738 THE ANIMAL KINGDOM. 



of the branch, the mouth transversely to that axis. Two mesenteries only 

 are complete, and they correspond to the long axis of the body : they have 

 mesenterial filaments, and carry the sexual products. Two short mesen- 

 teries correspond to each end of the shorter diameter, and two still more 

 reduced lie one on either side of each complete mesentery. There are con- 

 sequently ten mesenteries in all. A canal traverses the colony beneath the 

 bases of the zooids, but whether or not it is in connection with them is un- 

 certain. The surface of the colony secretes in Gerardia and some species 

 of Antipathes a viscid mucus which entangles foreign bodies, such as spicules 

 of Axifera, and Sponges, grains of sand. The nematocysts of the ectoderm 

 in the two genera named are aggregated into groups. The sexes appear 

 to be separate. 



The Madreporaria are simple or colonial, and differ most markedly 

 from other Zoantharia in the presence of a continuous calcareous skeleton 

 secreted by cells or calycoblasts, which either actually are, or represent the 

 basal ectoderm J . The skeletal structures are therefore, strictly speaking, 

 external to the animals to which they belong. The form of the skeleton is 

 very variable. In a typical simple, i. e. non-colonial Madreporarian, it consists 

 of a theca, which is as a rule attached by a base, large or small, to some 

 foreign object, but it may, as in Flabellum, &c., become free when adult. 

 The theca is excavated terminally by a depression, the calycle or calice in 

 which the animal is lodged, the soft parts extending over its lip to a variable 

 extent. The theca is sometimes flattened, e. g. Bathyactis, Ftmgia, and it 

 may then, as in the genera named, be covered completely on its lower sur- 

 face by soft tissues. The inner aspect of the calycle is not smooth, but 

 bears a number of radial calcareous ridges or septa, differing in size and 

 extent, and accordingly distinguishable into systems, primary, secondary, &c. 

 Certain of these septa may meet in the centre of the cup, where they form 

 a pillar or columella, which may be prolonged more or less upwards. They 

 may unite together in various degrees, and there may be at or near their 

 central ends one or more circles of plate-like upgrowths, known as pali. 

 The outer surface of the theca may be marked with ridges or costae, which 

 do or do not correspond to the septa, and are separated by intercostal 

 spaces : it may be covered to a greater or less height by an epitheca or 

 calcareous layer distinct from the theca itself, which is sometimes extended 

 into radicles 2 . In the colonial forms the shape of the colony depends on 

 the mode of growth and multiplication of the individual ; it is massive, 

 branched, lamellate, cup-shaped, &c. The soft parts of the zooids may 



1 Vegetable parasites are often found in the coralla of Madreporaria, Heliopora, and the 

 coenostenm of Millepora. See Moseley, ' Hydrocorallinae,' Challenger Reports, ii. p. 30 ; Martin 

 Duncan, P. R. S. xxv. 1877, and Quart. Journal Geol. Soc. xxxii. 1876. 



2 For definitions of descriptive terms relating to Corals, see Martin Duncan, J. L. S. xviii. pp. 

 200-2. 



