THE WHITE STONY CORALS. 295 



their external aspect. All have a home in the sea or in brackish water, and are usually brilliant in 

 colour, radiating in their construction, have tentacles around a mouth which leads to a stomach, and this 

 opens into a large lower cavity, called the perigastric. This communicates with the hollow tentacles, 

 arid has the reproductive organs in it. Portions of it are folded at the sides longitudinally, 

 and projections of its membrane, called mesenteries, develop on and within them the ova and sperma- 

 tozoa. The ova and young escape by the mouth as free ova, or as planulaj hatched from them, and 

 not as medusae. There is an outer skin, or ectoderm ; an inner in relation to the stomach, the 

 endoderm ; and a tissue between, the mesoderm ; and this last secretes, in some, a hard and even cal- 

 careous skeleton. The soft structures are usually very contractile, and they contain cellular structures, 

 muscular fibres, much connecting tissue, and scanty, rudimentary nervous elements. There is no special 

 circulatory, respiratory, or excretory system. But the ciliated cells of the outer and inner derms move 

 water over the surface ; and in some of the Corals, with a great number of individuals collected in one 

 mass, there are evidences of a water system, which appears to regulate the symmetry of the whole. 

 The derm is crowded with nematocysts, or thread -cells, different as a rule in their construction from 

 those of the Hydrozoa ; but some have spiny barbs on the thread, and this is often invaginated more 

 or less before extension. Large and small cells are also present, containing glairy mucus, which escapes 

 on pressure, and colouring matter. The muscular fibres are delicate, without striae, and are longitu- 

 dinal and transverse, or encircling. The whole soft structure appears to have a power of general or 

 amoeboid movement. 



ORDER ZOANTHARIA. 



The first sub-order of the Zoantharia is that of the White Stony Corals, or Madreporaria, and the 

 members of it, very numerous in genera and individuals, live on the floor of the sea at all depths 

 down to 3,000 fathoms, and cling to the shore from water level to twenty fathoms. Those which 

 form Coral reefs come under the last assemblage, and the more solitary and simple deep sea Corals 

 belong to the first. All contain in the mesoderm a quantity of hard matter, composed mainly of 

 carbonate of lime in a fibrous or long crystalline condition, called Aragonite ; and externally a 

 simple kind will resemble a Sea Anemone. There is a range of tentacles, or more than one, on the top 

 of the body, and a disc within the circle of tentacles, in the midst of which is a small mouth. A 

 coloured tissue, like that of the outside of the tentacles and disc, covers the outside of the body, which 

 generally assumes a cup-like form, or may be flat, bell-shaped, tubular, or compressed like a fan. The disc 

 is marked with coloured lines that appear to radiate from the mouth, and if it is touched with a hard 

 pencil it will contract slightly, and then beneath it is felt a hard structure, made up of a number of 

 plates placed vertically, with their edges upwards. Spaces exist between these septa interseptal 

 spaces in which there is a process of the under part of the disc, the mesenteric fold. 



On stripping off the disc, the tops of the numerous septa are seen, covered with a filmy structure, 

 and between each pair a soft mesenteric fold. 



In the middle of the top of the Coral, and just under the opening of the mouth, is a hard pro- 

 jection, or axis (the columella), or else one does not exist, and the stomach cavity occupies the place. 

 Above the columella, or in its place, is the stomach, lined with endoderm, and having the mesenteries 

 radiating on all sides. Moreover, the tentacles, which are hollow, open into the interseptal spaces, so 

 that the fluid of the stomach can pass around all the soft internal parts and up into the tentacles. 

 The mouth in the disc has muscular sides, and is extensible, and it passes at once by a narrow space to 

 the underlying digestive cavity. Nematocysts and glairy cells and ciliated cells abound in these parts. 



The structures outside of the Coral, which are continuous with those of the tentacles above, are 

 thin, coloured, and abound with the same kind of cells as those just mentioned. The whole is under 

 the influence of the moving water, and is aerated by it. Food, in the form of minute invertebrata, 

 comes accidentally in the way, is stopped by the secretion of mucus or by the action of nematocysts, 

 and is moved to the mouth by the tentacles, which grasp it, or by cilia, which simply move it onwards. 

 The mouth opens, and the prey disappears to be digested, and the indigestible parts come forth from the 

 mouth. The juices of the prey are circulated from cell to cell, and add to the bulk of the creature. But 

 the calcareous parts of the prey its shell, for instance, had it one and a certain amount of the salts 

 of lime held in solution by the water, are retained in the structures of the mesoderm of the body, and 

 they form the hard Coral. The hard part of the Coral is produced by a deposition of carbonate of 



