originate on a special offshoot (a quasi flower-stalk), which is now naked, and now protected by a transparent urn; sometimes they spring singly or in clusters from the body of the polypite; sometimes they pullulate from a portion of the common substance which links together and binds into one organism the many zooids composing a colony.

Let us examine a cluster of them, hanging from the body of one of the naked polypites. Commonly each bud is inclosed in a delicate capsule, which protects but only half conceals the exquisite structure that is ripening into perfect form and "tender grace" within it. In some cases, however, this envelope is wanting, and the bud is freely exposed through the course of its evolution to the surrounding water. In each cluster we find buds in all stages of development; but one is always much in advance of the rest, and attains maturity while some of its companions are merely rudimentary. And here let me pause for a moment to celebrate the beauty of the group of animal flower-buds, already showing traces of the vivid colouring that adorns the adult medusa; some closely folded up, and giving scarce a hint of the marvellous form that is being moulded within, some heaving with the pulsations of the imprisoned zooid, while one, it may be, has opened, and the fullblown medusa is visible hanging from its slender pedicle, and struggling with the bond that still detains it.

Through the investing capsule the general form and leading features of the contained zooid may be dimly traced; but so tightly is it packed in its little case, and so much are the parts compressed and thrown together, that it is difficult to realize its appearance until it emerges from its captivity. In the later stages of development, the contractile movements that are so characteristic of the tribe become frequent and vigorous, and at length the capsule is ruptured, and the medusa unfolds itself; the tentacles, which had been stowed away within the swimming-bell, are cast forth, the bell itself assumes its true proportions, and it hangs as if on the point of starting into free life. But it is still attached, and arduous and often protracted struggles must precede its final liberation.

Up to this point that portion of the bud which is to form. the digestive sac of the medusa has been in direct connection with the common cavity of the zoophyte, and has received from it constant supplies of nutriment. But the communication is now to cease, and all the energies of the zooid are engaged for the time in severing the link that binds it to the common life of the colony. (Vide Plate LXXXVIII., fig. 3; a marks the point at which the medusa was attached, and in communication with the general chymiferous cavity.) The struggle for freedom is often long, and, to the watching naturalist, wearisome enough; the bell contracts and expands rapidly, jerk succeeds jerk, tug

z 2

follows tug, these spasmodic efforts alternating with long periods of quiescence, until at last the connecting link suddenly gives way, and the medusa drops into the water.

Before we finally dismiss it from the colony, and while it still continues an integral portion of it, let us pause to consider the somewhat perplexing question of individuality with reference to these composite organisms, in which many zooids, similar and dissimilar, continuous and discontinuous, are the product of a single ovum.

Philosophically regarded, the whole series of forms evolved by budding, and intervening between two generative acts is, no doubt, the equivalent of the "individual" in other classes; and this is the case, even if some of these zooidal forms detach themselves, and lead an independent existence. In this sense the polypite and the medusa are as undoubtedly not "individuals." Yet it must be accounted unfortunate that this term should have been applied to the zoological conception. It would be better, surely, to speak of the life-series of the zoophyte, of which the various zooids are so many units, with the understanding that this corresponds with the "individual" of other tribes, than to perplex the ordinary mind by asserting that the free and independent medusa is not an individual at all, but that a hundred polypites and a company of a hundred medusæ together constitute an individual! The medusa is not the immediate and single product of an egg. It is developed as a bud from another structure, which is the immediate product of the ovum. It is not, therefore, an “individual,” biologically considered. But in the ordinary sense of the words, as Professor Allman has remarked,* “every zooid has an individuality of its own," which it is important to recognize. The medusa, with its original and distinctive manner of life, and its independent ways, has a very marked individuality of its own, which I shall endeavour to exhibit in the following pages.

To resume the history, the liberated medusa after a brief period of quiescence begins to move rapidly through the water, propelling itself by the alternate contraction and expansion of the gelatinous disc or swimming-bell, which constitutes its most striking feature. It certainly presents a remarkable contrast to the sedentary kindred from which it has lately parted company. Its whole organisation fits it for active locomotion, the polypite is rooted to one spot for life; is a restless floater, ranging widely through the waters of the sea, the latter is a fixture: it pursues its prey, the latter waits for it; it is mercurial, the latter vegetative; and yet after all it is but a

In his great work on the "Tubularian Hydroids," 1871.

polypite disguised in a dress which is suitable to the needs of a free existence. The general plan of structure will be best understood by a reference to the plate. It is of this kind. A delicate, more or less transparent disc or bell, serves as a float to which the various organs are attached; it is eminently contractile, and by a regular systole and diastole propels itself through the water. In figure it is variable, but always bounded by lines of beauty; sometimes it is almost globular, sometimes hemispherical, sometimes shaped like a watch-glass, and as translucent, sometimes of more fantastic form. Commonly it is of the slightest, filmiest material, often colourless, and so crystalline as to be hardly visible; as often tinted with the most delicate hues, which only the flower can match; frail as the bubble, and brilliant as the bubble when touched by the sunlight. Within the cavity of the bell, and suspended from its summit (Plate LXXXVIII., fig. 7, m), hangs the digestive sac, terminating below in a mouth, and at its upper extremity opening into a number of delicate canals (Plate LXXXVIII., fig. 7, c, c), which traverse the walls of the swimming-bell longitudinally, and empty themselves into a circular vessel running round its margin. We have here the simple nutritive system, which corresponds essentially with that of the fixed and plantlike elements of the hydroid colony. The digested and diluted food is forced from the pendent stomach into the radiating canals, and conveyed by them and the circular vessel that unites them throughout the organism, the stream flowing back periodically to the central depôt for fresh supplies. The number of the canals varies in different species; commonly it does not exceed four (Plate LXXXVIII., figs. 3, 4, 6,), but six (fig. 3), eight, ten (fig. 7), and twelve are met with, and in some cases as many as a hundred (fig. 1). The vessels are generally simple, but occasionally they bifurcate or are slightly branched, and in one rare instance give off short, lateral diverticula; never exhibiting, however, the remarkable complexity which occurs in the parallel group of the Discophores.* From the free margin of the bell hang a variable number of tentacles, some of which are a direct continuation of the radiating canals (Plate LXXXVIII., fig. 3); these which are first developed and always present, may be regarded as the primaries, and the spaces between them are often occupied by large numbers of secondary tentacular appendages (figs. 1, 2,), forming a beautiful fringe of delicate interlacing threads. The tentacle is of course a prehensile organ and instrument of offence; it is usually covered

