(126). On making a longitudinal section of one of the expanded polyps (fig. 26, 1), the main features of its anatomy become at once recognizable. The alimentary canal (c) is seen to be a cylindriform cavity with membranous walls, occupying the axis of the upper portion of the body, and extending from the mouth (b) to about the middle of the free portion of the protruded polyp, where it terminates by a distinct orifice (d.) Internally, the digestive sacculus presents eight longitudinal lines, and a multitude of minute transverse folds. Its inferior termination becomes suddenly contracted, as though the terminal orifice were closed by a sphincter muscle, and communicates with the wide abdominal cavity (e) that occupies the entire diameter of the lower portion of the polyp, and is prolonged inferiorly into the common body of the polypary. The calibre of the digestive tube is much smaller than that of the animal in the centre of which it is suspended; nevertheless it is firmly connected with the parietes of the polyp by the intervention of eight delicate membranous lamellae derived from its outer surface (fig. 26, 1 & 2, f) and extending along its whole length. The position of these septa corresponds with the intertentacular spaces; and as by their upper extremities they are united to the peristomal disk, they form the walls of eight longitudinal canals which are uninterruptedly continuous with the cavities of the corresponding tentacula (fig. 26,1, g.) These last-mentioned appendages are completely hollow, and moreover present on each side of their internal cavity a series of ten or twelve minute apertures (fig. 26, g') leading into the marginal pinnules, that are of similar structure.

* Professor Grant, Lectures on Comparative Anatomy. Lancet for 1833-4, vol. ii. p. 261.

1 Quoy et Gaimard, Zoologie du Voyage de l'Uranie. Paris, 1834.

2 Memoiresur un nouveau genre de la famille des Alcyoniens (genre Alcyonide.) Ann. des Sc. Nat. 1835.

Alcyonidium elegans (after Milne Edwards, Ann. des Sc. Nat. 1835, pl. 12. fig. 1.)

Fig. 25. Alcyonidium elegans (after Milne-Edwards, Ann. des Sc. Nat. 1835, pl. 12. fig. 1.) a, foreign body to which the polyp is attached; b, the hard portion, or coriaceous foot; c, the trunk, or membranous portion of the polypary; d, polypiferous ramifications of ditto; e, the soft parts of the trunk completely retracted into the coriaceous stem; f, yellow specks indicating the ova contained in the lower portion of the polypary.

(127). Inferiorly, the eight longitudinal interseptal canals communicate freely with the great abdominal cavity (e), and the vertical partitions whereby they are separated become continuous with the longitudinal folds (f) visible in its interior. The longitudinal plicae are apparently of the same structure as the vertical septa of which they are the continuations, only they are narrower, and their inner margin being free, they hang loosely in the abdomen of the polyp. On closer inspection they seem to be made up of two extremely thin membranous layers folded upon each other and continuous with the internal tunic that lines the parietes of the body. At the point of continuity the two laminae become slightly separated, so as to leave a little canal at the base of each fold; while superiorly, close to the termination of the stomach, there is a remarkable filiform and very flexuous organ (fig. 26, 1, k), apparently an appendage to the alimentary cavity.

Anatomy of Alcyonidium elegans (after Milne Edwards.)

Fig. 26. Anatomy of Alcyonidium elegans (after Milne-Edwards.) 1. A polyp opened longitudinally to show its internal organization: a, the tentacula; b, mouth; c, alimentary canal; d, inferior opening of ditto; e, upper portion of the abdominal cavity; f, longitudinal septa passing between the parietes of the body and the walls of the digestive cavity; f, continuation of the same into the abdominal cavity; g, canals formed between the septa, which are continuous with the interior of the tentacula; g', one of the tentacles opened, showing the holes by which its cavity communicates with those of the marginal pinnules; h, minute spicula situated at the base of the tentacles; k, filiform appendages to the alimentary tube. 2. Transverse section, showing the manner in which the longitudinal plicae are connected with the alimentary tube. 3. Section through the basilar portion of the polypary.

(128). As has been stated above, the common polypary consists of two portions differing widely from each other in texture, forming the trunk and the foot. By dissection it becomes immediately apparent that the softer portion, named the trunk, is made up of membranous tubes disposed longitudinally parallel to each other, and so closely connected together that it is difficult to separate them. The foot of the polypary is essentially nothing more than a continuation of these same tubes modified in structure: those situated near the centre of the stem have their walls only slightly thickened; but those placed near the periphery acquire a more solid consistence, from having their parietes incrusted with innumerable fusiform spicula composed of carbonate of lime imbedded in a cartilaginous substance; these are arranged longitudinally (fig. 26, 3), and give to the stem its solidity and peculiar aspect. Near the circumference of this portion of the polypary longitudinal fibres are perceptible, which appear to be the remains of tubes atrophied by compression (fig. 27,3, a).

(129). The tubes thus united in fasciculi are evidently analogous to the cavities into which the polyps of Alcyons, Corals, etc, are retracted: these have generally received the name of "polypiferous cells;" and some authors consider them as being quite distinct from the animals inhabiting them: in the zoophyte under consideration, however, a very superficial examination is sufficient to prove that they are really merely continuations of the bodies of the polyps themselves, no line of demarcation being distinguishable between them. It is not, therefore, into polypiferous cells that these little flower-like creatures retire, but become retracted into their own bodies by a species of invagination; and the entire polypary which seems to afford them lodging is nothing but a mass formed by the aggregated tubes of all the polyps belonging to it.