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.
(301). This kind of reproduction is effected by the development of stolons, gemmae, and bulblets from any portion of the surface of the polypoid animal, which in turn give origin to similar offsets (fig. 54, 12, 13, 14), precisely resembling, when mature, the original polypoid body.
(302). The next phasis in the development of these Acalephs is one of the most remarkable circumstances connected with their history, and, were it not for the accumulated testimony of numerous observers, might appear almost incredible. The polyp, much in the condition represented at fig. 54, 11, is immovably fixed by its basis to the surface of a Fucus, or some similar support; in length it is about 1/8-th, and in diameter 1/16th of an inch; its surface is smooth, and its texture altogether gelatinous; its tentacles are movable in all directions and exceedingly irritable, and its whole structure and appearance, in short, that of a gelatinous polyp or Hydra. But a great change is now in preparation: the body of the Hydriform polyp gradually increases in size; and transverse folds begin to make their appearance at equal distances, one below the other, partitioning off its body into numerous rings or segments (fig. 54,15).
Fig. 54. Development of Cyanea capillata (after Sars, 'Annalee des Sciences Naturelles' for 1841, plates 15 B, 16, & 17, pp. 19, 50.) 1. Young Acalephs newly hatched (natural size.) 2. One magnified, showing infusorial condition of development. 3, 4, 5, 6. The same animal now become attached by a pedicle, and gradually assuming the polypoid form. 7. A still more advanced condition, showing the mouth surrounded by numerous retracted tentacula: the mouth is dilated, exhibiting four longitudinal eminences, situated in the stomachal cavity. 8. The same individual cut open longitudinally, and spread out so as to show the longitudinal eminences in the interior: the transverse lines are caused by the contraction of the body. 9, 10. Two polypoid Acalephs, with stolons developed from the upper part of the body: in fig. 10 the stolon has become attached to the supporting surface. 11. Fully-developed polyp. 12. Another individual giving off a stolon, from which proceeds a second that in like manner gives off a third offset. 13. Stolons growing off from the base of the polypoid Medusa, which, creeping along the surface of the substance to which it is attached, give origin to new polyps, a, b. 14. Three young gemmae sprouting from the body of a polypoid Acaleph. 15. A polypoid larva magnified (the natural size is shown at 15 a), having its body divided by numerous transverse wrinkles.
(303). In the course of a short time the segments thus formed become surrounded with marginal rays dichotomously divided at their extremities. These rays or arms are free, having their apices directed upwards, and disposed with such regularity that the once polypoid body seems to be furnished with eight longitudinal ribs (fig. 55, 16).
(304). We now arrive at the fourth period of the process, when the different segments into which the original polyp has become divided separate from each other, so as to form so many distinct disks (Planulae, Dalyell), each of which on its separation becomes a complete animal. This separation commences at the upper extremity of the series of newly-formed beings, and is repeated, segment after segment, towards the base, each segment as it becomes detached presenting the form, characters, and attributes of a free Acaleph, and in this condition assuming an independent existence, under the appearance represented in fig. 55, 17, a.
Fig. 55. Transformation from the polypoid form to the third, or Acaleph, condition (after Sars.) 16. The polypoid larva (16 a, natural size) in a more advanced state, now divided into segments piled upon each other, each of which is a young Medusa, having its disk surrounded with radiating processes bifurcated at their extremities. These segments becoming detached one by one from the summit of the pile successively, assume the medusiform condition. 17. Another example, in progress of division, in which only four segments remain undetached, and of these the three uppermost are at the point of separation. 17 a. A segment of the preceding detached, now become a free Acaleph: it is represented as seen from below, and already exhibits in its centre the square oral orifice, round which are perceptible rudimentary tentacula, together with the radiating nutritive canals, etc. 18. The same, in a still more advanced stage, exhibiting the rudiments of marginal tentacula. 19. The same, arrived at its perfect form, furnished with its four buccal arms, now completely divided and pendent, and fully provided with the marginal tentacles of the adult.
(305). The now free Acaleph, the disk of which is not as yet much more than 1/8-th of an inch in diameter, exhibits, when magnified, the characteristic organization of a true Medusa, - the oral orifice (fig. 56,a), the positions of the ovaria b, the radiating nutritive canals, c, the circular marginal vessels, d, the oculiform points, e, the anal apertures, f, and the rudimentary tentacula surrounding the disk, g g, being all easily recognizable. The Medusa being thus far complete, its further advance is rapid, the rays become gradually shorter in proportion to the disk, the marginal tentacles are more and more developed, and at length the young Acaleph, complete in all its parts (fig. 55,19), will in time, by the production of multitudinous ova, give birth to another generation, destined, during their development, to exhibit a parallel series of changes.
(306). In some of the Medusae which are destitute of a central pedicle, such as Cuvieria carisochroma (fig. 57), the arrangement, both of the alimentary and generative apparatus, is considerably modified. In aequorea violacea, examined by Milne-Edwards*, the gastric cavity, which is very large, occupies nearly a third of the whole diameter of the disk; the oral aperture, instead of being pedunculated, is simply surrounded with a membranous border, hanging loosely down when in a state of repose, and so short as to be quite inadequate to close the opening of the mouth. Superiorly this membranous border is attached to a ring, slightly more callous in its structure than the rest of the body, immediately above which is a circle of tolerably wide orifices placed very close to each other, all of which lead into radiating canals that diverge towards the circumference of the disk. These canals, seventy-four in number, becoming narrower as they recede from the stomachal cavity, run in straight lines to terminate in a circular vessel that surrounds the disk near its margin, from which little canals are given off, apparently analogous to the emunctory vessels above described as existing in Medusa aurita. (307.) The generative system in aequorea is in relation with this arrangement of the nutritive canals.