porites and Madrepora. The two types have been shown to be somewhat 

 comparable with the metacnemic sequence and resulting arrangement in the Hexactiniae and 

 Ceriantheae among the Actiniaria, and I propose to make of them two Madreporarian groups 

 of nearly equivalent value as follows: 



Entocnemaria. — Madreporaria in which the mesenteries always arise in bilateral pairs, and 

 beyond the protocnemic stage the increase takes place within one or both of the directive 

 entocoeles. 



( 'yclocTu m, tri<i. — Madreporaria in which the mesenteries beyond the protocnemic stage arise 

 in isocnernic unilateral pairs within the primary exoneles. The mesenteries in the adult are 

 usually arranged in two or more alternating cycles. 



So far as our knowledge of the anatomy and development of coral polyps goes, the second 

 group will include the majority of recent forms, and fossil genera in which a regular multicyclic 

 disposition of the septa can be established, while the first will comprise Porites, Madrepora, and 

 probably certain fossil coral- exhibiting a bilateral arrangement of the septa. The Entocnemaria 



