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.
(157). At the point where the ovigerous laminae reach the tentacles a membrane is observable, which assumes the shape of a funnel when the animal retires into its shell; and at the open end of the funnel the membrane is seen to fold outwards and become continuous with the calcareous tube (fig. 33, 1, b); its inner surface, indeed, is prolonged under the form of a thin pellicle over all that part of the interior of the tube which is inhabited by the polyp, terminating at a kind of diaphragm composed of the same hard substance as the tube itself. The remains of these diaphragms are found in the interior of old tubes at various distances from each other.
(158). The funnel-shaped membrane does not terminate suddenly at its point of junction with the calcareous tube; the latter, indeed, is a continuation and product of the first, the calcareous substance being evidently deposited in this gelatinous membrane in the same manner as phosphate of lime is deposited in the bones of very young animals, changing its soft texture into hard, solid substance. The manner therefore in which this tube is formed cannot be compared to the mode of formation of the shells of mollusca: in the latter case it is a secretion from the skin, an epidermic product; but in these poly-paries there is a real change of soft into solid substance, which is effected gradually, but not deposited in layers.
(159). When the tube has acquired a certain height, the animal forms the calcareous horizontal plate which unites it to those around; the still membranous upper part of the tube extends itself horizontally outwards around the aperture (fig. 33, 2, b), doubling itself so as to form a circular fold: this part of the membrane is no longer irritable; its internal surfaces unite, so as not to interrupt the continuity of the tube; carbonate of lime is gradually deposited within it; and soon a prominent partition, composed of two lamellae, soldered together through almost their entire extent, surrounds the tubular cell. Generally many polyps of the same polypary form these partitions at the same time and upon the same plane. In this case the gelatinous margins of the folded membrane unite, no space is left, and they ultimately become most intimately soldered together, and the solid plane or stage (fig. 32) is formed. If the animal construct its partition against a tube already perfect and solidified, it fixes its collar to its sides, so that the point of junction is imperceptible; but when it is quite insulated, as at b, fig. 32, the horizontal collar is still formed, and it then assumes somewhat of an octagonal shape. The tube-forming membrane exhibits no appearance of vessels or other traces of organization.
Fig. 33. Anatomy of Tubipora. 1. a, Polyp partially expanded; b, flexible extremity of the tube; e, ovigerous laminae; f, calcareous portion of the tube. 2. b, Expanded extremity of the tube, still uncalcifled; c, polyp retracted; d, inflected membrane embracing the neck of the polyp. 3. Ovarian lamina detached. 4. Development of young: a, horizontal stage; b, c, growing offspring.
(160). When the polyp is withdrawn within its cell, its tentacles form a cylindrical fasciculus (fig. 33, 2, c), the papillae which partially cover them being laid upon each other like the leaflets of some Mimosce when asleep.
(161). The protrusion of the creature from its tube is accomplished by the contraction of the membrane, b, inserted into its neck.
(162). The germs, during the first period of their development, have no organs distinguishable, not even the rudiment of a tube; each appears to consist of a simple gelatinous membrane folded upon itself (fig. 33, 4, c), and forming upon the stage upon which it is fixed a little tubercle resembling a small Zoanthus or other naked zoophyte. This tubercle gradually elongates, and assumes the form of a polyp, provided with all its organs; but the sac which encloses it is still gelatinous at its upper part and membranous near the base (fig. 33, 4, b), where it gradually diminishes in thickness, and, becoming calcareous, gives to the animal the general appearance of its original.
(163). An extensive and important group of the Anthozoa, from the fibrous character which the substance of their bodies assumes, have been named by zoologists "Fleshy Polyps;" nevertheless, although the genera composing this division are exceedingly numerous, and vary much in their external characters, they will be found to conform, in the essential points of their organization, with the corals above described. The subject we have selected as the type of these beautiful zoophytes is a well-known Actinia, which being common upon our own coasts, the reader will have little difficulty in procuring specimens for examination, or for preservation in a marine aquarium, of which they will form conspicuous ornaments.
(164). The body of an Actinia, when moderately expanded (fig. 34), is a fleshy cylinder, generally found attached by one extremity to a rock, or some other submarine support, whilst the opposite end is surmounted by numerous tentacula arranged in several rows around the oral aperture (fig. 35.) When these tentacula are expanded, they give the animal the appearance of a flower, the deception being rendered more striking by the beautiful colours they not unfre-quently assume; and hence, in all countries, these organisms have been looked upon by the vulgar as sea-flowers, and distinguished by names indicative of the fancied resemblance. Their animal nature is, however, soon rendered evident by a little attention to their habits. When expanded at the bottom of the shallow pools of salt water left by the retreating tide, they are seen to manifest a degree of sensibility, and power of spontaneous movement, such as we should little anticipate from their general aspect. A cloud veiling the sun will cause their tentacles to fold, as though apprehensive of danger from the passing shadows; contact, however slight, will make them shrink from the touch; and if rudely assailed, they completely contract their bodies, so as to take the appearance of a hard coriaceous mass, scarcely distinguishable from the substance to which they are attached.