In some cases, as in the common Turkey Sponge, the horny skeleton consists of a network of solid but slender fibres, very tough and elastic, which branch and anastomose in every direction, at very short intervals, as you may see by looking at this atom, which 1 cut off from a dressing-sponge.

In the lime and flint Sponges, however, the continuity and cohesion of the skeleton does not depend upon the organic union of the constituent parts, as it does in the loose and open network of the Turkey sponge. For it is made up of an immense multitude of glassy needles, all separate and independent, between themselves, yet so contrived that they do hold together very firmly, and in a great number of cases are arranged on a prescribed plan, so as to give a certain form and outline to the aggregate.

If you have ever shaken up a box of dressing-pins, and have then endeavoured to take one out, you know how by their mere interlacement they adhere together in a mass,

A group of animals with shells, resembling in appearance those of the common nautilus ; and, like them, consisting of several chambers divided from one another by walls which are pierced with numerous minute holes, in Latin called foramina;—whence their name. 2 c so that by taking hold of one you may lift a bristling group of scores. Somewhat on the same principle are the calcareous and siliceous pins (spicula) of a Sponge held together by mutual interlacement. Yet their cohesion is aided by the tenacity of the living sarcode which invests them ; for I have found that specimens of Grantia (calcareous Sponges with needles of three rays), when long macerated in water, so that the sarcode is dissolved, have very slight power of cohesion among their spicula.

To understand the structure of a sponge we will shave a thin sectional slice from this Halichondria suberea. This when alive is of an orange colour ; and is always found closely investing turbinate, or top-shaped, shells which are inhabited by Hermit-crabs. We will macerate the slice in tepid water for a quarter of an hour, and then examine it in the live-box.

The surface is a thin layer of greater density than any other part, and is composed of coloured fleshy granules,—

SECTION OF SPONGE.

omitting for the present, the skeleton. Of the same substance is the whole slice composed, but looser and more open as it recedes from the surface. It is separated by

blank spaces which are larger towards the centre, smaller and more numerous as they approach the exterior.

These openings are sections of so many canals, by which the whole substance of a sponge is permeated. The surface is perforated with minute pores, at which the surrounding w^ater enters on all sides. These presently unite into slender pipes, which, irregularly meandering, are continually uniting into larger and yet larger canals ; of wThich the greater open spaces that you see are the oblique divisions. These have certain outlets, called oscula, on the surface, from which the stream is poured that has thus made the grand tour of the whole interior. Such oscula, as you perceive on the remainder of the Halichondria, are usually raised on slight eminences ; and resemble, especially when in living action, miniature volcanoes, vomiting torrents of water and granules of effete matter, instead of fire and ashes.

During life these granules were much more diffused, and formed a considerable portion of the living flesh, the remainder being composed of a glairy sarcode, almost fluid. The whole was maintained in position by the solid spicula of flint, which you see abundantly in this slice. These take a curious form, exactly that of the pins which w Te use on our dressing-tables ; each consisting of a cylindrical slender rod, pointed at one end, and at the other surmounted by a globular head, the whole formed of glass,—-flint glass literally. You see them bristling all round the edge of the section, being stuck into the surface of the sponge, exactly as pins are loosely stuck into a pin-cushion. The heads and points, too, project into the cavities; more, however, than they did during life, for you must make allowance for the shrinking of the soft parts ; and thus you perceive how the whole structure is permeated by these glassy pins, Avhich seem to be entangled together quite at random without rule or arrangement. And yet there is an 2 c 2 arrangement discernible here ; for the canals are formed by the manner in which these are grouped ; and this is seen much more clearly in the case of the three-rayed needles of lime in the Grantice. Mr. Bowerbank has shown that in G. compressa the substance is divided into very regular chambers in a double series, divided by a diaphragm, whose axis is at right angles to the axis of the sponge ; and that these chambers are denned by walls made up of the three-rayed needles in their mutual interlacement.

Chapter XXI. Infusoria

We will now resume our examination of the drop of pond-water, and the fragments of Myriophyllum, which have been waiting for us in the live-box.

Our attention then shall first be given to some elegant creatures of a brilliant translucent green hue, which are gracefully gliding about. They are of the genus Euglena, so called because each is furnished with a very conspicuous spot of a clear red hue, situated near the head, which Ehrenberg, on account of its resemblance to the lowest forms of eyes in the Rotifera, that are somewhat similar in colour and appearance, pronounced to be an organ of vision. More recent physiologists, however, doubt the correctness of the conclusion.

