HISTORY OF SERTULARIA POLYZONIAS.

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lower ramifications; nevertheless, the persistence of vigorous animated extremities may be long, and their reproductions numerous, while all the lower ramifications are quite dead.

Let us, however, illustrate the economy of these beautiful organisms by an example.

SERTULARIA POLYZONIAS.

The Hoop Sertularia.

Specimens of this zoophyte rise to about 4 inches high, by a stem which is slightly waved, and scantily provided with boughs and branches diverging to right and left. The cells are ranged alternately on both sides of the stem, the subordinate parts always originating from the convexities by which these are distinguished (Pl. II. fig. 6).

Each cell is occupied by a Hydra furnished with about twenty-four or twenty-six tentacula (Pl. II. fig. 6, a). The Hydra is large in proportion to the size of the cell, from which it protrudes by means of a long neck or body; and, when disturbed, retreats completely within the cavity. The whole zoophyte is of a green colour, of various shades and intensities, this colour being derived from the pith, as the horny sheath is perfectly transparent. The stem of adults or older specimens is occasionally yellowish or brown.

The ovaria or reproductive vesicles of this Sertularian are large in comparison with the cells, and of singular structure, appearing as if they were composed of hoops or belts united together (Pl. II. fig. 6, b).

Specimens with lively Hydræ, and bearing reproductive vesicles, were obtained by Sir John Dalyell in July. Only one vesicle appeared among twenty cells.

Those that were prolific extended about a line in length, and were filled with yellow, globular corpuscles, about twenty-four in each (Pl. II. fig. 6, b, d).

About fifty Planulæ issued from the different vesicles on the 8th of July. These were about a third of a line in length; the body plump, nearly round, but somewhat flattened below; of a smooth, uniform aspect, and darker in colour than straw-yellow. In the course of their escape they were evidently suspended from various parts of the specimen by an invisible thread; but, on reaching any solid surface, they advanced with an equal gliding motion, resembling Planaria (Pl. II. fig. 6, c).

Many Planule continued quitting the vesicles from the 8th to the 12th of July. They spread on the bottom, and crowded together on the sides of their vessels. Numerous dark green, thick, obtuse spines were observed rising from spots on the bottom of the glasses (Pl. II. fig. 6, g); and, on the 14th of the month, several of these had evidently enlarged as buds (Pl. II. fig. 6, f), which, next day, and on the day following, had in many instances become developed into Hydræ (Pl. II. fig. 6, g). The spine is at first dark green, thick and obtuse; when further advanced, the summit enlarging exhibits the Hydra as a green bud within its cell, the "spine" becoming the sustaining stalk. Also the green colour of the stem demonstrates the existence of a central pith.

Illustrations of the process of progressive growth may likewise be witnessed in this Sertularia in its carliest stages. An enlargement appears at the summit of a stalk, which is found to be invested by a deli

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cate thin film, including twin buds. These are unequally advanced; but as the lower one becomes matured, its higher companion forks off, and then another is developed from this latter in the same manner. The buds are nascent Hydræ in their respective cells, each having apparently its own peculiar integument within the common filmy involucrum.

Rapid growth ensues. A young specimen with only a single head on the 4th of October, had acquired six on the 20th, besides a seventh in embryo ; and of course, as the extent of the zoophyte increases, and the branches become more numerous, the evolution of additional polyps becomes proportionably more rapid.

The multiplication of these elegant zoophytes appears therefore to take place in three different modes: 1st, by cuttings, as in plants; 2ndly, by offshoots, or the formation of new branches bearing polyps; and 3rdly, by Planulæ, capable of locomotion.

The first mode strikingly resembles what is observed in the vegetable kingdom; for as every branch of the plant-like body contains all the parts necessary for independent existence, it can hardly be a matter of surprise, that any portion separated from the rest. will continue to grow and thrive as well as the entire colony.

The second mode of increase, namely, by the formation of new branches and polyps, seems more like the growth of a plant than the development of an animal; while the third is evidently specially intended for the diffusion of the species.

CHAPTER XIV.

CAMPANULARIAN ZOOPHYTES.

THERE is a great resemblance between these Zoophytes and the Sertularians described in the preceding chapter, insomuch indeed that the reader will find it difficult to distinguish them.

The CAMPANULARIÆ may, however, be recognized by the following characters. The polyp-stems, which are plant-like and horny, are rooted by a creeping tubular fibre, either branched or simple. The polypcells are thin and campanulate, that is, bell-shaped, and, moreover (instead of being, as in the Sertularians, sessile upon the sides of the stem and branches), are in the Campanulariæ terminal and elevated on foot-stalks, which are disposed either alternately or in an irregular manner. The germs of progeny are developed, as in the Sertularians, in horny, deciduous capsules, or "reproductive vesicles," as they are termed; but the young, instead of presenting the form of Planulæ, or resembling young Planariæ, make their appearance as Medusa, thus affording another remarkable example of Medusiparous birth.

We have already found among the Hydroid Zoophytes, that the Medusiparous mode of reproduction

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is by no means a phenomenon of unfrequent occurrence in this remarkable class, and yet, until a very few years ago, such a circumstance in their history was not only entirely unknown, but, when first announced to exist, the discovery was received by naturalists with as much hesitation and jealous caution, as if some visionary enthusiast had endeavoured to impose upon their credulity.

Animal magnetism and table-turning found ready advocates; the upholders of clairvoyance and spiritrapping had no difficulty in making converts to their mysteries; but to believe that a polyp could give origin to a jelly-fish, or a jelly-fish to a polyp, was esteemed to be an assertion so incredible, that nothing but the well-known simple truthfulness of its first discoverers could have obtained a hearing in support of statements apparently so monstrous and absurd.

"If, in a picture, Piso, you should see a horse's body with a fish's tail, and limbs of beasts of the most different kinds covered with feathers of all sorts of birds, would you not laugh, and think the painter mad?" was a question easily propounded, and still more readily answered, until at length the notion of these alternations of form in the Hydrozoa might have been scouted as incredible and preposterous, had not the evidence of our own senses, the aquarium and the microscope, asserted the reality of the fact, and compelled the belief of the most sceptical:

"Serpentes avibus geminentur, tigribus agni."

The subject of the present chapter affords us another opportunity of elucidating this wonderful pro• Horace, De Arte Poetica,-freely translated.