ANTHOZOA
460
A large number of corals, both aporose and perforate, are colonial. The colonies are produced either by budding or division. In the former case the young daughter zooid, with its corallum, arises wholly outside the cavity of the parent zooid, and the component parts of the young corallum, septa, theca, columella, &c., are formed anew in every individual produced. In division a vertical constriction divides a zooid into two equal or unequal parts, and the several parts of the two corals thus produced are severally derived from the corresponding parts of the dividing corallum. In colonial corals a bud is always formed from the edge-zone, and this bud develops into a new zooid with its corallum. The cavity of the bud in an aporose coral (Fig. 18, A, C) does not communicate directly with that ot the parent form, but through the medium of the edge-zone.
grow farther apart, this continuity is broken, each corallite has its own edge-zone, and internal continuity is also broken by the formation of dissepiments within each calicle, all organic connexion between the two zooids being eventually lost. Massive meandrine corals are produced by continual repetition of a process of incomplete division, involving the mouth and to some extent the peristome : the calyx, however, does not divide, but elongates to' form a characteristic meandrine channel containing several zooid mouths. , T( , Corals have been divided into Aporosa and F erf or at a, according as the theca and septa are compact and solid, 01
M.
Fig. 17.—Transverse section through a zooid of Cladocora. The corallum shaded with dots, the mesogloea represented by a thick line. Thirty-two septa are present, six in the entocceles of the primary cycle of mesenteries,! ; six in the entocoeles of the secondary cycle of mesenteries, II; f ur in the entocoeles of the tertiary cycle of mesenteries, II', only four pairs ot the latter being developed ; and sixteen in the entocoeles between the mesenterial pairs. D, D, directive mesenteries ; st, stomodieum. (After Duerden.) As growth proceeds, and parent and bud become separated farther from one another, the edge-zone forms a sheet of soft tissue, bridging over the space between the two, and resting upon projecting spines of the corallum. This sheet of tissue is called the coenosarc. Its lower surface is clothed with a layer of calicoblasts which continue to secrete carbonate of lime, giving rise to a secondary deposit which more or less fills up the spaces between the individual coralla, and is distinguished as coenenchyme. This ccenenchyme may be scanty, or may be so abundant that the individual corallites produced by budding seem to be immersed in it. Budding takes place in an analogous manner in perforate corals (Fig. 18, B), but the presence of the canal system in the perforate theca leads to a modification of the process. Buds arise from the edge-zone which already communicate with the cavity of the zooid by the canals. As the buds develop the canal system becomes much extended, and calcareous tissue is deposited between the network of canals, the confluent edge-zones of mother zooid and bud forming a coenosarc. As the process continues a number of calicles are formed, imbedded in a spongy tissue in which the canals ramify, and it is impossible to say where the theca of one corallite ends and that of another begins. In the formation of colonies by division a constriction at right angles to the long axis of the mouth involves first the mouth, then the peristome, and finally the calyx itself, so that the previously single corallite becomes divided into two (Fig. 18, E). After division the corallites continue to grow upwards, and their zooids may remain united by a bridge of soft tissue or coenosarc. But in some cases as they
Fig IS — A Schematic longitudinal section through a zooid and bud of llMora'di^taU. In A, B, and C the thick black lines represent the soft tissues ; the corallum is dotted, s, stomodseum ; c, c, ccenosarc , col, co!umella • 2' tabuhe. B, Similar section through a single zooid and bud ot Astraides ’calimlaris. C, Similar section through three corallites of LophoUelia ZolifeZ ez, edge-zone. D, Diagram illustrating the process of budding by unequal division. E, Sectionlnthrough a dividing cahcle Mima, showing the union of two septa . th.eh ^aneoriginal’- the the origin of new septa at right angles to them. (C, original, tne rest after von Koch.) are perforated by pores containing canals lined by endoderm. The division is in many respects convenient for descriptive purposes, but recent researches show that it does not accurately represent the relationships of the different families. Various attempts have been made to classify corals according to the arrangement of the septa,, the characters of the theca, the microscopic structure of the corallum, and the anatomy of the soft parts. The lastnamed method has proved little more than that thcie is a remarkable similarity between the zooids of all recent corals, the differences which have been brought to light being for the most part secondary and valueless for classificatory purposes. On the other hand, the study of the anatomy and development of the zooids has thrown much light upon the manner in which the corallum is formed, and it is now possible to infer the structure of the soft parts from a microscopical examination of the septa, theca, &c., with the result that unexpected relationships have been shown to exist between corals previously supposed, to stand far apart. This has been particularly the case with the group of Palieozoic corals formerly classed together as Rugosa. In many of these so-called rugose forms the septa have a characteristic arrangement, differing from that of recent corals chiefly in the fact that they show a tetrameral instead of a hexameral symmetry. Thus in the family Stauridce there are four chief septa whose inner ends unite in the middle of the calicle to form a false columella, and in the Zaphrentidce there are many instances of an arrangement, such as that depicted in lig. 19, which represents the
