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.
(1195). It is only within the last few years that microscopical researches have revealed to naturalists the real structure of a series of animals originally confounded with the simpler polyps, with which, as far as external form is concerned, they are indeed intimately related. The observations of Milne -Edwards 1, Audouin, Ehrenberg 2, and Thompson § gradually led the way to more correct and precise ideas concerning the more highly-organized genera; while Dr. Arthur Farre || and Van Beneden, by a series of investigations followed up with exemplary industry and perseverance, seem to have completed our knowledge of the anatomical details of these creatures in a manner which leaves few points of their economy unknown.
1 Ann. des Sci. Nat. for Sept. 1828 and July 1836. 2 Symbolsae Physicae.
§ Zoological Researehes and Illustrations, Memoir 5. Cork, 1830. ║ Phil. Trans., Part 2. for 1837.
(1196). We shall select an individual, named by Dr. Farre Bower-banhia densa, as an illustration of the general structure of the Polyzoa, partly from the complete manner in which its organization has been developed in the memoir alluded to, and partly because we have had frequent opportunities of verifying the accuracy of the observations recorded.
(1197). The tentacula of Bowerbankia (fig. 238) during the expanded state of the animal are kept quite straight and motionless, as represented in the drawing. Each tentacle is provided upon its outer aspect with a series of stiff and immoveable spines, probably serving to keep off any foreign bodies that by their proximity might interfere with the ciliary movements immediately to be described.
(1198). Besides the stiff spines, the tentacula are covered with an immense number of vibrating cilia, which, at the will of the animal, are thrown into most rapid movement, so as to produce strong and continuous currents in the surrounding fluid, whereby particles floating in the neighbourhood are hurried along with great velocity. From the direction of the streams produced by the cilia, namely towards the mouth, we at once perceive the utility and beauty of the contrivance, compensating to a great extent for the fixed condition of the Polyzoon: animalcules floating in the vicinity no sooner come within the influence of the currents so produced than they are forced towards the mouth, situated in the centre of the tentacular zone, and, being at once seized, are immediately swallowed.
(1199). The tentacula themselves, notwithstanding their immobility during the process of watching for prey, are highly irritable, and sensible of the slightest contact. No sooner does an animalcule impinge upon any part of their surface than the tentacle touched bends with extraordinary quickness, as if endeavouring to strike it towards the mouth; and if the object be sufficiently large to touch several at the same moment, all the tentacula simultaneously cooperate in seizing and retaining it.
Fig. 238. Anatomy of Bowerbankia densa (after Farre.) a, The animal with its tentacula expanded: - 1, pharynx; 2, oesophagus; 3, the gizzard; 4, the stomach; 5, the pylorus; 6, the intestine; 7, the anal aperture, b represents the Bryozoon retracted into its cell: - 1, 2, 3, muscular fasciculi, c, An imperfect gemma before the opening of the cell: - 1, stomachal cavity, d, A gemma sprouting from the common stem.
(1200). The existence of these cilia upon the tentacula would seem to be characteristic of the Polyzoa, and is invariably accompanied, as far as our information extends at present, with a digestive apparatus of far more complex structure than what we have seen in the unciliated polyps; for in the class before us, besides the stomach, there is a distinct intestinal tube and anal outlet. In the specimen under consideration the organization of the alimentary organs is rendered even more elaborate than is usual in the class, by the addition of a gizzard or cavity wherein the food is mechanically bruised before its introduction into the proper stomach. The mouth is placed in the centre of the space enclosed by the tentacula: it appears to be a simple orifice, incapable of much distention, through which the particles of food brought by the ciliary action pass into a capacious oesophagus (fig. 238, a, 1,2); this, gradually contracting its dimensions, ends in a globular muscular organ, to which the name of gizzard has been applied (3.) The walls of this viscus are composed of fibres that radiate from two dark points, seen in the figure; and its lining membrane is covered with a great number of hard horny teeth, so disposed as to represent, under the microscope, a tessellated pavement. The contractions of the gizzard are vigorous; and, from the structure of its interior, its office cannot be doubtful.
(1201). To the gizzard succeeds a stomach (fig. 238, a, 4), which is studded with brown specks, apparently of a glandular nature, and probably representing a biliary apparatus. The intestine leaves the stomach at its upper portion, close to the gizzard (5), and, running parallel with the oesophagus towards the tentacula (6), terminates at the side of the mouth (7), in such a position that excrementitious matter is at once whirled away by the ciliary currents. The whole intestinal apparatus floats freely in a visceral cavity that contains a transparent fluid and encloses distinct muscular fasciculi, to be described in another place.
The process of digestion in this minute yet highly-organized being is well described by Dr. Farre in the memoir above-mentioned.
(1202). The little animal, when in vigour, is seen projecting from its cell, with the arms extended and the cilia in full operation, - the upper part of the body being frequently turned from side to side over the edge of the cell, the extremity of which, from its peculiar flexibility, moves along with it. The particles carried to the mouth in the vortex produced by the action of the cilia, after remaining a little while in the pharynx, are swallowed by a vigorous contraction of its parietes, and carried rapidly down the oesophagus and through the cardia to the gizzard, that expands to receive them. Here they are submitted to a sort of crushing operation, the parietes of the organ contracting firmly upon them, and the two dark bodies being brought into opposition. Their residence, however, in this cavity is only momentary, and they are immediately propelled into the true stomach below, where they become mixed up with its contents, which, during digestion, are always of a dark, rich brown colour, being tinged with the secretion of its parietal follicles.