MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 487 



In the upper region of the calice the septal invaginations as a rule stretch but a short distance 

 radially into the gastro-ccelomic cavity, the portion of the polypal cavity included between one 

 septum ami another being spoken of as an interseptal chamber or loculus. As the lower regions 

 are approached, the septa extend farther and farther centrally, subdividing the cavity more and 

 more, until ultimately they unite in the middle to form or share in the columella. Where such 

 central fusion takes place, the gastro-ccelomic cavity is divided into interseptal chambers, which 

 in transverse section are wholly distinct from one another, laterally and centrally (figs. 55, 84). 

 If the septa of all the cycles, both entoccelic and exoccelic, extend as far as the center of the 

 polyp, each chamber is simple, and includes within it only one mesentery (Solenastrasa, tie-. 84); 

 hut when younger cycles extend only part way toward the center, each chamber is incompletely 

 subdivided, and may contain two or more mesenteries (figs. 54, 55). All stages toward the com- 

 plete isolation of the septal chambers are represented in passing a series of transverse sections in 

 review, from above downward, the primary septa being the first to unite centrally. Where the 

 septa do not completely fuse centrally, in other words, where they do not form a solid 

 columella, the interseptal polypal chambers remain in open communication at the middle 

 throughout the polyp (Mseandrina, PI. XXI, tig. 142). 



The actual manner in which the interseptal chambers terminate proximally is best studied 

 in entire, decalcified polyps. In some species they narrow gradually in an oblique manner, 

 while in others they terminate abruptly. Where the chambers are oblique, the polyps gradually 

 diminish in transverse area as the lower region is approached, the newer chambers ceasing in 

 advance of the older. This is characteristic of the genera Qculina, Favia, Agaricia, Maeandrina, 

 Mar$icina, and TsophyUia. In Orbicella, Solenastrmt, Sldi'r«xtr;i'n. Chuli>c<>r<t, Axtnmgia, Porites, 

 and most Madrepora, the polypal chambers are of practically the same sectional area from begin- 

 ning to end, though the truncation of the newer chambers may occur a little in advance of that 

 of the older. In mature polyps of the last series of genera, the basal floors of the mesenterial 

 chambers may all occur at practically the same level, as if all had been cut off by dissepiments 

 formed simultaneously. 



PI. V. fig. 42, representing transverse sections through Porites, shows how .the central cavity 

 is encroached upon by the spine-like, columellar projections; according to the level at which the 

 section is made, they may be either distinct or in continuity with the septa. The presence of 

 synaptic aila results in a similar encroachment on the outer regions of Siderastrasa, at the same 

 time leading to a disappearance of the peripheral portion of the mesenteries (PI. XXII). 



The extent to which the gastro-ccelomic cavity may be prolonged over the edge of the calicinal 

 wall, before it terminates at the line of communication with the adjacent polyps, varies greatly in 

 different species. In the older regions of colonies of Oculina, the interval between one polyp and 

 the next may be a centimeter or more; the mesenteries may. however, cease before the spiral 

 line of union of contiguous polyps is reached. In Cladocora the outside of the theca may be 

 covered for 5 or ti mm. by the flesh}- tissues, and the mesenteries subdivide the inclosed chamber 

 for practically the whole distance (PI. VII, fig. 54). The calicinal wall in Orbicella and 

 Solenastra&a is prolonged but a short distance above the level at which the contiguous polyps are 

 united with one another. Just as the calicular- portion of the polypal cavity is partitioned and 

 intruded upon by skeletal growths, so is the narrower extracalicular space; the mesenterial 

 continuations divide it into vertical chambers, and costal ingrowths usually alternate with the 

 mesenteries, corresponding with the septa internally. In Madrepora, where no perithecal 

 mesenteries occur, the space is broken up by costate ridges, which are more numerous than the 

 septa within. Here, however, the skeletal ridges actually come into contact with the superficial 

 wall and support it. and were it not for transverse communications the perithecal cavity would 

 be typically represented by a series of distinct vertical canals. 



SYNAPTICULA. 



Synapticula are solid calcareous bars of various form which unite adjacent septa across 

 the interseptal loculus. They are formed by the enlargement of granulations on opposite faces 

 of adjoining septa, growth continuing until the projections meet in the middle of the interseptal 



