(216). Zoophytes of this description are readily found on our own coasts, and the microscopic observer can scarcely enjoy a richer treat than the examination of them affords. In order to study them satisfactorily, it is necessary to be provided with several glass troughs, of different depths, in which the living animals immersed in their native element may be placed: in this situation, if the water be carefully renewed at short intervals, they will live for some time.

(217). Besides the cells which contain the polyps, others, specially destined to the development of the ova, exist at certain periods of the year; they are larger than the preceding, and of a very different shape; but of these we shall have occasion to speak more fully hereafter.

(218). The general stem of the polypary is entirely filled with a fleshy substance exactly resembling in its nature the tissue composing the body of the polyp, whereby all the individuals belonging to the common stock are brought into communication with each other. Internally it seems to be hollow, and to contain a fluid, in which numerous globules may be observed in active motion. It is from this central fleshy substance that the buds or lateral offsets derive their origin.

(219). On examining attentively the stem of a living Sertularia, the globules are all seen to follow each other in distinct currents, and sometimes may be observed to move in opposite directions in the same branch: on arriving at the bifurcation of a stem, some seem to stop, whilst others continue their course to the right or left. If the branch of a living polypary, while in a state of activity, be divided and slightly compressed, the globules that escape from the cut extremity still continue their movements for some considerable time, somewhat after the manner of zoosperms; and as this kind of motion, when observed externally to the polypary, resembles very closely that which they exhibit in its interior, it is apparently not dependent upon any pressure from the walls of the general fleshy substance, but seems to be inherent in the globules themselves. The general movement of the fluid contained in a branch, however, more especially as relates to its direction, depends upon the pressure exercised by the polyps; so that if several individuals on one side of a branch contract simultaneously, they sometimes even force the contained liquid through the mouths of those upon the opposite side.

(220). It has been generally stated that the living pith exudes from its surface the horny matter which, by its concretion, forms the tube or external skeleton investing the whole; the accuracy of such a supposition, nevertheless, may well be questioned. We have already seen, in the Tubipora musica, that the calcareous tube investing that polyp was produced by the interstitial deposits of earthy matter in the membrane that originally constituted its outer case. In the tribe of zoophytes we are now speaking of, we shall find the exterior tube to be formed in a way precisely similar. On referring to the diagram (fig. 45), the mode of its growth will be rendered intelligible. The soft part, or living axis of the polypary, is seen to be contained in two distinct layers: the inner one, a, being continuous with the digestive sac of the polyp, and immediately embracing the granular matter, seems to be the special seat of the nutritive process; the outer or tegumentary layer, b, after leaving the tentacula, may be traced down the sides of each polyp to the bottom of the cell, where its course is arrested by a slight partition, at which point it turns outwards, lining the interior of the cell as far as its margin, where, as in the Tubipora, it is seen to be continuous with the horny matter itself. It is this tegumentary membrane, then, which forms by its development the entire skeleton.

As it expands, it gives origin to the cells and branches characteristic of the species; and, from being at first quite soft and flexible, it gradually acquires hardness and solidity by the deposition of corneous matter in its substance.

(221). The cells thus formed are inhabited by polyps analogous to those that provide nourishment for the cortical families, though differing in the number and appearance of the tenta-cula, which are here studded with minute tubercles, but never provided with cilia. Few objects are more admirable than these polyps, when watched with a good microscope. Protruding themselves beyond the mouths of their cells, they inflect their bodies in all directions in quest of prey, waiting till some passing object impinges upon their tentacula, when it is at once seized and conveyed into the stomach with a rapidity and dexterity almost beyond belief.

(222). The tentacula in the Sertularian Hydrozoa are all arranged in a single row, and form a sort of funnel-like appendage to the oral orifice of the body. They are susceptible of considerable elongation and contraction, like those of the Hydra, but in a less degree. Their number is constant throughout the different periods of growth in each species, but varies in different genera. Internally they are not hollow as is the case in the Anthozoa, but under the microscope they have the appearance of being divided into compartments by delicate transparent diaphragms, giving them an appearance like that of some Confervse; they are throughout of equal thickness, and no movement of fluids is perceptible in their interior.

(223). In the centre of the tentacular circle may be observed a fleshy protuberance of variable shape, which might be compared to a proboscidiform elongation of the mouth; sometimes this appendage is elongated into the form of a tube, sometimes it shrinks into a globular mass, or occasionally it may be seen so completely contracted as simply to form a broad lip-like ring around the oral opening.

Diagram representing section of a Sertularian zoophyte, a, inner or nutritive layer.