ADULT COLONY. 35 



much subdivision and complexity of the lower part of the polypal cavity, as 

 compared with that of an actinian, where the basal disc retains its primitive 

 flatness. In the coral polyp radial parts of the basal disc are pushed vertically 

 upwards into the polypal cavity, concurrently with the deposition of calca- 

 reous matter, so that some portions of the aboral wall are of the height of 

 the septa. The earliest stages in the formation of the basal infoldings over 

 the septa are shown on plate 9, fig. 53. In the later processes of growth 

 the lower edge of the column wall is also carried upwards to about the same 

 height as the septal invagination,* hence it results that the basal wall is 

 represented by so many vertical lamellar infoldings, the column wall and 

 disc appearing as a mere superficial covering to these. 



In transverse sections the invaginations are found to occur between 

 every two mesenteries, that is, they are both entocoelic and exoccelic in 

 position, the two series being equal in number, the exoccelic extending less 

 centrally than the others (plate 6, fig. 34). In the upper region of retracted 

 polyps the invaginations do not extend as far centrally as the primary mesen- 

 teries, though in the stomodasal region they are continued beyond the secondary 

 and tertiary mesenteries. As the lower regions are approached the foldings 

 stretch centrally beyond all the mesenteries until, in the end, the walls of 

 adjacent invaginations unite with one another and thus completely inclose 

 each mesentery in a separate interseptal loculus, the skeletal deposit being 

 also continuous from the periphery to the middle (plate 7, fig. 39). 



As shown in the various figures, the septal invaginations do not remain 

 truly radial. At their inner extremities the exoccelic members turn laterally 

 to unite with an entoccelic invagination, and the entoccelic of the third cycle 

 are united with the entoccelic of the second. In this way continuity of the 

 invaginated walls in transverse section is broken, and the skeletal matter 

 within one invagination is likewise continuous with that in another. On plate 

 7, fig. 39, it is seen that some of the septa are continued radially as far as 

 the central columella, while the others extend centrally only indirectly by 

 fusing with the first. As a result of these many fusions the cyclic plan of 



* In many corals, e. g-., Cladocora, Afanicina, the proximal margin of the column wall is lower than 

 the upper edge of the septa, and the latter are generally united peripherally in the thecal wall; the septa 

 have grown upwards into the polypal cavity, leaving the lower boundary of the column wall behind. 

 The mesenteries still remain attached to the outer column wall, but internally they have been 

 pushed upwards, as it were, along with the upward growth of the thecal wall. Owing to this, parts of 

 the polypal cavity and mesenteries are peripheral or outside the calice ^extracalicular or pericalicular) 

 and the remainder is within the calice (intracalicular). The portion of the polyp outside and around the 

 calice is what is known as the "edge-zone" or "Randplatte," but, as noticed on p. 9, there is no edge- 

 zone in Siderastrea, the polypal cavity and mesenteries being wholly intracalicular when the polyps 

 are retracted. 



