MADREPORARIA The Madreporaria, zoologically known as "true corals," form a large group of Anthozoa which are characterized by their power of secreting a massive limy skeleton. The formation of a skele ton is not absolutely universal among them, but the number of species which have no hard parts is extremely small. The Madrepores contrast with the other Coelenterata which secrete a massive calcareous skeleton, in that although the actual size of individual polyps varies greatly, the average order of size is large ; moreover the structure of the polyps is distinctive. The Madreporaria tend to form colonies containing few or many polyps; but in addition to the co!onial species there is a large number of solitary forms in which a single polyp produces a single coral skeleton.
The polyps are similar in general build to Sea Anemones; but in their finer structure are recognizably different from the latter. Moreover, their life is necessarily a purely sedentary one since after they have once secreted a skeleton they are permanently anchored to it, and in correlation with this fact and with other of their characteristics, they are less muscular than anemones and exhibit in particular a less marked development of the retractor muscles. The variation of structure among coral-polyps is wide; but it hardly parallels the extraordinary diversity found among the anemones. In the latter, diversity of the individual reaches its height, whereas in the colonial corals equally great diversity affects the relations of the polyps to one another, and conse quently the form of the colony which they build up.
Corals are widely distributed in the seas of the world and both solitary and colonial forms may occur either on shore or in deep water. The true reef-building corals, however, which are in the main colonial forms, are restricted to the tropical and sub-tropi cal zones ; they flourish best in shallow water and their depth limit is about 5o metres. It is significant that rapid deposition of calcium by marine animals is favoured by high temperatures.
The Madreporaria are an ancient group geologically, dating from Triassic times, and vast numbers of fossil forms are known. Many other corals which may or may not have been ancestral to them (the Tetracorallia, etc.) are known from much earlier times. Corals form the largest bulk of fossils belonging to the Coelenterata and some limestones are composed almost entirely of their remains. In certain past epochs reef-corals had a far wider distribution than is now the case, their remains being plentiful in latitudes at which they cannot now maintain them selves.
The skeleton of a solitary coral-polyp such as Caryopjiyllia (a genus represented even in Great Britain by C. smithii, the "Dev onshire Cup-coral") is illustrated in fig. 1o. It consists of a num ber of parts which together build up a shape definitely related to the soft parts which have secreted it. There is a basal plate attach ing the whole structure to the substratum, a circular wall (theca) arising from this, and a number of radially arranged ver tical partitions (septa) which project inwards from the inner surface of the theca towards the centre, and which partially sub divide its cavity. The septa are not all equal but belong to definite grades of size, which alternate regularly.
The polyp is seated in the cup of skeleton, and during life its body extends well above and beyond the latter, overlapping also down the outside of the theca. If the polyp desires to swallow any considerable mass of food, it must necessarily extend itself above the skeleton in order to make room in its coelenteron for the food. The tissues of its column and base line the skeletal cup, and its mesenteries alternate with the septa; the septa, however, merely push the whole thickness of the polyp's column wall inwards, they nowhere penetrate into the coelenteron and are entirely external to the tissues of the animal. It will thus be understood (fig. i r) that although there is deep inter-penetration between skeleton and soft parts, morphologically speaking the skeleton is entirely an external structure. The skeleton is pro duced as a secretion of cells known as calicoblasts, which are formed by the ectoderm of the base and sides of the polyp.
The eggs of corals develop into planulae which attach them selves of ter a time to a foreign surface, assume the form of miniature polyps and begin to build up a skeleton, which first appears between the base of the polyp and its support. A few corals lie unattached and are anchored simply by the weight of their skeleton.
For an account of the formation of coral-reefs see CORAL REEFS. Some idea of the variety which the skeletons exhibit may be derived from the Plate. It must be understood that the growth of a colony is due to the continued deposition of calcareous material by the polyps, and as more and more is added the skeleton must necessarily increase in size, growing upwards or outwards or both. The form which any skeleton assumes is dictated by a number of factors affecting the polyps—by the way in which new polyps arise with relation to those already in existence ; by their rate of formation and growth ; by the relation between growth in height and growth in width; by the angle and plane of divergence between the polyps as the colony grows; by the presence or absence of a secretion of skeleton in the intervals between the polyps, and similar considerations. Each species of coral has inherent within its polyps the ability to develop in a given manner, and the form of the colony is also affected by the degree of its exposure to wave-action.
Little is known about the details of colony-formation from actual observation, although measurements of the growth-rate, affecting increase in size of the colony regarded as a whole, have been taken. The older accounts concerning the processes by which the polyps increase in number should be read with re serve, since the ideas in vogue on this subject have until recently been largely speculative, and theories of colony-formation have been too much based on the study of the skeleton. It is obvious that the skeleton is merely an imperfect if permanent record of the activities of the polyps, and study of the latter reveals the fact that in some cases at least conclusions as to the manner of growth, drawn from the skeleton, may be entirely erroneous. A recent study by Matthai of the soft parts of a number of corals from which the calcareous matter had been dissolved away, leads to the conclusion that in the cases investigated (and probably in the group in general) polyps are formed in two ways.
The classification of Madreporaria is a vexed question. The vital parts of the organisms are the polyps, and as yet too little is known of the structure and potentialities of these throughout the group to make possible the construction of a satisfactory system.
(T. A. S.)