§ 249. In the Platyhelminthes good examples of the connexions between forms and forces occur. The Planaria exemplifies the single bilateral symmetry which, even in very inferior forms, accompanies the habit of moving in one direction over a solid surface. Humbly organized as are these creatures and their allies the Nemertidæ, we see in them, just as clearly as in the highest animals, that where the movements subject the body to different forces at its two ends, different forces on its under and upper surfaces, and like forces along its two sides, there arises a corresponding form, unlike at its extremities, unlike above and below, but having its two sides alike.
The Echinodermata furnish us with instructive illustrations—instructive because among types that are nearly allied, we meet with wide deviations of form answering to marked contrasts in the relations to the environment. The facts fall into four groups. The Crinoidea, once so abundant and now so rare, present a radial symmetry answering to an incidence of forces that are equal on all sides. In the general attitudes of their parts towards surrounding actions, they are like uniaxial plants or like polypes; and show, as those do, marked differences between the attached ends and the free ends, along with even distributions of parts all round their axes. In the Ophiuridea, and in the Star-fishes, we have radial symmetry co-existing with very different habits; but habits which nevertheless account for the maintenance of the form. Holding on to rocks and weeds by its simple or branched arms, or by the suckers borne on the under surface of its rays, one of these creatures moves about not always with one side foremost, but with any side foremost. Consequently, averaging its movements, its arms or rays are equally affected, and therefore remain the same on all sides. On watching the ways of the common Sea-urchin, we are similarly furnished with an explanation of its spherical, or rather its spheroidal, figure. Here the habit is not to move over any one approximately-flat surface; but the habit is to hold on by several surfaces on different sides at the same time. Frequenting crevices and the interstices among stones and weeds, the Sea-urchin protrudes the suckers arranged in meridional bands over its shell, laying hold of objects now on this side and now on that, now above and now below: the result being that it does not move in all directions over one plane but in all directions through space. Hence the approach in general form towards spherical symmetry—an approach which is, however, restrained by the relations of the parts to the mouth and vent: the conditions not being exactly the same at the two poles as at other parts of the surface. Still more significant is that deviation from this shape which occurs among such of the Echinidea as have habitats of a different kind, and consequently, different habits. The genera Echinocyamus, Spatangus, Brissus, and Amphidotus, diverge markedly towards a bilateral structure. These creatures are found not on rocky shores but on flat sea-bottoms, and some of them only on bottoms of sand or mud. Here, there is none of that distribution of surfaces on all sides which makes the spheroidal form congruous with the conditions. Having to move about over an approximately-horizontal plane, any deviation of structure arising accidentally which leads to one side being kept always foremost, will be an advantage: greater fitness to function becoming possible in proportion as function becomes fixed. Survival of the fittest will therefore tend to establish, under such conditions, a form that keeps the same part in advance—a form in which, consequently, the original radial symmetry diverges more and more towards bilateral symmetry.
§ 250. Very definite and comparatively uniform, are the relations between shapes and circumstances among the Annulosa: including under that title the Annelida and the Arthropoda. The agreements and the disagreements are equally instructive.
At one time or other of its life, if not throughout its life, every annulose animal is locomotive; and its temporary or permanent locomotion, being carried on with one end habitually foremost and one surface habitually uppermost, it fulfils those conditions under which bilateral symmetry arises. Accordingly, bilateral symmetry is traceable throughout the whole of this sub-kingdom. Traceable, we must say, because, though it is extremely conspicuous in the immense majority of annulose types, it is to a considerable extent obscured where obscuration is to be expected. The embryos of the Tubicolæ, after swimming about a while, settle down and build themselves tubes, from which they protrude their heads; and in them, or in some of them, the bilateral symmetry is disguised by the development of head-appendages in an all-sided manner. The tentacles of Terebella are distributed much in the same way as those of a polype. The breathing organs in Sabella unispira, Fig. [260], do not correspond on opposite sides of a median plane. Even here, however, the body retains its primitive bilateralness; and it is further to be remarked that this loss of bilateralness in the external appendages, does not occur where the relations to external conditions continue bilateral: witness the Serpula, Fig. [261], which has its respiratory tufts arranged in a two-sided way, under the two-sided conditions involved by the habitual position of its tube.
Figs. 260–261.
The community of symmetry among the higher Annulosa, has an unobserved significance. That Flies, Beetles, Lobsters, Centipedes, Spiders, Mites, have in common the characters, that the end which moves in advance differs from the hinder end, that the upper surface differs from the under surface, and that the two sides are alike, is a truth received as a matter of course. After all that has been said above, however, it will be seen to have a meaning not to be overlooked; since it supplies a million-fold illustration of the laws which have been set forth. It is needless to give diagrams. Every reader can call to mind the unity indicated.
Figs. 263–270.
While, however, annulose animals repeat so uniformly these traits of structure, there are certain other traits in which they are variously contrasted; and their contrasts have to be here noted, as serving further to build up the general argument. In them we see the stages through which bilateral symmetry becomes gradually more marked, as the conditions it responds to become more decided. A common Earth-worm may be instanced as a member of this sub-kingdom that is among the least-conspicuously bilateral. Though internally its parts have a two-sided arrangement; and though the positions of its orifices give it an external two-sidedness, at the same time that they establish a difference between the two ends; yet its two-sidedness is not strongly-marked. The form deviates but little from what we have distinguished as triple bilateral symmetry: if the creature is cut across the middle, the head and tail ends are very much alike; if cut in two along its axis by a horizontal plane, the under and upper halves are very much alike, externally if not internally; and if cut in two along its axis by a vertical plane, the two sides are quite alike. Figs. [263 and 264] will make this clear. Such creatures as the Julus and the Centipede, may be taken as showing a transition to double bilateral symmetry. Besides being divisible into exactly similar halves by a vertical plane passing through its axis, one of these animals may be bisected transversely into parts that differ only slightly; but if cut in two by a horizontal plane passing through its axis, the under and upper halves are decidedly unlike. Figs. [265, 266], exhibit these traits. Among the isopodous crustaceans, the departure from these low types of symmetry is more marked. As shown in Figs.[ 267] and 268, the contrast between the upper and under parts is greater, and the head and tail ends differ more obviously. In all the higher Arthropoda, the unlikeness between the front half and the hind half has become conspicuous. There is in them single bilateral symmetry of so pronounced a kind, that no other resemblance is suggested than that between the two sides. By Figs. [269 and 270], representing a decapodous crustacean divided longitudinally and transversely, this truth is made manifest. On calling to mind the habits of the creatures here drawn and described, it will be seen that they explain these forms. The incidence of forces is the same all around the Earth-worm as it burrows through the compact ground. The Centipede, creeping amid loose soil or débris or beneath stones, insinuates itself between solid surfaces—the interstices being mostly greater in one dimension than in others. And all the higher Annulosa, moving about as they do over exposed objects, have their dorsal and ventral parts as dissimilarly acted upon as are their two ends.