MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 485 



GASTRO-COELOMIC CAVITY. 



The term gastro-coelomic is applied to the whole of the internal, endoderm-lined cavity of 

 coral polyps, including any outgrowths or continuations which it may possess. In some ways 

 the designation is preferable to the terms gastro-vascular cavity or ocelenteron, generally 

 employed for the polypal cavity in Anthozoa. For from the considerations of van Beneden 

 (1891), and E. B. Wilson (1884), there seems some evidence to support the view that the space l>ut 

 incompletely inclosed by the mesenterial filaments is the morphological equivalent of the gastric 

 cavity, or enteron, of the higher Metazpa, while the remainder of the internal cavity, partitioned 

 by the mesenteries, is the morphological representative of the coelom of the Enteroccela. 



Among colonial corals the gastro-coelomic cavities of all the polyps in actual union with one 

 another arc in communication, and the nutrient fluid can pass from one to the other. This is a 

 persistence of the conditions consequent upon asexual development, whether this takes place by 

 gemmation or by fissiparity. In gemmation new polyps arise wholly or in part from the column 

 wall of other polyps, and the internal cavities of the two are common for a time. Thus the 

 developing bud of Madrepora, shown in the series of figures on PI. III. arises altogether from 

 the ccenosarcal wall of the colony, and its ccelomic cavity during the primary stages is repre- 

 sented by one of the superficial canals of the colony. In the developing polyp of Solenastrsea, 

 represented in section in PI. XII, fig. 87, a distinct partition wall, lined with endoderm on 

 both sides, now partly separates the bud from the parent; but interruptions occur at more or 

 less regular intervals, which permit of a circulation between the two cavities. 



The mode of communication of the various polypal cavities in a colony varies somewhat in 

 different forms. In genera like Pontes, Siderastrsea, and. Agarieia, in which the polyps are 

 separated from one another merely along a common calicinal wall, intermesenterial apertures 

 remain along the line of union, while the polyps are partitioned mesenterial ly. In Siderastrsea 

 septal partitions also occur, at any rate during retraction; for peripherally the column wall 

 comes to rest directly upon the septal covering, so that only a very narrow space is left on each 

 side between the mesenterial and the septal wall (PI. XXIII, fig. 156). The channels of commu- 

 nication of four adjacent polyps of Porites over the thecal edge are represented on PI. Ill, fig. 31, 

 taken from a section through the superficial region of a colony in which the polyps were in a 

 retracted condition. The fragments of the corallum seen are the slightly exsert septa, and the 

 canals pass over and around them. 



Adjacent polyps of Orbicdla and Solenastrsea are also placed in communication intermesen- 

 terially at the superficial line of union of the polyps. During retraction the apertures are not 

 connected directly with the main cavity, but through the intermediation of its perithecal 

 prolongations. The same method of superficial intermesenterial communication holds for the 

 contiguous rows in the genera reproducing by incomplete discal fission, e. g. , Mssandrma 

 (PI. XX. tig. 138), Colpophyllia, Manicina, Tsophyllia. In these, however, the polyps which 

 are united in the same discal system have no independent cavity, the one continuous chamber 

 is shared in common. The polyps do not attain true individuality; they can best be understood 

 as so many mesenterial and stomodseal systems within a general cavity. 



The numerous polyps constituting a colony of Madrepora are likewise in communication by 

 means of the superficial canals, which are continuous over the thecal edge with«the maiu gastric 

 cavity (PI. I. fig. :.'); but in the great group to which Madrepora and also Porites belong — the 

 Porosa— there is another and more complicated system of communication than that afforded by 

 the superficial canals. Anastomosing radial canals are given off by the basal (skeletotrophic) 

 part of the body wall, in such a way that they appear as if penetrating the corallum which 

 separates one polyp from another, and thus place the different ccelentera in union; in Madrepora 

 they further come into communication with the superficial canals. 



The radial canals are given off very closely in both Porites and Madrepora, and are disposed 

 both mesenterially and intermeseuterially, without any apparent regularity. As many as live or 

 six may be seen in a single transverse section (PL I, figs. 3-6). They are not so numerous in the 



