Fig. 45. Diagram representing section of a Sertularian zoophyte, a, inner or nutritive layer; b, outer or tegumentary layer; e, oral tentacles of the polyp; d, e, gemmules; f, polypiform external capsule; g, polypiferous cell; A, reproductive cell.

(224). The stomach, as in the Hydra, is a simple cavity excavated in the interior of the body, without any proper parietes, which inferiorly communicates immediately with the fleshy substance contained in the common polypary; so that the contents of the stomachal sac may not unfrequently be seen to pass into the living pith, and in like manner the globules there circulating to return into the stomach.

(225). The multiplication of these beautiful zoophytes appears to take place in three different modes: - 1st, by cuttings, as in plants; 2ndly, by offshoots, or the formation of new branches bearing polyps; 3rdly, by Planulce capable of locomotion.

(226). The first mode strikingly resembles what is observed in the vegetable kingdom; for, as every branch of the plant-like body contains all the parts necessary to independent existence, it can hardly be a matter of surprise that any portion, separated from the rest, will continue to grow and perform the functions of the entire animal.

(227). The second mode of increase, namely, by the formation of new branches and polyps, seems more like the growth of a plant than the development of an animal. We will consider it under two points of view: - first, as regards the elongation of the stem; secondly, as relates to the formation of fresh cells containing the nutritive polyps. On examining any growing branch, it will be found to be soft and open at the extremity, and the soft tegumentary membrane (above described as forming the tube by its conversion into hard substance) is seen to protrude through the terminal orifice; the skeleton is therefore not merely secreted by the enclosed living granular matter, but it is the investing membrane, which continually shoots upwards, and deposits hard material in its substance as it assumes the form and spreads into the ramifications peculiar to its species.

(228). Having thus lengthened the stem to a certain distance, the next step is the formation of a cell and a new polyp, which is accomplished in the following manner*. The newly-formed branch has at first precisely the appearance and structure of the rest of the stalk of the zoophyte (fig. 46,1), being filled with granular matter, and exhibiting in its interior the circulation of globules (already described) - moving towards the extremity along the sides of the tube, and in an opposite course in the middle; the end of the branch, however, before soft and rounded, soon becomes perceptibly dilated. After a few hours the branch is visibly longer, its extremity more swollen, and the living pith is seen to have partially separated itself from the sides of the tube, the boundaries of which become more denned and undulating (2.) The growth still proceeding, the extremity is distinctly dilated into a cell, in which the soft substance seems to be swollen out, so as to give a rude outline of the bell-shaped polyp (3), but no tentacula are yet distinguishable; a rudimentary septum becomes visible, stretching across the bottom of the cell, through the centre of which the granular matter, now collected into a mass occupying only a portion of the stem, is seen to pass. The polyp and cell gradually grow more defined (4, 5, 6), and the tentacula become distinguishable; the cell, moreover, is seen to be continued inwards by a membranous infundibular prolongation of its margin (7), that immediately reminds us of the funnel-shaped membrane of Tubi-pora (§157), and its office is no doubt similar.

As the development proceeds, the tentacles become more perfect (8,9), and the polyp at length rises from its cell to exercise the functions to which it is destined.

* Lister, Phil. Trans, loc. cit.

Diagram illustrating the mode of growth of a Sertularian polypiclom.

Fig. 46. Diagram illustrating the mode of growth of a Sertularian polypiclom.

(229). The mam feature that distinguishes the Sertularian Zoophytes from the Hydrce seems to consist in the fact that, whereas in the latter each newly-formed offset becomes detached from the parent stock and enjoys a separate existence, in the former each new sprout remains permanently adherent, the successive generations being united into a ramified stem, which is common to the entire group. The Hydra, having no polypary or outer covering, when it dies, entirely perishes: but in the Sertularidse, every sprout, when it dies, leaves its horny integument attached to the general community; and thus, in time, there results an elaborately-branched stem, the complexity of which increases with the age of the colony.

(230). The third mode of multiplication, or that by Planulw, seems to be specially adapted to the diffusion of the species; and as it presents many points of peculiar interest, we shall dwell upon it at some length. At certain periods of the year, besides the ordinary cells adapted to contain nutritive polyps, others are developed from different parts of the stem, which may be called female or fertile polyps, although usually simply termed the vesicles. The cells of this kind are much larger than the nutritive cells, and of very different form (fig. 48, a); they are moreover deciduous, falling off after the fulfilment of the office for which they are provided. They are produced in the same manner as the rest of the stem, by an extension of the tegumentary membrane (fig. 45, b), which, as it expands into the form of the cell, becomes of a horny texture; it may be traced, however, over the opening of the cavity, where it sometimes forms a moveable operculum. The cell being thus constructed by the expansion and subsequent hardening of the tegumentary membrane, it remains to explain the origin of the reproductive germs which soon become developed in its interior.