THE ANATOMY OF THE MADHEPORARIA. 
591 
already described by Semper ( 12 ), but certain corrections are 
to be made in his account relative to the arrangements of the 
septa. Beautiful figures of the colony will be found in his 
paper, which contains much valuable and curious information 
about the group Madreporaria. 
The corallum of a polyp is about 30 cm. in height ; the calyx, 
which is as usual oval in outline, measures about 18 mm. in 
the longer axis, and 9 — 13 mm. in the shorter. Fresh polyps 
may be budded off from the side, or, more rarely, from the 
calyx. 
The theca has the porous appearance characteristic of the 
Perforata, and is marked on the external surface by distinct 
spinous costae, or ridges; each of which corresponds exter- 
nally to the attachment of a septum on the interior surface of 
the theca (fig. 14). 
Both exosepta and entosepta occur in this form. Of true, 
i.e. entocoelic septa, there are only three orders, with occa- 
sional traces of a fourth ; from the sides of each primary and 
secondary entoseptum grows out au exoseptum (fig. 14), and 
the relations of these two classes to each other are rather 
complicated. Such a system as a — a in fig. 22, shows, in a 
transverse section taken high up in the polyp, the arrangement 
diagrammatised in fig. 19, consisting of five true entosepta 
(each of which lies between a pair of mesenteries), and four 
exosepta alternating with them. In a lower section (fig. 20), 
the two exosepta which grow out from the sides of adjacent 
primary and secondary entosepta, fuse over and with the inter- 
mediate tertiary septum into one. Lower yet (fig. 21), the two 
compound septa thus produced in each system meet over and 
with the secondary septum ; so that the columella is due to 
the irregular fusion (fig. 15) of twelve primary entosepta, 
distinct for their whole length, and twelve other septa thus 
elaborately compounded. 
ii. Anatomy. — In Rhodopsammia, which, like all the other 
forms as yet described, bears a close resemblance to an Actinia, 
the mouth-disc, unlike the case in Flabellum, passes into a dis- 
tinct external body wall of ectoderm, mesoderm, and endo- 
