372 E A L S base free and pelagic, or living buried to the lips in mud or sand. No corallum, but occasionally a membranous epidermic tube of investment. (Ilyanthus, Peachia, Edwardsia, Cerian- thus, Halcampa.} Fain. III. ZOANTHIDJE. Polypes adherent, united by a creeping or crust-like ccenosarc, rarely solitary, and never capable of locomotion. No true corallum, but generally a pseudo- skeleton formed by adventitious particles of sand or stone imbedded in the ectoderm. (Zoanthus, Epizoanthus, Paly- thoa. ) Zoantharia sclerobasica. The " Black Corals," or Anti pathidce, which comprise this group, are always composite, consisting of a number of polypes united together by a thin fleshy coenosarc, which is spread over and supported by a simple or commonly branched horny axis, or "sclero base." The tissues are not furnished with calcareous secretions, and the polypes have in general six simple tentacles. The Antipathidce form colonies which are attached by the base to some foreign object, and are generally more or less branched and plant-like. The colony consists of a thin fleshy crust or ccenosarc, in which the minute polypes are imbedded at intervals. The polypes are furnished with six simple conical tentacles each, though in the genus Gerardia as many as twenty-four of these organs may be present. The soft tissues appear to be wholly destitute of calcareous secretions of any kind, such as are found in the Gorgonidce. The entire coenosarc, with its imbedded polypes, is sup ported by a horny corallum, which is generally black in colour, and forms an axis or stem covered by the soft parts. The corallum is secreted by the ccenosarc, and is wholly external to the polypes, for which reason it constitutes what is technically called a " sclerobasis " or " sclerobasic coral lum" ("foot-secretion" of Mr Dana), in contradistinction to the true tissue secretions of so many other Actinozoa. In some instances (Cirrhipathes] the sclerobasis is simple and unbranched, and may attain a length of several feet ; but it is more commonly branched in a more or less plant- like and complicated manner (Antipathes, ArachnopatJies, Rhipidopathes}. The surface of the corallum may be smooth (Leiopathes) ; but it is more commonly covered with minute spines (Antipathes}, being thus readily distin guished from the grooved or striated sclerobasis of the Gorgonidce. In composition the corallum is horny. FIQ. 3. Portion of the colony of Antipathes anguina, Edw. and H., in its living condition, enlarged. (After Dana.) The Antipathidae constitute the only known family of the Zoantharia sclerobasica, as most naturalists are now agreed that the " Glass-rope Zoophytes " (Ilyalonemadce), with their twisted siliceous axis, are truly referable to the sponges. As regards their distribution in space, the Antipathidce are principally inhabitants of warm seas, and are, therefore, most abundant in the neighbourhood of the equator. Several species are known from the Mediter ranean ; they have been found at various points in the North Atlantic ; and they have even been recorded from the coasts of Greenland. They occur in depths of from 10 feet up to several hundred fathoms. As regards their distribution in time, the Antipathidce are not known to have come into existence during the Palaeozoic or Mesozoic period. They appear for the first time in the Miocene Tertiary, where they are represented by a single species (Lciopatkes vetusta). Zoantharia sclerodermata (Madreporaria This group includes the majority of the coralligenous zoophytes of recent seas. They may be simple, consisting of a single polype only, or composite, consisting of many polypes united by a fleshy ccenosarc. They always possess a cor allum, which is partially or wholly developed within the tissues of the polypes themselves (" sclerodermic "), which does not consist simply of scattered spicules, and in which the parts are so very generally disposed in multiples of six as to justify the name of Hexacoralla applied to the group. The anatomy of the soft parts of the simple Zoantharia sclerodermata may be considered as practically identical with that of the Sea-Anemones ; and the compound forms may be regarded as being essentially composed of a number of actinoid polypes united by a common flesh or coenosarc. It will, therefore, be unnecessary to treat here of more than the leading peculiarities of the hard parts, or " corallum," from which these organisms derive their common name of " corals." An ordinary simple coral of this group may, then, be regarded as being essentially a Sea-Anemone, in which a more or less complicated skeleton has been developed. As in the Sea- Anemones, the animal possesses a column, a base, and a disc, the margin of the disc supporting the tentacles, and its centre being perforated by the aperture of the mouth. The mouth, often more or less proboscidiform, opens into a stomachal sac, the walls of which are connected with the parietes of the body by vertical folds of the meso- derm and endoderm (" mesenteries "), and which communi cates freely below with the general cavity of the body. Within the mesenteries are contained the reproductive organs; and the disc, with its tentacles and dependent gas tric sac, is permanently soft and capable of retraction and expansion. Below the stomach the soft tissues of the animal are strengthened and supported by a more or less perfect calcareous skeleton or corallum. This is composed of calcareous matter ("sclerenchyma") deposited by and in the tissues themselves, and the corallum is therefore within the polype, and is said to be " sclerodermic." It is thus a true " tissue-secretion," and differs very conspicuously from the " sclerobasis " of other Actinozoa (" foot-secretion " of Dana), which is secreted by the ccenosarc, and is not formed by a calcification of the soft parts of the polypes themselves. A typical simple corallum may be regarded as a cone, sometimes extremely depressed, sometimes so elongated as to be almost a cylinder, with an outer wall and an internal included space. The wall of the cone is known as the " theca," and it may be very imperfect, or it may be covered externally more or less completely with a secondary calcareous investment (the " epitheca "). The theca encloses a space which is known as the " visceral chamber," is variously subdivided inferiorly, and superiorly presents itself as a shallower or deeper cup-shaped depres sion (the " calice "). The centre of the calice is hollowed out for the reception and protection of the stomach-sac of the polype, but the theca generally rises round its margins nearly to the level of the disc. Below the calice the visceral chamber is divided into a series of vertical com partments (the " interseptal loculi ") by a series of upright partitions or "septa," which spring from the inner surface of the theca and are directed inwards towards the centre. The septa are of different breadths. Some of them are much wider than the others, and often extend far enough inwards to meet in the centre of the visceral chamber. These are the " primary septa ;" but there are others which fa 1 ! short of the centre by a greater or less distance, and these are known as the " secondary " and " tertiary " septa, according to their width. The centre of the visceral cham ber may or may not be occupied by a variously-formed structure known as the " columella." In its most typical form the columella is a calcareous rod, which extends from
the bottom of the visceral chamber to the floor of the calice,