little spherical masses of sarcode which may be derived from the nucleus by fission, or may be produced by a segmentation of the endosarc, the animal having previously become torpid, and the nucleus and contractile vesicle having disappeared. These little masses, however produced, develop themselves when liberated into ordinary Amoebae. This last method of reproduction is obviously very closely analogous to the production of pseudonavicella' in an encysted Gregarina.

The remaining members of the Amabea are constructed more or less closely after the type of the Amoeba itself. In the nearly allied Difflugia the sarcode forming the body of the animal is invested with a membranous envelope or 'carapace,' strengthened by grains of sand and other adventitious solid particles, and having a single aperture at one extremity, through which the pseudopodia are protruded (fig. 2b). In Arcella there is a discoid or bason-shaped carapace, secreted by the animal itself, and likewise possessing but a single pseudopodial aperture, placed in this case on the flat surface of the body.

In Pamphagus there is no carapace, but the pseudopodia are nevertheless protruśible from one extremity only of the body, the remainder of the surface appearing to be of too resistant a consistence to allow of this. The common sunanimalcule (Actinophrys sol) is another well-known Rhizopod which is usually placed in this order (fig. 3). It consists of a spherical mass of sarcode, about 1-1,300 of an inch in diameter, and usually covered with long, radiating, filamentous pseudopodia, which are

much less mobile than in the case of the Amoeba. The division of the substance of the body into ectosarc and endosarc is tolerably evident, and the latter contains numerous granules and vacuoles. The

a Diatom.

them, and they exhibit a circulation of granules along their edges, though this is not nearly so marked a feature as in the case of the Foraminifera. A nucleus and contractile vesicle are also present.

The Amabea may be divided into two sub-orders: 1. Amobina, including those forms which have the body naked; and 2. Arcellina, comprising those in which the body is protected by a carapace.

CHAPTER III.

FORAMINIFERA.

THE Foraminifera may be defined as Rhizopoda in which the body is protected by a shell, or 'test,' usually composed of carbonate of lime; there is no distinct separation of the sarcode of the body into ectosarc and endosarc, and the nucleus and contractile vesicle are both absent. The pseudopodia are long and filamentous and interlace with one another to form a net-work.

The Foraminifera are specially characterised by the possession of a 'test' or external shell, which is usually composed of carbonate of lime, but is sometimes membranous. (If Lieberkühnia is to be regarded as a Foraminifer, the possession of a test cannot be looked upon as essential, since this animalcule is naked.) The test is usually composed of an aggregation of chambers or 'loculi,' and its walls are usually pierced by numerous pores or 'foramina' through which the pseudopodia are protruded; the place of these being in some forms supplied by the large size of the terminal, or 'oral,' aperture of the shell.

As regards the soft parts of the Foraminifera, the body is composed of extensile and contractile sarcode-usually reddish or yellowish in colour-which not only fills the interior of the shell, but generally invests its outer surface also with a thin film, from which the pseudopodia are emitted. The sarcode is not differentiated into a distinct ectosarc and endosarc, and is devoid of a nucleus and contractile vesicle, and, indeed, of any organs or specialised parts of any kind. From this uniformity in its composition there seems some reason to conclude that the Foraminifera-in spite of the complexity and mathematical regularity of many of their shells-should be looked upon as the lowest forms of the Rhizopoda, or even of the Protozoa.

The pseudopodia in all the Foraminifera are filamentous and protrusible to a great length, and they possess the singular property of uniting together in various directions so as to form a kind of net-work, like an 'animated spider'sweb.' (Hence the name Reticularia applied to the order by Dr. Carpenter.) This property, however, is not peculiar to members of this order, but is seen also in Actinophrys and in the Thalassicollida, though to a less extent. Further, throughout the entire net-work formed by the inosculating pseudo

podia there is a constant circulation of granules in different directions. This singular phenomenon is in many respects analogous to the circulation of granules which is seen in many vegetable cells, and it is believed by Dr. Carpenter that 'the conditions of the two sets of phenomena are essentially the same.'

The shells of Foraminifera may be classed in three divisions, termed respectively the 'porcellanous,' the 'hyaline' or 'vitreous,' and the arenaceous.' The porcellanous shell is quite homogeneous in its composition, is opaque-white when seen by reflected light, and is not perforated by pseudopodial foramina. In these forms (e.g. Miliola) the pseudopodia are emitted solely from the mouth of the last-formed segment of the shell. The vitreous shell is transparent and glassy in texture, and its walls are perforated by numerous pseudopodial apertures. The arenaceous shell is properly speaking not a true 'test,' since it is simply composed of particles of sand united together by some unknown cement. Its walls may or may not be traversed by pseudopodial foramina.

