370 CORALS an adventitious coat of fragments of shell, grains of sand, and small pieces of stone. The mesoderm is essentially composed of two layers of muscular fibres, those of the outer layer having a circular direction, whilst those of the inner layer are longitudinal in the column and become radial in the base and disc. The endoderm is likewise double in its composition, its inner stratum being formed of ciliated epithelial cells. The " thread-cells " (cnidae or nematocysts), which are so abundantly developed in the integument of the Sea- Anemones, are microscopic organs of offence and defence. Though differing very much in size and in the details of structure in different species, the thread-cells consist essentially of an elastic double- walled sac, one extremity of which is invaginated and carries a long, often serrated or spinose filament, which lies coiled up in the interior of the sac. On the slightest pressure the sac is instantaneously everted, and the lasso-like thread in its interior is shot forth with the rapidity of lightning, having the power of pene trating any soft body with which it may come in contact, and apparently inflicting an envenomed puncture. FIG. 2. Morphology of the Actinidje. A, Ideal representation of a Sea-Anemone, vertically bisected; it, tentacles ; p, peristomial space; m, mouth; s, stomach; 6, interior of the general body- cavity, below the stomach, showing the free edges of the mesenteries ; me, one of the mesenteries; o, ovary; a, acontium. B, Thread-cell of Caryophyllia Smithii (after Gosse), greatly enlarged. C, Thread cell of Tealia crassicornis (after Gosse), with the filament everted, greatly enlarged. The organs of prehension of the Sea-Anemones are the " tentacles " (tentacula). These are hollow, smooth, coni cal or filiform organs, arranged around the margin of "the disc in one or more successive circles. The walls of the tentacles are formed of the general integuments of the body, and each communicates inferiorly with an inter- mesenteric chamber, and is thus filled with fluid derived from the general cavity of the body. They are capable of extension and retraction, and their extremities are com monly perforated, though sometimes swollen and impervious (Corynactis, Caryophyllia}. They are abundantly furnished with thread-cells, and are, therefore, organs of touch as well as of prehension. Except in some cases, where one or more tentacles may be aborted, the number of the tentacles seems to be primitively six, and remains some multiple of this during life, the number of these organs being increased by the development of successive cycles arranged in concentric and alternating circles. The primi tive cycle consists of six tentacles, the second cycle also of six, the third cycle of ttvelve, the fourth cycle of twenty-four. the fifth of forty-eight, and so on, the number of each cycle being invariably double that of the preceding cycle, except in the case of the second cycle. Though it has been generally accepted that the number of the tentacles is primitively six in the Zoantharia (hence often called Hexactinice), and that their increase is as above stated, grave doubts have of late arisen as to the correctness of this view. According to Lacaze-Duthiers, the primitive tentacles are first two in number, then four, then six, then eight, and finallj 7 twelve (in the Actiniae}. According, also, to com petent observers (Gosse, Fischer, and others) the adult Sea-Anemones by no means invariably possess tentacles which are a multiple of six, or even of five. On the contrary, various species have tentacles which are a multiple of eight, whilst in others the numerical arrangement of the tentacles seems to belong to an indeterminate type. Though showing a marked radiate arrangement, it will be sub sequently shown, in speaking of the mesenteries, that even the tentacles occasionally show distinct traces of bilateral symmetry. Internal to the circle of tentacles, the upper surface of the disc exhibits a more or less conspicuous flattened area ("peristomial space"), which is destitute of appendages, but is marked with converging lines (" radii"), which start from the bases ol the tentacles and meet round the mouth, and which represent the upper attached edges of the mesenteries. In the centre of the peristomial space, often at the summit of a kind of proboscis, is placed the opening of the oval or fissure-like mouth. The angles of the mouth are furnished with grooves ("goniclial grooves"), which serve as channels for the conveyance of the ova to the exterior. The mouth opens by the intervention of a short corrugated and folded gullet into a membranous stomach, with thin muscular walls, usually descending about one- third of the distance towards the base. The stomach, when distended, is of a globular form, and it opens inferiorly directly into the general cavity of the body, by a wide patulous opening. When not in use, the walls of the stomach are in contact ; its sides exhibit the downward con tinuation of the oral gonidial grooves. In some forms, a layer of coloured fat-cells is developed in the walls of the stomach towards its upper portion, and this is conjectured by Mr Gosse to represent the liver. No other distinct alimentary organs exist, and indigestible matters are got rid of through the mouth. The general cavity of the body (" somatic cavity ") freely communicates with the external medium through the stomachic sac and mouth, and is bounded externally by the integuments, and lined by the endoderm. The space thus formed is subdivided into a series of chambers or compart ments by a number of radiating vertical membranous laminae, to which the name of "mesenteries" is given. The mesenteries are essentially double, each being composed of an inward reduplication of the muscular mesoderm, covered by the endoderm, and they vary greatly in width. The first-formed and widest mesenteries (" primary mesenteries") are attached by the whole of their outer edges to the column-wall, by their upper edges to the disc from its margin to the mouth, and by their loioer edges to the base from the circumference to the centre. The inner edges of the primary mesenteries are attached to the sides of the stomach, from the mouth almost to its inferior opening ; but below this point they present a free, curved margin, which looks inwards towards the centre of the visceral chamber, being ultimately continued to the centra of the base. Between the primary mesenteries are developed other shorter laminae, which agree with the preceding in being attached externally along their whole length to the column-wall, but which do not extend sufficiently far inwards to reach the walls of the stomach^ According to their width these are known as " secondary " and " tertiary " mesenteries. The primitive number of the primary mesenteries is normally six, and the development of the remaining mesenteries is effected by the same law aa
governs the development of the tentacles. Thus the second