DUMB-BELLS IX HOLOTEVHIA. good many specimens occur in which the two dumb-bells can be quite distinguished from each other. The calcareous matter that solders the elements together seems abundant, and has the appearance that would be presented if they had been made of solid glass, and united by glass in a state of fusion ; the latter having apparently run together, so as to smooth and round angles and fill up chinks, even where, as is often the case, the globes themselves have only mutually approximated, and not come into actual contact.

The average dimensions of these oval aggregations may be .004 inch in length, and a little more than .002 in width ; but some specimens occur which are a little larger, and others a little smaller than this ; while the irregular plates are sometimes three times the length. Some of the more worm-like members of this class have, however, a skeleton composed of pieces imbedded in their skin, of even more remarkable shapes than these. One of these is the CJdrodota violacea ; a native of the southern coasts of Europe. We have indeed a British

WlIKhL UN IH1KODOTA. species of the same genus, a

specimen of which is in my possession, but I have vainly examined the skin for any structure analogous to this.* In the Mediterranean species the skin, especially of the belly-side, is described as filled with plates exactly resembling broad and thin wheels of glass, supported by four, five, or six radiating spokes, and having the inner edge of the hoop cut into teeth of exceeding delicacy.

Another animal remarkable for the furniture of its skin is the genus Synapta, which is very similar in form, and closely allied, to the Chirodota. It is very common in the Adriatic and Mediterranean seas, but has not yet been taken on the British coasts. I would counsel you, however, to have your eyes open if you have the opportunity of searching our coasts ; for, as M tiller found one species, the Synapta inhcerens, on the shores of Denmark, it is not at all unlikely that we may possess either it or some other. Should it ever come into your hands, slit open the skin of the belly, where you will find, imbedded in little papillaor warts, some highly curious spicula or calcareous forms. Each consists of an oblong plate, perforated with large holes in a regular manner, and having a projection on its surface near one extremity, to which is jointed a

AXCHOB-PLATE I2T STXAFTA.

second piece, having the most singularly true resemblance to an anchor. The flukes of this anchor project from the skin, the shank standing obliquely upward from the plate, to which it is articulated, where the ring would be, by a dilatation which is cut into teeth, f

The most careful and repeated search has not availed me to find in the skin the least trace of calcareous atoms ; but this may possibly be because I had unfortunately preserved my specimen in acetate of alumina, and the acetic acid has perhaps dissolved the lime.

*T Since the printing of the above, an admirable paper has appeared in the proceedings of the Zoological Society of London, by Messrs. Woodward and Barrett, " On the Genus Synapta." These authors in-

Among the multitude of transparent creatures that swim in the open sea, few are more interesting than the infant state of the very animals that we have lately been examining ; the Sea-Urchins and their allies. It is a productive way of obtaining subjects for microscopic research, to go out in a boat on a quiet summer's day, especially in the afternoon, when the sun has been shining, or when evening is waning into night, and, with a fine muslin net stretched over a brass ring at the end of a pole, skim the surface of the smooth sea. At intervals you take in your net, and having a wide-mouthed glass jar ready, nearly filled with sea-water, invert the muslin in it, when your captures, small and great, float off in the receiver. After a few such essays, unless you have very bad success indeed, you will see even with the naked eye, but much more with a lens, that the water in your jar is teeming

form us that two species of Synapta are found in the British seas, one of which, the Chirodota digitata of E. Forbes, is the species referred to in the text as being in my own possession. Its skin proves to be studded with anchors, not with wheels, and it is on this ground referred to the genus Synapta.

Still later (See Qu. Jourti. Micr. Scl. for 1862, p. 131), Dr. Wyville Thompson has obtained Synapta inhazrens in the Irish Loughs of Belfast and Strangford. In his admirable paper, elaborately illustrated, on the infancy and development of this curious creature, he has traced the gradual formation of the calcareous spicula, from their first indication. The anchor first appears as a straight rod sharp at both ends. One end gradually becomes knobbed ; the knob extends on each side, giving the rod a T form ; and soon the arms curve backward, giving the anchorform. Then the basal end becomes dilated. Hitherto no trace of the plate has appeared ; but now a small glassy needle is seen lying across the shank near the base. Each extremity of this now branches, and these again branch, the points meeting and uniting here and there, until at length the plate is gradually mapped out. As yet the openings left by the uniting network are smooth-edged ; but the glassy network gradually strengthens, and the edges of the holes, now uo longer meshes of a net, but orifices in a plate, are studded wit i teeth all

ound. I refer my readers, for much information of he deepest interest, to this very valuable memoir.

with microscopic life ; and though many of your captives will not long survive the loss of their freedom, still meanwhile you may secure many an interesting object, and examine it while yet the beauty and freshness of life remain. And, moreover, with care and prudence, some selected subjects may be maintained in vigour, at least long enough to afford you valuable information on the habits, economy, metamorphosis, and development of animals, of which even the scientific world knows as yet next to nothing.

