As the hydra fusca and the hydra grisea are considerably larger than the hydra viridis, it is more easy to observe the manner of their producing their young. It is upon these, therefore, that most of the observations here recited have been made. If one of them be examined in summer, when the animals are most active, and more particularly prepared for propagation, it will be found to shoot forth from its side several little tubercles, or knobs, which grow larger and larger every day; after two or three days inspection, what at first appeared but a small excrescence, takes the figure of a small animal, entirely resembling its parent. It does not inclose a young polype, but is the real animal in miniature, united to the parent, as a sucker to the tree.
When a young polype first begins to shoot, the excrescence terminates in a point, as at e, [Plate XXIV. B.] Fig. 24; so that it is rather of a conical figure, and of a deeper colour than that of the body. This cone soon becomes truncated, and in a little time appears cylindrical. The arms then begin to shoot from the anterior end c i. The tail adheres to the body of the parent, but grows gradually smaller, till at last it only adheres by a point b, Fig. 23, it is then ready to be separated; for this purpose the mother and young ones fix themselves to the glass, or other substance upon which they may be situated. They have then only to give a sudden jerk, and they are divided from each other. There are some trifling differences to be observed now and then in their performing this operation, which it would be too tedious to enumerate here. A polype, a b, Fig. 20, with a young one, c d, places its body in an arch of a circle a d b, against the sides of the glass, the young one being fixed at the top d of the arch, with its head also fixed against the glass; so that the mother, by contracting the body, and thus becoming straight, loosens herself from the young one.
The young ones shoot in proportion to the warmth of the weather, and the nature of the food eaten by the mother; some have been observed to be perfectly formed in twenty-four hours, while others have required fifteen days for the same purpose; the first were produced in the midst of summer, the latter in a cold season.
The tail of the young polype communicates with, and partakes of the food from the parent in the same manner as its own arms do, and the food lies in the same manner as in the arms. When this fœtus is furnished with arms, it catches its prey, swallows, digests, and distributes the juices thereof even to the parent body; every good is common to each. Here then we have evident communication between the fœtus and the mother; this communication was further proved by the following experiment. A large polype, one of the hydra fusca, was placed on a slip of paper, in a little water; the middle of the body of the young one was cut, and the superior part of that end which remained fixed to the parent was found to be open. The parent polype was then cut on each side of the shoot. Thus a short cylinder was obtained, which was open at both ends. This being viewed through a microscope, the light was seen to come through the side slip, or young one, into the stomach of the old one. For further conviction, the cylindrical portion was cut lengthways; on observing these parts, not only the hole t of the communication, [Plate XXIV. B.] Fig. 17, was distinctly seen, but one might see through the end o of the young one. On changing the situation of these two pieces of prepared polypes, and looking through the opening e, Fig. 18, the day-light was seen through the hole of communication i.
This communication, between the parent polype and its young ones may be seen on feeding them; for, after the parent a b, [Plate XXIV. B.] Fig. 22, has eaten, the bodies of the young ones swell, being filled with the aliments as if they themselves had been eating. In the hydra fusca the young ones do not proceed from the tail part b c, [Plate XXIII. B.] Fig. 16, but only from the part a c, with this exception, there is no particular part of the body before the rest, on which they produce their young. Some of them have been so closely observed, and have so greatly multiplied, that there would be scarce any impropriety in saying they produced their young ones from all the exterior parts of their body. A polype puts forth frequently five or six young ones at the same time. Trembley has had some that have produced nine or ten at the same time, and when one dropped off another came in its place.
Though this gentleman had for two years thousands of them under his eye, and considered them with the most scrupulous attention, he never observed any thing like copulation. To be more certain on this head, he took two young ones the instant they came from their parent, and placed them in separate glasses; they both multiplied, not only themselves, but their offspring, which were separated and watched in the same manner to the seventh generation; nay, they have even the faculty of multiplying while they adhere to the parent. The arms of the young ones do not sprout till the body has attained some length.
Several excrescences or buds often appear at the same time on a polype, which are so many polypes growing from one trunk; whilst these are developing, they also bud, which buds again put forth little ones, the parent and the young ones forming a singular kind of animal society, in which all participate of the same life, and the same wants. In this state, the parent appears like a shrub thick set with branches. Several generations are often thus attached to one another, and all to the parent polype; after a time, this tree of polypes or hydræ is decomposed, and gives birth to new generations, or fresh genealogical trees. Here we see a surprizing chain of existence continued, and numbers of animals naturally produced, without any union of sexes; every polype raising a numerous posterity by a kind of animal vegetation.
From Fig. 16, [Plate XXIII. B.] the reader may form an idea of the promptitude with which these creatures increase and multiply; the whole group formed by the parent and its young was about an inch and an half long, and one inch broad, the arms of the mother and her nineteen little ones hanging down towards the bottom of the vessel; the animal would eat about twelve monoculi per day, and the little ones about twenty among them, or rather more than thirty for the group.
OF THE RE-PRODUCTION OF THE HYDRÆ.
So strange is the nature of this creature’s life, that the method by which other animals are killed and destroyed becomes a means of propagating these. When divided and cut to pieces in every direction that fancy can suggest, it not only continues to exist, but each section becomes an animal of the same kind.