Page:Encyclopædia Britannica, Ninth Edition, v. 6.djvu/401

C O R A L S 371 cycle of mesenteries agrees with tlie first in being six in number, the third cycle is twelve, the fourth twenty-four, the fifth forty-eight, and so on. In some Sea-Anemones only twelve mesenteries are developed (Peachia), but ordinarily the number of these organs present in the adult is much greater. As already indicated with regard to the tentacles, it cannot be regarded as certain that the hexameral arrangement of the mesenteries, which is so conspicuous a feature in many of the Zoantharia, is by any means universal in the order, or even in the Zoantharia malacodermata in particular. Many of the adult forms, at any rate, appear to invariably possess mesenteries which are not a multiple of six in number. According to the researches of Rotteken and Schneider, the mesenteries are invariably of three orders in the Actinice, each individual mesentery being double, and the smallest number of each order that was observed being six of the first cycle, six of the second, and twelve of the third order. It is chiefly to the star-like disposition of the mesenteries and tentacles that the Sea-Anemones owe their conspicuous radial symmetry ; but indications are not wanting of true bilaterality. Thus a single tentacle may be of a different colour from, or a larger size than the others ; the two radii of the disc which correspond with the gonidial grooves and run to the angles of the mouth are often more con spicuous than the other radii ; the mouth itself runs fore- and-aft, and divides the body into a right and left half ; there may be only a single mouth-angle and gonidial groove (Actinoloba) ; and lastly, two of the mesenteries, correspond ing with the opposite mouth angles, are commonly developed before the rest. Even more conspicuous traces of bilateral symmetry which will be subsequently alluded to, are recognizable in many corals. The entire body-cavity, with the intermesenteric chambers and tentacular diverticula, is filled with a transparent fluid (" chylaqueous fluid "), which is to be regarded as the representative of the blood. It consists of sea-water mixed with the products of digestion, containing albumen in solution along with numerous floating corpuscles, represent ing the " blood-corpuscles " of the higher animals. There are no proper circulating organs, but a free circulation of the chylaqueous fluid through all parts of the body is effected by means of the richly ciliated endoderm which lines all parts of the somatic cavity. No distinct respiratory organs are present, as a rule, the function of respiration being discharged by the ciliated endoderm, as well as by the currents in the external water maintained by the cilia covering the tentacles. In some species, however, which live half-buried in the sand, there are found lobed and frilled organs attached to or beside the tentacles, and these have been conjectured to be branchial (Verrill). In the Zoanthidce, aiso, there are found curious paired organs covered with cilia, and attached to the primary mesenteries a little below the stomach ; and these may probably be regarded as gills (Dana). The free edges of the mesenteries below the stomach are thickened, and constitute a puckered and convoluted mar ginal cord (" craspedum "), which is richly furnished with thread-cells. Also attached to the free edges of the mesenteries are sometimes found the organs known as " acontia." These are long, thread-like filaments, which are only attached by one end to the mesentery, and are crowded with thread-cells. The acontia seem to be undoubtedly organs of offence and defence, as they can, on irritation, be rapidly shot forth from the mouth, as well as from certain minute orifices in the body-wall (cinclides) which appear to be specially intended for their emission. Specialized organs of the senses are either wanting in the Sea- Anemones, or only present in a rudimentary condition. Tactile sensibility, though well developed, is generally diffused over the -surface, residing more particularly in the tentacles. Organs of hearing are wholly unknown. As regards the sense of sight, many species possess round the margins of the disc a series of brightly-coloured bead-like bodies (" marginal spherules," " bourses marginales," or " chromatophores "), which are said to be furnished with nervous filaments, and which may with great probability be regarded as imperfect organs of vision. The condition of the nervous system is still a matter of great obscurity, and its very existence has generally been considered as doubtful. According, however, to recent researches (Duncan), it would appear that a plexiform arrangement of nerve fibres can be detected in the base of Actinia, a similar nervous apparatus probably existing in the disc aa well. The reproductive organs are in the form of thickened bands, of an orange or pink colour, enclosed in the mesenteries near their free edges, and the ovaries and testes are similar to one another in form and structure, differing only in their contents. As a rule, the sexes appear to be distinct, but in some forms they are united in the same individual. The reproductive elements escape into the body-cavity by dehiscence of the reproductive glands, but the precise manner in which the ova of the dioecious species are fertilized has not been determined. In addition to true sexual reproduction, increase is sometimes effected, nori-sexually, by gemmation or fission. Gemination is rare amongst the Sea-Anemones, the new polypes being budded forth from the sides of the parents close to the base, and being finally detached as independent animals. Fission ia not by any means so rare, and may either take place by a longitudinal cleavage of the original polype into two wholly or partially independent individuals (Antheus and Actinia), or by the separation of portions of the margins of the base of the parent, and by the development of these into new polypes (Actinoloba, Sagartia). As regards their development, the fecundatea ovum becomes converted into an ovate, ciliated, actively locomo tive embryo ("planula"), with a double wall enclosing a central cavity. A depression next appears at one extremity, indicating the future mouth, and the embryo passes into the " gastrula " stage, by the opening up of a communication between its central cavity and the exterior medium, or by invagination on itself, it being still uncertain which of these modes is employed. The gastrula now fixes itself by one extremity to some foreign body, and the primitive mesenteries and tentacles are developed. These are originally six in number ; but according to Lacaze- Duthiers the first and second cycles (twelve in all) are developed by passing successively through the numbers two, four, six, and eight. The remaining cycles of tentacles and mesenteries are rapidly added, until the animal attains the full number of these organs proper to the adult. As to the geographical distribution of the Zoantharia malacodermata little need be said, as the members of this group, taken as a whole, are cosmopolitan in their range. They are, however, pre-eminently characteristic of the littoral and laminarian zones, only very few forms, and these not typical Sea-Anemones, extending to depths of over 500 fathoms (Palythoa), and one genus (Arachnactis) being pelagic. As to their distribution in time, nothing at all can be said, as, from the soft nature of their bodies, they have left no traces of their past existence. The Zoantharia malacodermata may be divided into the following three families : Fara. I. ACTINID.E. Polypes essentially simple, the base form ing a flattened sucker by means of which tlie animal ad heres at pleasure to foreign bodies. No corallum. (Actinia, Sagartia, Bunodes, Tcalia, Boloccra, Anlheus, Phymactis, Adanisia, Cancrisocia, Corynactis, <fcc.)

Fam. II. ILYANTHID.E. Polypes simple, destitute of an adherent