Living creatures which are capable of moving or being freely moved about, present us with similar but more marked differences.
Certain aquatic creatures drift passively about (borne by streams or currents) with no permanent relation between any fixed portion of their bodies and the medium which transports them. Such creatures being equally acted on on all sides by surrounding agencies might be expected (like the subterranean truffle) to exhibit a spheroidal figure, with only one kind of surface upon their whole exterior. This is just what we find to be the case in a variety of more or less minute organisms, such, e.g. as Myxastrum radians and Magosphæra planula.[65]
The former of these consists, at one stage of its existence, of a small globular mass of protoplasm, from the whole periphery of which a multitude of fine pseudopodia radiate. When about to reproduce, the creature retracts its pseudopodia, and forms around its exterior a structureless coat or cyst, an action which takes place frequently in lowly organisms, and is called their process of encystment. The contents of the cyst then divides into separate bodies, which escape by the rupture of the cyst. Each of these bodies is enclosed in a silicious case with an aperture at one end, whence its contained protoplasm issues, and, having so issued, assumes a spherical shape.
Magosphæra is another small creature which goes through a remarkable series of changes, the greater number of which exemplify the ball-like shape of body alike on all sides.
Wherever the surface of the body is covered by pseudopodia, those processes, inasmuch as they have a power of spontaneous movement, enable the creatures possessing them slightly to aid or to resist the drifting action of the water in which they float.
But a living organism may be devoid of any definite shape whatever, as in Protamœba,[66] which consists of a mere particle of protoplasm, from which irregular-shaped processes of unequal size are irregularly protruded in every direction, so that the form of the creature may be said to be quite indeterminate.
The bodies of almost all organisms have, however, more or less definite forms, which may be all classed under seven morphological categories.
(1). The simplest form of all exemplifies spherical symmetry, and is that which we have seen in the truffle, the radiolarian, the volvox, Myxastrum and Magosphæra. In this spherical form any number of axes drawn through the creature in any direction are equal.
(2). The next organic form is one in which the body sphere is more or less elongated at its poles, the latter being equal and similar. In such an organism we have one axis longer than any one of the others and central, while from this axis symmetrical radii can be drawn in all directions. This form may be said to exemplify equipolar symmetry, and such is found in some radiolarians, in some small parasites (Gregarinida),[67] and others.
(3). The next morphological category may be spoken of as unipolar symmetry. Bodies which exemplify it are like those included in the last category, save that the two poles of the body are not alike.