To return to the living specimen on the leaf : you see seated in the angles of the branches, here and there, elegant urn-shaped cells, larger than the polype cells, each with a sort of shoulder and a narrow neck. The common marrow passes from the joint into the bottom of these, and then extends through the centre till it reaches the mouth. In some of the urns this forms merely a slender column, expanding at the mouth; but in others it enlarges at irregular intervals into large knobs or masses of granular flesh, which are confusedly grouped together, eight or ten in one capsule. This latter is the most interesting condition; let us watch it.

While doing so, let me inform you that these urns are the reproductive organs ; and the fleshy masses are embryos of peculiar character, which are developed out. of the nutrient medulla. The largest of those now under observation is, as you see, moving, and slowly working its way out of its glassy prison. Two or three flexible finger-like bodies are protruding from the orifice of the urn, and more are joining them : we see they are tentacles, protruded in a loose bundle, just as the polype emerges from the cell.

It is a somewhat slow process; but at length the fleshy mass squeezes itself forth, as if pushed out by some contractile force behind; while we see the fluids, carryinggranules, run into the parts of the tentacles which are already free. The embryo is liberated.

For a few seconds it appears helpless, and falls through the water in a collapsed state, so that we cannot discern its proper form. It gives a spasmodic contraction or two, feeble at first, then more vigorous; the tentacles lengthen, the body expands, and—lo ! it is not a Polype, but a Medusa !

And now take your eye for a moment from the microscope, and glance at this glass jar, in which the oarweed with its colony of Zoophytes has been standing for a few hours. Hold it between your eye and the light; do you not see that the water is alive with tiny dancing atoms 1 Hundreds are there, playing and pumping through the fluid with a sort of napping motion, which, when you get one sidewise in clear view, will not fail to remind you of the flagging flight of some heavy-bodied, longwinged bird. These are the Medusa-shaped progeny of the Laomedea.

But let us return to the one of which we have just witnessed the birth, and which is still flapping to and fro in the narrow glass trough. You see a pellucid, colourless disk or umbrella of considerable thickness, about onesixtieth of an inch in diameter in its average state of expansion. Its substance has a reticular appearance, probably indicating its cellular texture. Internally, the disk rises to a blunt knob in the centre, whence four vessels diverge to opposite points of the margin. These form elevated ribs, the surface being gradually depressed from each to the centre of the interspace. Externally, the centre of the disk is produced into a fleshy peduncle, having a narrow neck, and then expanding into a sort of secondary disk, of a square form, with the angles rounded. This organ, which is capable of varied, precise, and energetic motions, corresponds to the peduncle of a true Medusa, the angles being the lips. These lips, which correspond in their direction to the four internal ridges, are very protrusile; and, when the little animal is active, are continually being thrust out in various directions, sometimes everted, but more commonly made to approach each other in different degrees ; sometimes one being bent-in towards the centre, sometimes all closing-up around a hollow interior. These four lobes, thus perpetually in motion, and changing within certain limits their form and their relation to each other, remind one of the lips or the tongues of more highly organised animals. The substance of this peduncle appears to be delicately granular ; but there is a very manifest tendency to a fibrous character in its texture, the fibres being directed from the exterior towards the interior, supposing the lobes to have their points in contact.

Let us now look at the margin of the disk. Here are attached twenty-four slender tentacles, six in each quadrant formed by the divergent ribs, or radiating canals. Each tentacle springs from a thickened bulb, which is imbedded in the margin of the disk ; it is evidently tubular, but the tube is not wider in the bulb than in the filament. The general surface is rough, with projecting points, which in some assume a very regular spinous -appearance, and the tentacle terminates in a blunt point. The discal part of the bulb is fringed with a row of minute bead-like spherules. Around the edge of the circumference •of the disk, on the exterior, are arranged eight beautiful and conspicuous vesicles, or organs for hearing. They are placed in pairs, those of each pair being approximate, and appropriated to each of the quadrants of the circle. Each of these organs consists of a transparent globe, not enveloped in the substance of the disk, but so free as to appear barely in contact with it: it contains a single otolithe, of high refractive power, placed, not in the centre, but towards the outer side. The inexperienced naturalist, on first seeing these organs, would unhesitatingly pronounce them eyes, and the otolithe, or ear-stone,* the crystalline lens. They are, however, pretty certainly, rudimentary organs of hearing; the crystalline globule or otolithe being capable of vibration within its vesicle. Their •exact counterparts are found in many of the smaller Medusae, as we lately saw in the Thcmmantias.

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

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