296 NATURAL HISTORY. 



lime in long or short and slender needles, or prisms, in the interstices of a peculiar connective tissue 

 of the mesoderm. Hence these Corals are called Sclerodermic Zoantharia. 



By placing a Coral in weak hydrochloric acid and water, effervescence of carbonic acid ensues, and 

 the lime combines with the acid and is dissolved. At last a film remains of the shape of the Coral, 

 and it represents the organic basis of it. An old piece of Coral, when cut in slices for microscopic 

 examination, shows numerous radiating lines once occupied by the organic matter, and starting from 

 them, on all sides, are bunches and masses of the prisms and irregular-shaped needles of carbonate of 

 lime. In some Corals this texture is very dense, and in others very lax and porous ; and in these 

 last the texture of the hard part is very spicular, the ends being joined to form a kind of cellular 

 structure. Hence the two great divisions of the Madreporaria : the Aporosa and the Porosa. 



The hard parts of the Coral are remarkable for their regular radiation and numerical 

 arrangement. They consist of a theca, or wall, which forms the cup of the Coral, which is closed below 

 at the base, and open at the opposite end, at the calice. The septa, or vertical plates, pass from the 

 inside of the cup towards the central axis. They are free above at the calice, and are sometimes not 

 joined or attached to anything in the centre of the cup, but there may be a columella there, which 

 starts from the bottom of the cup and grows upwards ; or it may be formed by the ends of the 

 septa. The interseptal spaces are open from top to bottom in some Corals, and in others there are 

 thin pieces of carbonate of lime, which cross them more or less, and cut off the lower parts from the 

 upper. The animal secretes these, and lives above the upper one. They are called dissepiments. 



Outside the cup there are longitudinal ridges in relation to the septa within, which are called 

 ribs or costae ; and they may be united by ci'oss bars. 



Some Corals are always simple and separate; others, and especially the reef-builders, are 

 compound : that is to say, they propagate by budding from the parent, and then the buds form a 

 succession of buds. A little projection appears on the side of a cup, and soon a few tentacles are 

 seen there. It grows outwards and upwards, and resembles the parent. This is a bud. Other buds 

 arise, and all grow upwards in a bush-like form, and then the buds begin to bud, and so on. There 

 is a symmetry in the growth, and either this bush shape remains, or else structures are grown between 

 the buds and the parent, connecting the whole in a solid mass, called exotheca. They are composed 

 of layers of hard tissue arranged in cellular compartments or cross bars, so as to give great bulk, 

 lightness, and strength to the Coral. 



The Corals of all kinds produce ova, which escape from the mouth, and hatch into long ciliated 

 bodies, or planulae. These fix themselves and develop, becoming like the parent. The growth of the 

 individual is accompanied by an increase in the number of mesenteries within the body, and of solid 

 septa between each pair of mesenteries. Two great series of Corals develop septa differently. In one 

 six septa are followed by six smaller, then twelve still smaller are formed, one in each of the 

 already made interseptal spaces, and then twenty-four, and so on, the increase being by cycles in 

 multiples of six. In another series there is a more or less distinct increase by fours, and the position 

 of one or more of the first, or primary septa, is sometimes occupied by a groove. Or there may be a very 

 numerous collection of septa, which appear to be without any rule, and to be alternate in size. In some 

 Corals no definite order can be distinguished. Certain appendages to the septa, between them and the 

 columella, are called pali, and they appear to have reference to a fresh circlet of tentacles.* 



The first series are the Hexactinellids, and the second are the Rugosa. 



The Corals which live on the southern coasts of England are simple, and do not form reefs, and 

 others of the same genera are found in deep water on the floor of the great oceans. Pressure and 

 temperature seem hardly to influence them, and they flourish in the great depths, in water not much 

 above freezing-point. But the reef-building Corals require a warm sea, and highly aerated, pure 

 sea water, containing an abundance of living things. These conditions are only to be obtained in those 

 parts of the world where the surface temperature of the sea is never less than 68 Fahr., and indeed 

 some Corals require a much higher temperature, such as 72 Q to 86 W . Moreover, the necessary purity 

 of water and freedom from sediments can only be got in the neighbourhood of islands standing in deep 

 water. As the temperature of the sea diminishes rapidly with depth, that of 68 to 86 is not 



* An individual Coral, perfect in itself, is a Corallum ; a member or an individual which has budded or divided off, and yet 

 still remains as part of a whole, is a Corallite. 



