Fig. 96. Gromia oviformis.
In the Gromiæ the granular particles in the semi-fluid protoplasm are in constant motion. In the finer filaments there is but one current, and a particle may be seen to be carried to the extremity, and return again bringing back with it any granules that may be advancing; and should particles of food adhere to the filament they take part in the general movement. In the broader filaments two currents carrying particles pass backwards and forwards in opposite directions at the same time, and the network in which these motions are going on is undergoing continual changes in its arrangements. New filaments are put forth sometimes from the midst of the ramifications, while others are retracted; and occasionally a new centre of radiation is formed at a point where several threads meet. The food consists of diatoms and morsels of vegetable matter; but the Gromiæ have no vent, so that the indigestible matter collects in a heap within them. However, as the form of the test is such that the animal cannot increase its size, it leaves it when it becomes too small for its comfort and forms another, and it is supposed to get rid of the effete matter at the same time. The Gromiæ have no nucleus or contractile vesicle.
Class II.—Foraminifera.
The geological importance of the Foraminifera, their intrinsic beauty, the prodigious variety of their forms, their incredible multitude, and the peculiarity of their structure, have given these microscopic organisms the highest place in the class of Rhizopods. The body of these animals consists of a perfectly homogeneous sarcode or semi-fluid protoplasm, showing no tendency whatever to any film or surface-layer. It is inclosed in a shell; and the only evidence of vitality that the creature gives, is a protrusion and retraction of slender threads of its sarcode, through the mouth or pores of the shell, or through both according to its structure. [Fig. 97] shows some of their forms.
By far the greater number of the Foraminifera are compound or many-chambered shells. When young, the shell has but one chamber, generally of a globular form; but as the animal grows, others are successively added by a kind of budding in a definite but different arrangement for each order and genus of the class. When the creature increases in size, a portion of its semi-fluid sarcode projects like a bud from the mouth of its shell. If it be of the one-chambered kind, the bud separates from its parent before the shelly matter which it secretes from its surface consolidates, and a new individual is thus produced. But if the primary shell be of the many-chambered kind, the shelly secretion consolidates over the sarcode projection which thus remains fixed, and the shell has then two chambers, the aperture in the last being the mouth, from which, by a protrusion of sarcode, a third chamber may be added, the new chamber being always placed upon the mouth of its predecessor, a process which may be continued indefinitely, the mouth of the last segment being the mouth of the whole shell.
Fig. 97. Various forms of Foraminifera:—A, Oolina claxata; B, Nodosaria rugosa; C, Nodosaria spinicosta; D Cristellaria compressa; E, Polystomella crispa; F, Dendritina elegans; G, Globigerina bulloïdes; H, Textularia Mayeriana; I, Quinqueloculina Bronniana.
By this process an ovate shell with a mouth at one extremity may have a succession of ovate chambers added to it, each chamber being in continuity with its predecessor so that the whole shell will be straight and rod-like, the last opening being the mouth. If the original shell be globular, and if all the successive gemmæ given out be equal and globular, the shell covering and uniting them will be like a number of beads strung upon a straight wire. Sometimes the successive gemmæ increase in size so that each chamber is larger than the one which precedes it; in this case the compound shell will have a conical form, the primary shell being the apex, and the base the last formed, the aperture of which is the mouth of the whole shell; a great many Foraminifera have this structure. The spiral form is very common and much varied. A series of chambers increasing in size may coil round a longitudinal axis, like the shell of the snail; but if each of the successive chambers, instead of being developed exactly in the axis of its predecessor, should be directed a little to one side, a curved instead of a straight axis would be the result; there is a regular gradation of forms of Foraminifera between these two types. The convolutions are frequently flat and in one plane, but the character of the spiral depends upon the successive enlargement or not of the consecutive chambers; for when they open very wide and increase in breadth, every whorl is larger than that which it surrounds; but more commonly there is so little difference between the segments after the spiral has made two or three turns, that the breadth of each whorl scarcely exceeds that which precedes it.
However varied the forms may be, the mouth of the last shell is the mouth of the whole, either for the time being or finally. For all the chambers are connected by narrow apertures in the partitions between them. Each chamber is occupied by a segment of the gelatinous sarcode body of the animal, and all the segments are connected by sarcode filaments passing through the minute apertures in the partitions between the chambers, so that the whole constitutes one compound creature.