The animals are of several kinds. The most numerous is an active little thing of about T^ of an inch in length when extended, though from its extreme versatility it is as difficult to assign to it a definite size as a definite shape. It seems to be the E. sanguinea, so called because it is said to occur sometimes of a deep red hue y and in such vast profusion as to give the waters the appearance of blood. I have never seen it, however, other than as it now appears, rich emerald green in the body, with the two extremities perfectly clear and colourless. I might, perhaps, describe its ordinary form as spindle-shaped, with a pointed tail, and a blunt, rounded head ; but it is remarkable for the variableness of its shape. It is capable of assuming an appearance very diverse from what it had half a minute before, so that you would hardly identify it, if you were not watching its evolutions. Whether this ability to prove an alias be at all dependent on the remarkable clear-headedness of the subject, I leave for those who are skilled in metaphysics to determine. Away they go, tumblingover and over, revolving on the long axis as they proceed, which they do not very rapidly, with the blunt extremity forward.

Here is another kind, a little larger than the former, but much more slender; yet from the slowness and steadiness of its movement more easy of observation. It is named E. acus, or "the Needle Euglena." This is an animalcule of great elegance and brilliancy ; its sparkling green hue, with colourless extremities, and its rich pale crimson eye, are very beautiful. It commonly swims extended, with a slow gliding motion, turning round on its long axis as it proceeds, as may be distinctly seen by the rotation of certain clear oblong substances in its body. These, then, are seen not in the interior, but near the surface, as they would appear if imbedded in the flesh around a hollow centre. The interior is probably not hollow, but occupied with pellucid sarcode. These were assumed by Ehrenberg, but on no adequate grounds, to be organs connected with reproduction. They vary in number in different individuals, and those which contain the greatest number are thereby more swollen. They appear to be separated into two series, one anterior, the other posterior. The animal is capable of bending its head and body in various directions, but is most beautiful when straight. The front is furnished with a slender thread-like proboscis. This species affords us a good opportunity of observing the red spot, which, for convenience sake, we may still term an eye. It seems to be an irregular oblong vacuole, or excavation in the sarcode, tilled with a clear ruby-red fluid. The red spot in the Rotifera is connected with a well-defined crystalline lens, whose definite form and high refractive power may in many cases be distinctly marked ; but here nothing of the kind is seen ; the spot itself has no certain shape, and does not appear to be bounded by a proper wall. Some forms, which are by general consent admitted to be plants, have similar spots ; and hence it has been, rather too hastily, I venture to think, concluded that they have no connexion with vision. I think it still possible that a sensibility to the difference between light and darkness may be the function of the organ.

I have found that this animal, when allowed to dry on a plate of glass, retains its form and colour perfectly ; but in about two days the eye-spot, which at first becomes much larger in the drying, gradually loses all traces of its brilliant colour, probably by the evaporation of the contained fluid.

Another pretty species you see gliding along among the rest, called E. triqrietra, or the Three-sided. It bears a resemblance to a broad rounded leaf, with the footstalk forming a short transparent point, and the mid-rib elevated into a sharp ridge. The under side seams slightly concave. This is equally attractive with the others. It is persistent in form, and appears not to be even flexible. Its motion is slow, and as it goes it rolls irregularly over and over in all directions, THBEE-SIDED EVGLENA. not revolving on its long axis, and thus giving you very satisfactory views, though only momentary, of the keel with which the back is furnished. It is in the turnings of such minute creatures that the microscopist often gets a glimpse of peculiarities of form, which a view of the animal when in repose, however long continued, fails to reveal. Longitudinal interrupted lines

are seen running down the body of this pretty leaf, which do not appear to mark irregularities of the surface, and therefore are probably internal. Ehrenberg calls these and similar collections of granules " ova," or eggs ; but this is to cut the knot instead of untying it. There is no sufficient reason to believe that these animals increase by ova.

About the front of all these Euglence, you may discern now and then a slight nickering or quivering in the water. The power we are using, though best for the general display of the form, is insufficient to resolve this appearance : I will put on a higher objective. You now see that there proceeds from the frontal part of the body a long and very slender filament, which is whisked about in the manner of a whip-lash. This is considered to be the organ of locomotion ; but I rather doubt that such is the function ; the smooth and even gliding, often rotating, action of the creature, seems more like that produced by minute and generally-distributed cilia, than that caused by the lashings of a single long thread.

Yet two more species of this extensive genus we discern in this well-stocked drop of water. They have received the appellations of the Pear (E. pyrum) and the Sloth (E. deses). The former is the most minute we have yet seen and seems to be scarce ; but it is highly curious and interesting in appearance. It much resembles in outline a fish of the genus Balistes ; the muzzle being somewhat protruded and truncate, and the form rhomboidal ; it terminates in a slender pointed tail. The body is obliquely fluted, which gives a very singular effect ; for, from the transparency of the tissues, the lines of the opposite side can be discerned crossing those next the eye, and dividing the animal into lozenge-shaped areas. The colour is sparkling green, but the tail and the edges of the body are clear and colourless, and there is a bright red eye. At other times this Euglena takes the form of a claret-bottle or an oil-flask ; the muzzle being broadly truncate or even indented.

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