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.
(186). But perhaps the most remarkable feature in the history of the Hydra is its power of being multiplied by mechanical division. If a snip be made with a fine pair of scissors in the side of one of these creatures, not only does the wound soon heal, but a young polyp sprouts from the wounded part; if it be cut into two portions by a transverse incision, each speedily developes the wanting parts of its structure; if longitudinally divided, both portions soon become complete animals; if even it be cut into several parts, every one of them will in time assume the form and functions of the original.
(187). It has recently been discovered that the Hydrae, at certain seasons of the year, are reproduced from real ova*, at which period various observers have proved them to be possessed of a male apparatus of a most remarkable character. This strange organism makes its appearance under the form of two, three, or four minute conical tubercles, which become developed from the sides of the body, at a short distance below the tentacula; and in these, under the microscope, a glandular-looking body and innumerable active particles are seen to be contained.
* "On a species of Hydra found in the Northumberland Lakes," by A. Hancock, Esq., Ann. and Mag. of Nat. Hist, for 1850.
(188). The conical eminences, which constitute the spermatic capsules, appear to derive their origin from the greater degree of development of one or more of the superficial cells in the vicinity of the base of the arms. These capsules sometimes occur in considerable numbers, as from eight to sixteen, on the brown polyp; but in the green species only two or three are generally seen, placed on opposite sides of the body, and invariably situated somewhere in the vicinity of the oral extremity (fig. 40,1, b b.) The interior of the capsules has a slightly ribbed or striated appearance; and at the summit a small aperture is sometimes perceptible, through which, when the development is complete, the spermatic filaments are observed to issue. On breaking up the capsule, under the microscope, large numbers of these filaments are seen united in bundles by their minute globular heads, the filamentous part being free, and vibrating with great rapidity in the manner which is known to be characteristic of these bodies in all animals (fig. 40,3.) These spermatic capsules were observed by Dr. Allen Thompson* on many individuals in which no ova existed. The ova are developed in the lower portion of the body, which, at the time when the male apparatus makes its appearance, becomes considerably enlarged, presenting an opake swelling, in the interior of which an ovum makes its appearance (fig. 40, l, a); when mature, this ovum becomes detached from the parent animal and fixes itself to some foreign body.
Fig. 40. Oviparous reproduction of Hydra viridis. 1. Body of Hydra magnified: a, the ovum contained in the ovigerous capsule sprouting from the side of the polyp; b, b, spermatic capsules. 2. Mature ovum of Hydra crushed, its contents escaping. 3. Spermatic capsule broken by pressure, showing the contained spermatozoa.
* Edinburgh New Philosophical Journal, 1846.
(189). The ovigerous capsule is, when fully developed, of such a size as to be seen with the naked eye. It is attached to the side of the polyp, nearer to the foot than the spermatic capsules, and is distinguished from the rest by its spherical form and yellowish-brown colour. In the Hydra viridis only one of these ova appears to be developed on the body of the polyp at the same time; but a number, varying from five to seven, have been occasionally observed upon the Hydra fusca.
(190). The ovum appears, at first, as a small granular mass in the thickness of the wall of the animal. As the spherical yelk-mass enlarges, it projects from the side, seeming to carry along with it the outer or clearer layer of the animal's body; then the cells of this layer grow thinner, and recede from the outer covering or capsule enveloping the egg-like mass, which at the same time becomes much thicker, and is left attached to the animal only by a narrow portion or pedicle. As the development proceeds, a similar atrophy of the cells of the pedicle is followed at last by the separation of the spherical mass, which is thus detached from the parent polyp.
(191). From various observations it would seem that while some of the individuals of the Hydrce are hermaphrodite, others produce the organs of one sex only; but generally both kinds are developed from the same Hydra.
(192). The fertilization of the ova cannot take place until after the rupture of the spermatic cyst and that of the ovisac also; so that the parent has no more participation in it than has the Fucus in the analogous fertilization of its germ-cells after their discharge. Although the production of a new Hydra from such an egg has not yet been witnessed, there seems no reason to doubt the fact. It would seem that this alternation of reproduction between the gemmiparous and the sexual is greatly influenced by temperature, the eggs being produced at the approach of winter and serving to regenerate the species in the spring, while the budding process naturally takes place only during the warmer seasons of the year, but may be made to continue through the whole by a sufficiently high temperature.
(193). Nearly related to the Hydrae is the remarkable group of Gelatinous Polyps, as they are named by Cuvier in his classification of the Animal Kingdom, constituting the genus Coryne (Corine of Gaertner.) One of these little animals, seen with the naked eye, observes Mr.Gosse*, to whom we are indebted for the following particulars of their history, looks like a very slender branching plant. It is altogether about as thick as fine sewing-cotton, creeping along a frond of sea-weed, or other substance upon which it grows, like an irregularly-winding thread. This creeping root sends off frequent rootlets, which, crossing each other, appear to anastomose, making a sort of network from which free stalks shoot up here and there, sometimes to the length of three inches or more, sending forth the polyp-branchlets irregularly on all sides. The creeping fibre, the stalk, and the branchlets are seen under the micros-scope to be tubular; and the two latter are marked throughout their course with close-set rings or false joints, apparently produced by the annular infolding of a small portion of the integument.