As regards the form of the shell, the Foraminifera may be conveniently, though arbitrarily, divided into two sections: the Monothalamia and the Polythalamia. In the first of these sections (fig. 4), comprising the so-called 'simple' or 'unilocular' Foraminifera, the shell consists of a single chamber, and the animal is, in fact, nothing more than a little mass of sarcode enveloped in a calcareous covering. Lagena, with its beautiful flask-shaped shell, may be taken as the type of this division. In the Polythalamia, or multilocular' Foraminifera,

Fig. 4.-Monothalamous Foraminifera. a. Lagena sulcata. b. Adult form of the same. c. Lagena malo.

the shell is composed of many chambers separated from one another by divisional walls or 'septa' (fig. 5), each of which is perforated by one or more openings, 'septal apertures,' by means of which the sarcode occupying the different chambers is united into a continuous and organic whole, the connecting bands being called 'stolons.' Complex as their structure often is, the compound Foraminifera are, nevertheless, formed by a process of continuous gemmation or budding from a single primordial segment' in every respect identical with

the permanent condition of a simple species. They commence their existence, therefore, as Monothalamia, and are converted into Polythalamia merely by a process of vegetative' or 'irrelative repetition.' As their development proceeds, the primitive

Fig. 5.-Polythalamous Foraminifera. a. Nodosaria rugosa. b. N. longisulcata. c. N. spinicosta. d. N. hispida.

mass of sarcode, or 'primordial segment,' throws out fresh segments in the form of buds according to a determinate law; and it is upon the direction in which these segments are evolved that the ultimate form of the shell depends. The more important variations in this respect are as follows:-If the additional segments are added to the primordial chamber in a linear series, so as to form a straight or slightly curved line, we obtain respectively a Nodosaria (fig. 5) or a Dentalina. When the new chambers are added in a spiral direction, each being a little larger than the one which preceded it, and the coils of the spiral lying in one plane, then we get the nautiloid' shell, so, common amongst the Foraminifera (fig. 6 a).

Fig. 6.-a. Robulina echinata, a 'nautiloid Foraminifer.' b. Globigerina bulloides.

This type of shell is so closely similar to the shape of the pearly nautilus, that the older naturalists were long in the habit of classing these forms along with the Cephalopoda, or Cuttle

fish order. In the true nautiloid shell the convolutions of the spiral lie in a single plane, as in Rotalina, and the shell is said to be 'equilateral.' In other cases, however, the spiral passes obliquely round a central axis, and the shell becomes conical or turreted, when it is said to be inequilateral' or 'trochoid,' In other forms, such as Nummulites and Orbitolites, the structure of the shell, though regular, is much more complicated. Besides these symmetrical forms, there exist others in which the arrangement of the segments is very irregular, as is seen in Globigerina, Acervulina, &c. (fig. 6 b).

Besides the true pseudopodial foramina with which the walls of the test in most of the Foraminifera are pierced, there exists in some forms an additional system of complicated branching and anastomosing tubes, which are distributed between the laminæ of the shell, and establish a communication between its external and internal surfaces.

CLASSIFICATION OF FORAMINIFERA. The classification of the Foraminifera has hitherto proved a matter of extreme difficulty, and probably none of the arrangements as yet proposed can be considered as more than provisional. The following is the classification adopted by Dr, Carpenter, who is one of the greatest living authorities upon the group.

ORDER RETICULOSA. (FORAMINIFERA.)-Rhizopods showing no differentiation, or a very imperfect one, into ectosarc and endosarc; no nucleus or contractile vesicle; pseudopodia filamentous, minutely subdivided, and inosculating freely to form a net-work,

Section 1, Imperforata.-Envelope membranous or calca, reous, the walls not perforated by apertures for the pseudopodia, which are emitted solely from the single or multiple aperture of the shell,

Families, 1. Gromida. Test membranous.

2. Miliolida, Test porcellanous,

3. Lituolida. Test arenaceous.

Section 2. Perforata,-Envelope calcareous (hyaline or vitreous) or rarely arenaceous, its walls traversed by numerous foramina for the emission of pseudopodia.

The following classifications by D'Orbigny and Schultze are founded merely upon the form of the shell, and, as such, are purely arbitrary. Of the two Schultze's arrangement is probably the more satisfactory.

VOL. I.