Another genus somewhat similar is Arcella, but it differs in being furnished with a more or less rounded shell (lorica), like a little box. In examining the matters that adhere to the stems of duckweed, and other water-plants, we frequently observe little circular bodies of a yellowish or reddish-brown colour, some much darker than others, but all having a central round spot paler than the rest. On first examination they seem inert and dead, but if we closely watch one, we perceive that it is endowed with the power of motion ; and we directly discern, thrust out from its edge, variable processes, in the form of arms, of clear, perfectly colourless, and most delicate jelly, sometimes pointed, sometimes blunt, which slowly change their form and position. By the aid of these, a feeble and irregular motion is given to the box, which is sometimes turned partly over; when we perceive that its under side is flat, or probably concave, and that its outline is cut into facets. The lorica is somewhat flexible, for the edges at two opposite points are sometimes bent down towards each other, so as to give the creature the form of a crescent. The internal viscera are dimly discernible through the coloured lorica, and resemble those of Amoeba. A dark oval ring is commonly seen at one side, which is probably the outline of the contractile bladder. It may, in fact, be considered as an Amoeba, whose external surface has the power of secreting a symmetrical shell of horny or chitinous substance. The lorica is about T * ^th of an inch in diameter. This species is named Arcella vulgaris.

Laying aside our live-box with its contents for the present, we will have recourse to the tanks of sea-water for one or two other objects of intermediate interest. On the green and brown mossy sea-weed which covers the rocks on the bottom, you see many white specks clinging to the filaments; and there are several adhering to the sides of the tank. These are little living shelled animals of the class Iforaminifera, and these which you see include several species. By bringing your eye, assisted by the lens, to bear upon one of these latter, you perceive that it is a little discoid spiral shell, of very elegant form, marked with curved diverging grooves. This is the pretty little Pobjstomella crispa, a fair sample of its class, and though not more than -g^th of an inch in diameter, it is a giant compared with the Arcella.

There is more, however, than the shell to be seen ; though so filmy and shadowy, that I wonder not at your overlooking it. Extending from two opposite sides of the shell to a distance each way considerably exceeding its diameter, you discern fine threads of clear jelly, running out in long points. The power you employ is not sufficient to enable you to resolve their detail : and for this I will try to secure a specimen for the microscope.

In this other live-box, then, I inclose one of the white specks from the moss-like clothing of the stones. It is, I see, of another species, namely, Pohjmorphina oblonga; but it will answer our purpose equally well.

At present we see only the shell, the removal of the animal having induced it in alarm to withdraw the whole of its softer parts within the protection of its castle. We must have a few minutes' patience.

Now look again. From the sides of the opaque shell we see protruding tiny points of the clear sarcode; these gradually and slowly,—so gradually and slowly that the eye cannot recognise the process of extension—stretch and extend their lines and films of delicate jelly, till at length they have stretched right across the field of view. The extension is principally in two opposite directions, corresponding to the long axis of the shell; though the branched and variously connected films often diverge considerably to either side of these lines, giving to the whole a more or less fan-shaped figure.

These films are as irregular in their forms and sizes as the expansion of the sarcocle of Amoeba, with which they have the closest affinity. Their only peculiarity is their tendency to run out into long ribbons or attenuated threads, which, however, coalesce and unite whenever they come into mutual contact, and thus we see the threads branching and anastomosing with the utmost irregularity, usually with broad triangular films at the points of divergence and union.

There can be no doubt that the object of these lengthened films, which are termedpsevAopodia* is the capture of prey or food of some kind ; perhaps the more sluggish forms of minute animalcules, or the simpler plants. These the films of sarcode probably entangle, surround, and drag into the chambers of the shell, digesting their softer parts in temporary vacuoles, and then casting out the more solid remains, just as the Amoeba does.

Though this beautiful array was so very deliberately put forth, it is, as you perceive, very rapidly withdrawn on any disturbance to the animal, as when we agitate the water by slightly moving or turning the cover of the livebox. Another fact, of which you may convince yourself, by watching manifest, though small, changes of position in the shell, while under observation, is, that it is by means of the adhesion and contraction of the jiseitdopodia, that the animal drags itself along a fixed surface. This

Literally, false feet, from the Greek iptudog (pseudos), a falsehood, and ttovq (pous), genitive 7rodoQ (podos), afoot.

it can effect so assiduously, that I frequently find them in the morning adhering to the tank-sides three or four inches from the bottom, though, on the previous evening, none were visible on the glass. Thus they must crawl, on occasion, from a hundred to a hundred and fifty times their own diameter in a night.

The structure of a Sponge is much the same as that of these animals, with the exception that its solid part or skeleton is not a continuous covering by which the sarcode is invested, but consists of fibres or points or rods of varying form, which are clothed with the sarcode. This loose sort of skeleton may be of horny or chitinous matter, like that of Arcella, or calcareous like that of theForaminiferdt* or it may be siliceous,—that is, composed of flint (silex).

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.

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