A radial type of structure is often found in fixed forms, and in some floating forms, like the jellyfish. In a fixed form, a sea-anemone, for instance, the conditions around the free end and the fixed end of the body are entirely different, and we find that these two ends are also different. The free end contains the special sense-organs, the mouth, tentacles, etc.; while the fixed end contains the organ for attachment. It is evident that the free end is exposed to the same conditions in all directions, and it may seem probable that this will account for the radial symmetry of the anemone. There are also a few free forms, the sea-urchin for instance, that have a radial symmetry. Whether their ancestors were fixed forms, for which there is some evidence, we do not know definitely; but, even if this is true, it does not affect the main point, namely, that, although at present free to move, the sea-urchin is radially symmetrical. But when we examine its method of locomotion, we find that it moves indifferently in any direction over a solid surface; that is, it keeps its oral face against a solid object, and moves over the surface in any direction. Under these circumstances the same external conditions will act equally upon all sides of the body. In contrast to these common sea-urchins, there are two other related groups, in which, although traces of a well-marked radial symmetry are found, the external form has been so changed that a secondary bilateral form has been superimposed on it. These are the groups of the clypeasters and the spatangoids, and it is generally supposed that their forefathers were radially symmetrical forms like the ordinary forms of sea-urchins. These bilateral forms move in the direction of their plane of symmetry, but we have no means of knowing whether they first became bilateral and, in consequence, now move in the direction of the median plane, or whether they acquired the habit of moving in one direction, and in consequence acquired a bilateral symmetry. It seems more probable that the form changed first, for otherwise it is difficult to see why a change of movement in one direction should ever have taken place.