MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 413 



tion, there is no Randplatte, and one can hardly employ the term homologous in connection with 

 structures which are merely continuations of one another. 



Coenpsarc then, no more than Randplatte, is a polypal structure "sui generis;" the two are 

 merely special regions of the column wall and underlying skeletotrophic layer, in the latter case 

 provided with mesenterial continuations, and in the other de\ oid of them. Coenenchyme likewise 

 is inseparable from the portion of the thecal wall laid down by the extrathecal layer of the 

 morphological basal disk, under whatever name it may be known. The terms have merely a 

 topographical, not a morphological, significance." In the following pages column wall will 

 generally include the whole of theexternal body wall, from the line at which it passes below into 

 the skeletogenic tissues to the outer margin of the tentacular zone above. 



Fowler, in his studies of various species of corals, has given much attention to the relation- 

 ship of the peripheral part of the column wall to the skeleton, particularly to the manner in 

 which it may be said to be supported. At first it appeared that in species without coenenchyme 

 the column wall was supported upon only the perithecal continuations of the mesenteries 

 (••peripheral lamella?"), while in species with ctenenchyme the wall was directly supported 

 upon only echinulations of the skeleton. PI. VII, tig. 54, and PI. XIX, fig. 132, will serve 

 as examples of the former, and PI. I, figs. 2-<>, taken from Madrepora, are instances of the 

 latter method. Later, however, Fowler found that no such rule could be maintained; that the 

 two methods of support — mesenterial and echinulate — might co-exist in the same form, e. g., 

 Madracis, AmphiJu I'm. 



Where mesenterial continuations occur, the perithecal portion of the polypal cavity exists as 

 a series of simple vertical canals; but where mesenteries are absent, and the column wall rests 

 directly upon skeletal ridges or echinulations {Madrepora), the cavity is usually broken up into a 

 complicated system of canals. 



FORM AND ANATOMY. 



Externally the column wall of coral polpys presents few structural modifications compared 

 with the same region in the Actiniaria. There is an entire absence of the simple or complicated 

 columnar outgrowths often displayed in the latter group, and nothing comparable with a capitulum 

 or cycle of acrorhagi has been observed, the column always passing uninterruptedly into the 

 tentacles. Practically the only external distinction in this direction concerns the surface of the 

 column, whether smooth or verrucose. The latter condition is brought about by the presence of 

 teeth or spines on the edges of the costse and septa. Where these occur the polypal walls on 

 retraction come to rest upon them, and the areas over the projections become slightly raised 

 above the general surface, assuming a warty appearance; and even on fullest expansion, when 

 free from the corallum, the tubercle-like character rarely entirely disappears. Sometimes the. 

 verruca' are indicated by a slight color distinction, and often give a coarse appearance to the 

 polyps. Where the edges of the costa 1 and septa are smooth, or only finely toothed, the surface 

 of the outer polypal tissues is Likewise smooth. Histologically the verrucse present no differences 

 from the rest of the column wall, except that their constituent layers are generally thinner. 

 They are thus to be distinguished from the verrucse of Actinia', which are slightly modified 

 evaginations of the wall, or more often take the form of vertical rows of suckers, with a 

 strongly marked histological modification. The verrucse in corals are characteristics dependent 

 upon the form of the skeleton, rather than a structural differentiation of the soft tissues. 



Corresponding with the costse and septa, the verruca' are arranged in vertical intermesente- 

 rial rows, larger and smaller rows often alternating, in agreement with the large and small skeletal 

 partitions. This is readily seen in species of Orbia II, i, Favia, and Manicina, while in Mseandrina 

 all the rows are equal. The verruca 1 in any single row are somewhat irregular in size and height 



" The study of the Ooenopsammia from Lifu has fea<l Gardiner ( 1900, p. 361 ) to define ccenosarc in such a way as 

 to make it much more embracive than would either Bourne or Miss Ogilvie. Tims: "The Ccenosarc is that part of the 

 /:< lyps in n colony which lies outside but not abovt • i ■■■ in expand* d state) the thecm of the several coraUites. The 'Randplatte' 

 of von Heider and von Koch, the 'edge-zone' of Miss Ogilvie, is then that part of the ccenosarc which lies over the 

 free portions of the coraUites." 



Vol. 8 -No. 7 2 



