23 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 visi1)le through the translucent tissues. 



In certain " serial " corals, such as Diploria cerchriformis} the edge of the disc gives 

 exit io prehensile cirrlii, and these organs are to be seen projecting from the rim of the 

 disc in Caryopliyllia clavits." They are very thread-like, and have prehensile powers. 

 The mici'oscopic anatomy of these cirrhi has not been studied. 



The tubular structures, " cordons pelofonnes" Avhich 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 structiu-es 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 Caryophjllia clavus the outside of the corallum is covered by soft tissues 

 from its narrow base to its calicular margin and the inside also. The wall, the costse, 

 the septa, the pali, and the columella are covered by a membrane which sends processes 

 into their dense structm-e. 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 groAvth 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 costse 

 and wall are as dead as the lower parts of the interior of the corallum. This 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. Each portion of the endofheoa, 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. \7. ■ Plate II, fig. 1 1. ^ Plate II, fig. 2. 



^ Plate I, fig. 17, diagram. 



