This section is from the book "General Outline Of The Organization Of The Animal Kingdom, And Manual Of Comparative Anatomy", by Thomas Rymer Jones. Also available from Amazon: A General Outline of the Animal Kingdom and Manual of Comparative Anatomy.
(1223). Beneath the first enlargement, the digestive apparatus becomes narrower, but immediately expands again, and offers at this point a certain number of filiform appendages, which appear to be free and floating in the interior of the cell. To the second cavity succeeds a narrow canal, opening into a third dilatation, generally of a spherical form. From the last-named viscus issues a kind of intestine, which soon bends upon itself and becomes attached to an organ of a soft and membranous texture, having the appearance of a caecum, and which seems to be continuous superiorly with the digestive tube; the latter continues its progress towards the upper part of the cell, and ultimately terminates by a distinct anal aperture upon the upper aspect of the tentacular sheath.
(1224). The operculum which closes the cell in Flustrce and Escharw is moved by two muscular fasciculi inserted into the internal face of this valve by the intermedium of two filaments analogous to tendons; by their inferior extremity these muscles are attached to the walls of the cell; and when, by its own elasticity, the operculum is turned back, and the mouth of the cell thus opened, they, by their contraction, can close it like a door.
(1225). The existence of nervous ganglia has been satisfactorily detected in many genera of the Polyzoa: it consists of a nervous ganglion, situated immediately above the oesophagus, from each side of which proceeds a nervous cord forming a collar around that tube, as well as other filaments distributed to the muscular system.
(1226). No organ of special sensation has been discovered in any animals of this class, either in their adult state, or during the earlier periods of their development.
(1227). From what is known concerning the propagation of the Polyzoa, it would appear that their reproduction is effected in several different ways.
(1228). The most ordinary is by the development of gemmae, or buds, that sprout from the parent stem in the branched species, or, as in the Flustrce and Escharce, are derived from the sides of contiguous cells.
(1229). In Pedicellina Belgica, the phenomena attending the gemmi-parous mode of reproduction are the following*: - First, there sprouts from the common stem of the Polyzoon, without any determinate situation, a minute tubercle, which is simply a prolongation from the stem itself; this tubercle gradually extends outwards, becomes more prominent, and soon swells into a vesicle, which is the first appearance of the new individual. Up to this period the interior of the vesicle is organized precisely in the same manner as the stem itself, of which it is only an extension; but now a cellule becomes visible in its centre, which forms the point of departure whence the development of the embryo proceeds.
(1230). Around this primitive cell a series of other very small cellules soon group themselves, which seem to constitute the parietes of the primitive vesicle, or blastoderm, the original cell representing the vitelline cavity. The bud now enlarges; and as its growth proceeds, the internal tissue becomes thickened, so as to fill it completely; subsequently an indentation becomes apparent on each side of the little cavity, separating the embryo into two halves, the inferior of which will form the stomach, the superior the intertentacular chamber.
(1231). In Laguncula repens, the reproductive gemmae sprout from the creeping stems which connect the individual animals, appearing at first as a slight prominence that soon expands into a rounded tubercle, which is the commencement of a new cell. On close inspection, each gemma is found to consist of a transparent envelope, that is, in fact, a continuation of the general investment of the animal, lined throughout with a soft membrane, having its inner surface studded with minute globules, by the accumulation of which the polyp is ultimately formed. The bud itself is hollow, and communicates with the parent stem: it therefore has nothing in its composition resembling that of an egg; neither distinct vesicle nor vitellus. The newly-formed cell soon grows taller, and its lining membrane becomes thicker, indicating the first appearance of the intestinal canal, which is at first a simple cavity bounded by the thickened lining of the cell. This cavity once formed, the development of the different organs proceeds rapidly. First there appears a longitudinal fold, resembling two lips, that, as they approach each other, divide the cavity of the body into an anterior and posterior compartment.
The two lips, which have a valvular appearance, become very regularly indented along their margins, and are soon recognizable as the rudiments of the tentacular circle.
* Van Beneden, "Recherches sur l'Anatomie, la Physiologie, etla Developpeinent des Bryozoaires qui habitant la Cote d'Ostende" (Bulletin de l'Acad. Roy. de Bruxelles, torn. xix.).
(1232). At this epoch, it must be remarked, the polyp presents two cavities distinct from each other: there is a space between the walls of the body and the parietes of the future alimentary canal, the interspace being in communication with the stem of the parent polyp, and filled with a fluid that is analogous to the blood of higher animals; superiorly this cavity likewise passes into the tentacles, and the fluid which bathes the exterior of the alimentary canal thus finds admission even to the extremities of those organs. The second cavity, which is the intestinal canal, has as yet no communication with the external world. As the formation of the tentacles proceeds, the portion which is situated in front of them will become the sheath, and the other part the intestine. As the tentacula are formed by the prolongation of the tubercles, which were their first rudiments, the cavity of the stomach and the rest of the intestinal tube gradually become apparent; and at the same time some globules are visibly disposed around the cul de sac of the former viscus, which gradually become arranged into fibrillae, and constitute the retractor muscles.