22 BRITISH FOSSIL CORALS. 
elevated above the calicular margin ; its tentacules are stretched out and overlap the hard 
parts, whilst the conical mouth is barely visible. Under other circumstances the disc is 
contracted, the mouth open, the tentacules more or less retracted, and the outer part of all 
the septa is visible through the translucent tissues. 
In certain “‘ serval” corals, such as Diploria cerebriformis, the edge of the disc gives 
exit to prehensile cirrhi, and these organs are to be seen projecting from the rim of the 
dise in Caryophylha clavus.” ‘They are very thread-like, and have prehensile. powers. 
The microscopic anatomy of these cirrhi has not been studied. 
The tubular structures, “ cordons pelotonnés,” which are attached to the juncture of 
the mesenteric folds with the pylorus,’ float about in the visceral cavity, and especially 
near the inner margin of the smaller septa ; their lower end is unattached and often rises 
on to the top of the columella. These tubular structures are very much _ twisted, 
hollow, and contractile, and are covered with cilia. They often contain ova. ‘The relation 
between the mesenteric folds and these tubular structures in the physiology of repro- 
duction requires further examination. 
The hard parts of the corallum are included in and nourished by soft tissues.‘ This 
is invariably the case in every species up to a certain period of growth. In some it is 
true during all the stages of their development, whilst in many species only the upper 
part of the corallum is in contact with the soft tissues after a certain height has been 
attained. 
Thus, in the Caryophyllia clavus the outside of the corallum is covered by soft tissues 
from its narrow base to its calicular margin and the imside also. ‘The wall, the costae, 
the septa, the pali, and the columella are covered by a membrane which sends processes 
into their dense structure. The nutrition, growth, and in some instances the absorption of 
the hard tissues, are carried on by means of the membrane and those processes, and so 
long as the hard and soft parts are in contact, the first cannot be said to be independent 
of the latter. . 
In corals where the growth is accompanied by the formation of dissepiments in the 
interloculi, the whole of the interior of the corallum below the dissepiments nearest the 
calice, is not in contact with the soft parts; it has ceased to be nourished by them, and it 
is to all intents and purposes dead. Moreover, the external membrane does not descend 
for any considerable distance below the calicular margin, and the lower parts of the coste 
and wall are as dead as the lower parts of the interior of the corallum. ‘his is the case 
in most of the large and luxuriantly growing compound corals, and only a few lines on 
their surface may be living, the rest is dead. Hach portion of the exdotheca, as it springs 
from the septa or wall, is formed by the fine membrane and is included in it; as growth 
proceeds the curved, straight, horizontal, or vertical dissepiment is lined on each surface 
1 Plate II, fig. 17. 2 Plate II, fig. 11. 3 “Plate If, ig. 2 
4 Plate I, fig. 17, diagram. 
