22 MADREPORARIA. 
advantageous. The cross sections, in that case, show a system of stout concentric rings of 
tissue, thus yielding a stronger support than a stem of radiating trabecule could possibly 
supply. 
The remarkable growth method, described in Vol. IV. as the “expanding sheaf” form, 
occurs in this genus also, but it does not form columns so frequently as in Gonzopora (compare 
P. Great Barrier Reef 25 with figs. 6,7 on Pl. XVL., which show its top and side calicles). 
An exaggeration of this principle of growth may perhaps be seen in the remarkable expanding 
table-top forms (cf. Pl. XIII. fig. 8) found in the Fiji and Ellice Islands (see, e.g. P. Hilice 
Tslands 5). On the other hand, it is always possible to regard this latter growth-form as 
purely accidental, and due to the coral approaching too near the surface of the water, which 
would tend to encourage lateral rather than vertical growth. 
Only two other points need be mentioned. In all the growth-forms an epitheca supports 
the edge of the coral up to its furthest margin. This fact, as already pointed out, has some 
morphological significance, and differentiates the Poritide from the Madreporid, in which 
the septal skeleton in explanate forms may grow out some distance beyond the epitheca. 
We have already called attention to this relatively superior importance of the epitheca, as 
indicating the rudimentary character of the Poritid skeleton. 
The old controversy as to which are the older and which the younger calicles in Goniopora, 
might just as well have been applied to Porites. Its solution is the same in both cases (see 
Vol. IV., pp. 11, 12, 27). 
V. THE LIVING POLYP. 
Figures of the polyps of Porites have been given by Lesueur,* Dana,t Duchassaing and 
Michellotti,t Agassiz,§ Mr. Saville-Kent,|| and Dr. Duerden.{ 
Dana shows a retracted polyp, but was, I believe, the first to point out that the polyps 
when protruded differed from those of most other Madreporaria in standing high up above their 
calicles like stalked flowers, their skeletons being mere basal secretions, 
The stinging cells first described and figured by Agassiz (l.¢.) are arranged in hemi- 
spherical batteries at the tips and down the sides of the tentacles, while very large cells, 
40 » in length, containing a thick coiled thread, occur in the endoderm. These are also 
thought to be stinging cells; probably, while the small kind are aggressive, the large are 
purely defensive, and only released when the colony is injuriously attacked. 
* Mem. der Museum, vi. (1820), pls. 16, 17. 
+ Zooph. (1848), pl. liv. figs. 5 w and ) (polyp retracted). 
t Mem. sur les Cor. des Antilles Suppl. (1864), pl. viii. fig. 2. 
§ “Florida Reefs” (1880), pl. xvi. 
|| Great Barrier Reef (1893), chromoplate viii. figs. 7, 8, 9. 
{ Mem. Nat. Acad, Sci., viii. (1902) p, 415. 