Vide a paper by the author in the POPULAR SCIENCE REVIEW for April 1871, in which the points of agreement and contrast between the two groups of the Hydroida and Discophora are fully presented.

with prominent groups of poison-bearing thread-cells, which give it a pretty beaded appearance and a deadly touch (fig. 7, t). When the medusa swims, the arms are coiled in a spiral, and are borne "tightly twisted like a corkscrew;" when it is at rest they hang passively pendent, or are cast out to several times the length of the bell, and float in undulating lines through the water. The opening of the bell below is closed by a membranous film (the velum), with a circular orifice in the centre, through which the water finds access to the interior. This veil is a continuation of the muscular layer which lines the entire cavity of the swimming-bell, and endows it with its remarkable contractility.

There only remain to be noticed the simple organs of sense with which the locomotive zooids of the Hydroid are generally furnished, but which of course are altogether wanting amongst the fixed and vegetative members of the colony. These are of two kinds: the first consists of a collection of pigment-cells, forming a coloured spot or ocellus, and inclosed by a delicate membrane (figs. 7, 8). In some cases a crystalline body, a refracting lens, is embedded in the pigment-mass, and we are naturally led to regard the whole structure as an eye. Even when the lens-like body is wanting, the elements that remain may possibly constitute a light-perceiving organ of the simplest and most rudimentary kind. The ocelli, which are often darkred or black, are borne on the bulbous enlargements from which the tentacles spring (fig. 3). The remaining organs of sense that occur on the medusa are certain minute sacs developed on the margin of the swimming-bell, in each of which one or more refractile spherules are inclosed (fig. 2; in this species they are very numerous). They have been regarded as organs of hearing, from their supposed resemblance to the auditory sacs that occur in other classes; but there seems to be no real analogy between the two, and accordingly some of the ablest observers assign them a visual function. It may be impossible to determine the precise significance of these primitive structures; but we shall hardly err in viewing them as sense-organs of the simplest type correlated with the habits and needs of a free and active existence. Though almost universally present, there are a few known cases in which they seem to be wanting. The medusa represented in our fig. 3-a remarkable form, which I have lately obtained, and which is still undescribed-is destitute, at least in its earliest stage, of both ocelli and marginal sacs.*

The nervous system of the medusa, if such should exist in any specialised form, has certainly not been detected in the vast majority of known Hydroids; and though such a system

So far as is known at present, the two sense-organs (the ocellus and the marginal sac) never exist together on the same medusa.

has been described as occurring in a few cases, we are justified, I think, in waiting for further testimony before coming to any positive conclusion on the subject. As sense-organs make their appearance along with a free existence, we should naturally look for the dawn of the nervous system at the same point.

So much may suffice as to the general plan on which the Hydroid medusa is organized. I wish to avoid unnecessary detail, and to fix attention on the cardinal points of structure, and especially to emphasize what may be called the artistic and aesthetic aspects of my subject. No technical description, however minute and accurate, can enable us to realize the distinctive beauty of these fragile beings. Not merely are the forms graceful, and the curves faultless, and the colours vivid, but an additional charm is imparted to all of them by the delicate and transparent material in which they are presented.

Perhaps the portrait of an individual carefully executed may give the best idea of the kind of beauty that belongs to these "wandering buds," and may serve to justify the enthusiasm of the naturalist about them. I will select as my "sitter" a medusa, certainly one of the most lovely of its kind, which made its appearance for two successive years in a friend's tank, but which we were never able to trace to the parent stock (Plate LXXXVIII., fig. 2). No doubt in some chink or cranny a charming little colony of polypites was concealed, which supplied the annual medusan brood, but it baffled our keenest search. I am not aware that the pedigree of the form which I am about to describe has ever been made out; it is probably one of the unattached. Imagine, then, a minute crystal globe, the surface of which is thickly dotted over with thread-cells, making it appear as if delicately frosted; from the free margin hangs a graceful fringe, composed of twenty beaded tentacles, springing from as many bosses of a brilliant green with metallic lustre, which girdle it as with a belt of emeralds; amongst them are placed a number of the supposed eyes, each with its refractile corpuscle; the pendulous sac in the centre of the dome is dyed with the richest carmine, and tipped with the purest white, while the ovaries on the course of the radiating canals, shower masses of pink through the walls of its translucent bell. The colours are wonderfully brilliant, and the green tentacular bulbs almost glitter like gems. It was the prettiest sight to watch this little hydroid, with its painted and jewelled disc, now darting through the water with contracted arms; now sinking slowly, like a balloon, with some of its tentacles extended laterally as if to regulate its descent, and some hanging

*It is, no doubt, the reproductive zooid of a Campanularian Hydroid, and may belong to the Genus Campanulina, or a near ally.