(140). Prom the above details it becomes easy to explain how a single polyp by its reproductive powers can form the complicated mass of the compound polypary of the Alcyonidao, as well as the means whereby an organic continuity is established between all the individuals of the community; also how the abdominal cavity of the primitive zoophyte becomes common to all the young ones that sprout from it; in short, how the little beings thus united together rather resemble a multiple animal than an assemblage of distinct individuals. But with the advance of age this intimate union gradually ceases. The communication between the abdominal cavities of the different polyps, whose basal portions reach as far as the foot of the polypary, is first of all interrupted by the ova with which the lower part of these cavities becomes filled (fig. 26, 3); and subsequently, by the pressure of the surrounding parts, the wall becomes confused, and all communication between the polyp whose abdominal tube is thus obliterated and the polyp from which it sprung is intercepted. The polypary, instead of resembling a tree, all the flowers of which hold together and communicate by common parts, may now be compared to a bouquet made by cutting off the more or less branched twigs of a plant and collecting them in a bundle. The different groups of polyps united in the same polypary become thus independent of the neighbouring groups, and, as may readily be conceived, in time each polyp can become individualized.

(141). In the Alcyons properly so called, a vascular system is very distinctly developed; and in Alcyonium stellatum more especially, M. Milne-Edwards was able to study it with facility. In this species (fig. 28, l) he was enabled to detect, upon the parietes of the abdominal cavity of the polyp, a variable number of minute apertures irregularly dispersed, which are in immediate communication with a system of capillary canals that traverse in all directions the spongy portion of the polypary, formed by the external tunic of its component animals; for in Alcyonium it is very easily seen that while the internal tunic lines the abdominal cavity of the polyp, the external layer, instead of being confounded with the former, as in the protractile portion of the animal, becomes perfectly distinct from it where it begins to enter into the composition of the polypary, at which point its thickness is considerably augmented, its texture spongoid, and in its substance are deposited a number of irregular crystals, composed of carbonate of lime mixed with a little colouring matter. In the tegumentary mass thus formed the vascular canals ramify, anastomosing freely among themselves, so as to constitute a vascular network. These vessels are formed of very attenuated membrane of a yellowish colour, which is continuous with the internal tunic of the polyps, and is perfectly distinguishable from the dense tissue with which it is surrounded.

The distribution of these canals is best displayed by cutting a thin slice of the mass of the Alcyon, and removing the crystals with which it is filled by immersion in some dilute acid; it is then seen that the canals are most numerous and of the largest size towards the extremities of the branches of the polypary, and that they establish frequent communications between the abdominal cavities of the diiferent polyps of the Alcyon. The fluids with which their bodies are filled must thus necessarily circulate in the entire mass of the polypary; and if each of the polyps has, on the one hand, an individual sensibility and a distinct digestive cavity, on the other there is a vascular system common to them all.

1 & 2. Sections of Alcyonium stellatum. 3. Ovigerous membrane of the same, isolated.

Fig. 28. 1 & 2. Sections of Alcyonium stellatum. 3. Ovigerous membrane of the same, isolated.

(142). The Alcyons, like the Alcyonides, are reproduced by ova, which are formed in membranous ovaria of precisely similar construction; and also by gemmae, which are developed around the pre-existent polyps, and thus augment indefinitely the number of individuals united upon one stock. There is, however, a very important difference observable between these two genera of zoophytes, in other respects so similar. In the Alcyons the abdominal cavity of the young polyps is not directly continuous with the abdominal cavity of their parent, and it is only by the intermedium of the vascular system, described above, that they are placed in communication with each other - a modification which depends upon a difference in the mode of formation of the reproductive gemmae. When an Alcyon stock is about to put forth a new branch, the spongy part of the polypary (that portion which is formed by the external tunic of the polyps, and permeated by the vascular network) begins to increase in size at some determinate point of its periphery, and soon produces a tubercle of greater or smaller size, into which the vessels spoken of above are continued, and form numerous anastomoses with each other.

At this early period of development the new branch presents no trace of polyps; but its vascular tissue is nevertheless already studded with calcareous crystals, and exactly resembles that situated in other parts of the common mass, between the abdominal cavities of the adult polyps; it must therefore necessarily be traversed by the currents which circulate in the general vascular system. On dissecting one of these newly formed branches, the vestiges of young polyps may be distinguished; and if the sprouts examined are still further advanced, it is easy to distinguish the young animals within, already possessing the form they will afterwards exhibit, but having not yet established a communication with the exterior (fig. 28,1.) At length, however, this communication is effected; the newly formed polyp only differs from the pre-existing ones in its small size; and as it grows, its increase causes the enlargement of the polypary. In this case it is very evident that the part which gives birth to the reproductive gemmae is no portion of the individual polyps of the Alcyon, but is common to them all. The generative tissue surrounds these little beings with a sort of living sheath, and produces in the interior of its own substance new polyps, quite independently of those previously in existence. These polyparies might therefore be compared to a sort of common ovary, the products of which are never completely individualized, but remain permanently lodged in its substance, and minister to the support of its existence and the aggrandizement of its tissue.