The disk is endowed with an energetic power oc contraction, hy which the margin is diminished, exa 3tly like that of a Medusa swimming; and the tentacles have also the power of individual motion, though in general this is * See note on page 56.

languid, their rapid flapping being the effect of the contraction and expansion of the disk, whereby the margin is moved quickly backwards and forwards, carrying the tentacles with it. Occasionally, however, all the tentacles are strongly brought together at their tips, with a twitching, grasping action, like that of fingers, which is. certainly independent of the disk, and may be connected with the capture of the prey.

Now every detail of the structure here, as well as the general form, appearance, and habits, agrees with the small naked-eyed Medusae, so closely that if we had not witnessed the birth of the little creature from the reproductive cell of a Laomedea, we should, w^ith unhesitating confidence, have pronounced it a true Acaleph. The peduncle, it is true, seems out of place, being on the outside of the dome, instead of hanging suspended from its interior ; but this difference is only apparent, and arises from the circumstance that the disk is reverted. If you suppose the edge of the disk to be turned in the opposite direction, you will have the peduncle in its proper place : the umbrella in these specimens is carried within, and the sub-umbrella without ; an inversion which is probably accidental, or, at least, unimportant.

Comparing now this strange production of a Medusa by a Polype, with Avhat I lately told you of the production of Polypes by a Medusa (as in the case of the lovely little Turris), you will have some acquaintance with the wondrous phenomena which have of late years been surprising and interesting naturalists, viz., those of the Alternation of Generations ; in which, as Chamisso, the first discoverer of the strange facts, observed,—" a mother is not like its daughter, or its own mother, but resembles its sister, its grand-daughter, and its grandmother." The Polype gives birth to a generation of Medusre which lay eggs, which develop into Polypes. The Medusa, on the other hand, lays eggs (gemmules), which develop into Polypes,"which at length divide themselves into colonies of Medusse.*

At first you will perhaps see nothing remarkable in another object which I collected in my rock-ramble today. A Hermit-crab in an old Natica shell ; both common things enough. Yet look more narrowly. The greater portion of the shell is not smooth, has no such porcelain-like polish as the Natica usually has, but is clothed with a sort of downy nap, a coarse sponginess of a greyish hue, splashed with yellowish and pink tints. The shell is invested with Hydractinia.

We restore the strange partnership,—shell, fleece, and crab,—to the glass of sea-water ; where we soon see the whole tumbling about the bottom in uncouth agility. Assist your eye with this pocket-lens, and look again. The shaggy nap upon the shell now bristles with tall slender polypes, crowded and erect, like ears of corn in a field.

No high magnifying power is necessary to furnish us with considerable entertainment from this populous colony. The polypes stand individually nearly half-aninch in height : each consists of a straight slender column, surmounted by eight straight rod-like tentacles, four of which stand erect, slightly diverging, and the other four alternating with these at their origin, extend horizontally like the arms of a turnstile.

The rough jolting of the crab over the stones the expanded polypes bear with equanimity ; they are used to

Since this work was published, many efforts have been made to explain, and to bring into harmony with what were supposed to be the laws of generation, these strange facts, which seem to subvert all our notions of individuality. Steenstrup's phrase is now by common consent abandoned : there is no alternation of generation proper. That hypothesis which regards the phenomena as those of metamorphosis, the fixed condition (Laomedea) being analogous to the larva, the free-swimming (Medusa) to the adult, is one of the most plausible ; but an attempt to apply it in all the cases shows it to be untenable. (See Greene, Coelenterata, p. 76.) it; and though their tentacles wave and stream hither and thither, they are not retracted on this account. But just touch with the point of the pencil in your hand any part of the shaggy fleece, and instantly the whole colony retire together, as if by a common impulse, apparently shrinking into the substance of the shell. Yet they soon re-appear, one after another quickly protruding its closed tentacles, which are presently expanded as before.

The explanation of this phenomenon is, that the whole colony of polypes are but the free points, or feeding mouths, of a common living film, which invests the shell; just as, in Laomedea, the polypes that inhabit the vase-like cells are the off-shoots or free points of the common medulla.

The investing film will sometimes in captivity spread upon the glass side of a tank, and there develop all the polypes and organs proper to the complete organism. When this is the case, an admirable opportunity is presented for studying with ease and precision the economy of the creature; and it is to the skill with which Dr. T. Strethill Wright has availed himself of such an opportunity* that I am indebted for the chief part of the facts that I am going to tell you, connected with the form and appearance, of which you can here judge for yourself.

The spreading film or polypary is a thin coat of transparent jelly, slightly coloured with various tints, which secretes and deposits within its substance a still thinner horny layer of chitine. This rises here and there into numerous spines and points, which are curiously ridged with toothed keels; and these ridges run in various directions over the horny layer also, making a fine network over it. The investing flesh, however, fills up all the cavities and areas so inclosed.