I have just been so fortunate as to obtain in this way one of our Sea- Urchins in its larva state ; and have it now in the thin glass trough which is on the stage of the microscope. It is just visible to the unassisted sight as a slowly moving point in the clear water, when the vessel is held up to the light ; but with the low power which I am now using, it is distinctly made out in all its parts, and is an object of singular elegance and beauty.

It is, as you see, somewhat of the figure of a helmet, the crest rising to a perpendicular point, which is rounded, the vizor or mask descending far down, and ending in two points, and a long ear hanging down on each side, so as to reach the shoulders of the wearer. Of course such comparisons are fanciful, but they assist one in intelligible description.

Now, the entire helmet is composed of a gelatinous flesh of the most perfect transparency, so that we can see with absolute clearness everything that is within it And the first thing that strikes us is, that a framework or skeleton of extreme delicacy, composed of glassy rods, supports the whole structure. Look carefully at this, and mark its symmetry and elegance. There is, then, first, a rod which passes through the crest perpendicularly, and carries at its lower extremity a horizontal ring. To the opposite sides of this ring are soldered two other very slender rods, passing down nearly in a perpendicular direction, but a little diverging ; and two other shorter rods pass down from the front of the ring, parallel to these. After a while each lateral pair of rods is united by a short cross-piece, and the result is four lengthened rods, two of which go down through the vizor into the chin-points, and two larger and stouter ones through the ears into the shoulder- points. This, then, is the solid skeleton, the interest of which is much enhanced, when we observe that it is formed, on the common plan, out of perforated lime-glass, the two ear-rods and the crest-rod being pierced with a regular series of oval holes, and bearing on their edges corresponding projecting points.

Now, to turn again to the gelatinous flesh. The inner surface of the vizor, or that which would be in contact with the face of the wearer, supposing it to be a real helmet, has a great squarish orifice with a thickened margin, which we see by its movements to be highly sensitive and contractile. This square orifice is the mouth of the larva, and it leads into a cavity in the upper part of the vizor, which is the gullet ; and this in its turn terminates in a narrowed extremity, which passes into the orifice of a greater and higher cavity, the lip of which embraces it just as the bunghole of a barrel receives and embraces the tube of a funnel. The latter cavity occupies the chief part of the volume of the helmet, the four rods diverging to inclose it. It is the stomach.

It adds to the beauty of the little helmet-shaped creature that, while the greater portion of the substance is of the most colourless transparency, the summit of the crest and the tips of the shoulder-points are tinged with a lovely rose red. The whole exterior surface is, moreover, studded with those minute and glandular specks, with which every part of the adult Urchin is covered ; and the light is reflected from the various prominences with sparkling brilliancy.

The little creature moves through the water with much grace and with a dignified deliberation ; the crest being always uppermost, and the perpendicular position invariably maintained. It does not appear capable of resting, its movements depending on incessantly vibrating cilia. These organs we perceive densely clothing the long ear-pieces, but more especially accumulated and more vigorous, in a thickened, fleshy band, which passes partly round the whole helmet, at the origin of these pieces.

You do not discern the slightest resemblance of form between this little slowly-swimming dome and the spined and boxed Urchin which crawls over the rocks ; and you wonder by what steps the tiny atom of one-fortieth of an inch in length is led to its adult stage. Fortunately I can satisfy your curiosity on this point, not indeed from my own observations, but from those of Professor Johann Miiller, whose discoveries of LAEVA OF SEA-TTBCHIN'. the developments of these and kindred animals are among the most interesting, because the most startling, of the marvels which modern zoology has revealed to us. The whole process is full of surprising details, to which the change of the caterpillar to a chrysalis, and the chrysalis to a butterfly, presents no parallel, wonderful as those changes of form appear and are. There we have but modifications of outward form, produced by the successive moults or castings of the external skin, and the gradual growth of the animal, which has from the first been present, though veiled. But the construction of the Sea-Urchin is by no means a process of skin-casting, nor has it any recognised parallel in the whole economy of natural history. It is a development perfectly unique. I will endeavour to make you acquainted with the results

Page 47 of 66