The mode in which the polypary increases is by throwing out from its edge a creeping band, exactly analogous to the root-thread of the Laomedea. This "propagative

See "Edin. New Philosophical Journal," for April, 1857.

stolon, after leaving the point of its origin, increases rapidly in diameter, and sends out irregular branches. The tips of these branches are covered with a glutinous cement, by which they attach themselves tenaciously to glass, or other surface near them. Having attached themselves, they expand laterally, at the same time throwing out finger-like prolongations, which, as they come in contact with each other, coalesce, until a fleshy plate is found adherent to the glass. Polypes are developed both from the loose branches and the attached polypary; and the latter is clearly seen to be permeated by a beautiful system of anastomosing canals, connected with the hollow bodies of the polypes. Within these canals may be detected an intermittent flow of fluid, containing particles, the dancing motion of which indicates the presence of ciliary action ; and which, having passed in one direction for a short time, are arrested, and, after a slight period of oscillation, begin to flow in an opposite direction."

The polypes which are developed from this living carpet are not all of the same form. ISTo fewer than five distinct sorts exist, at one and the same time ; and I doubt not we shall be able to find and to identify them all, on this well-grown specimen.

First, there are the alimentary polypes, which we have already cursorily glanced at. Within the space inclosed by the two circles of tentacles, there is a mouth with soft protrusile lips, which can be pushed out and folded back, so as to hide tentacles, column, and all.

Scattered among these we see numerous polypes, which agree in general form with these, but with some remarkable subtractions and additions. They have no mouth nor stomach, and the tentacles are reduced to the smallest possible warts or protuberances like small teeth on the dilated tip. But the additions are still more peculiar. From the middle part of the column a number, from four to nine, of great oval sacs project, each attached hy one end, while the other stretches out horizontally, thus surrounding the slender column. Each of these sacs is an ovarian capsule, and contains several ova of a brilliant yellow or crimson hue. Thus we have the second form, that of the reproductive polypes.

In some places single ovarian capsules stand up alone from the fleshy carpet, agreeing in every respect with those which we have just examined, except that they are sessile, instead of being carried by a polype.

The fourth form is that of the tentacular polype. Here and there, from amidst the forest of shorter polypes, both alimentary and reproductive, white threads are seen protruding, which extend to a length four or five times as great as theirs, and hang down or loosely float in the water. They are found on the outskirts of the whole compound structure, and at each extremity of the long diameter of the mouth of the supporting shell ; so that they must, in their natural condition, reach to the ground, along which the crab-tenanted shell is carried, enabling the Zoophyte to seize and appropriate the atoms scattered by the crab whenever he takes his meals. The tips of these organs are covered with a dense pavement of large thread-cells; and they must doubtless perform the office of general purveyors to the composite animal.

But still more remarkable, more extraordinary than all we have been considering, are the objects which are now in view in the field of the microscope. You see a number of bodies, which Dr. Wright calls ophidian or spiral polypes ; and which, as he truly observes, are " like small white snakes, closely coiled in one, two, or three spirals, and grouped immediately round the mouth of the shell." The habits of these polypes are still stranger than their forms. " When touched they only draw their folds more closely together. But if any part of the polypary, however distant from them, be irritated, the spiral polypes uncoil, extend and lash themselves violently backwards and forwards, and then quickly roll themselves up again ; and that not irregularly or independently of each other, but all together, and in the same direction, as if moved by a single spring. A violent laceration of the polypary causes these polypes to remain extended and stretched like a waving and tremulous fringe across the mouth of the shell, for several minutes. The ophidian polypes (evidently a barren modification of the reproductive polype) are never found in any other situation on the polypary than in that before described, or round the margins of accidental holes in the shell. They have no mouth and the tentacles are rudimentary. The walls of the body are very transparent, from the extreme vacuolation of the inner tissue. The muscular coat, as might be expected, from the active movements of the polypes, is highly developed, and forms a beautiful object on the dark polarised field of the microscope, each spiral coil shining out as a bright double ring divided by four dark sectors. The outer tissue of the whole body and tentacles is crowded with the larger thread-cells. The ophidian polypes are, doubtless, organs of defence or offence, like the motile spines and bird's head processes of the Polyzoa, or the pedicellarise of the Echinodermata ; but it is difficult to assign a reason for their peculiar situation. They vary much in number and size, in different specimens of Hydractinia, but are rarely altogether absent."*

The reflections of the able zoologist who first called attention to these varied developments, and his comparisons of them with those of another polype-form which we have lately been observing, are so interesting and instructive that you will not deem it needful that I should apologise for citing them. " In our consideration of the Hydractinia" he observes, " our attention is arrested by the multitude of objects grouped together to constitute a single animal, their variety in form, and the sympathy * Dr. Wright, op. cit.

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