Thus each group has a special life and motion, controlled by a general life and motion; strong individual muscular power controlled by general muscular power; yet no nervous system has as yet been discovered, so this animal activity must for the present be attributed to a strong, inherent, contractile power in the muscular fibre. The Praya is seldom complete, on account of the ease with which it casts off its great bells. None of the Diphyidæ have special organs for respiration; their juices are aërated through their delicate tissues. They are diœcious, and invariably produce perfect male and female medusiform zooids; they are situated among the groups of the polypites and their appendages, and are attached to the axis of the animal. When free, they swim away by the contraction of their bells; the eggs are fertilized, and produce young Diphyidæ, male and female; so these animals, like most of the oceanic Hydrozoa, have two alternate stages of existence. _Physograde Acalephæ._ The Galeolaria lutea (fig. 118, frontispiece) is similar to the Praya diphys in having a slender, tubular body, with groups of sterile polypites and their appendages hanging at intervals along its under-side like a fringe, and also in having two swimming-bells at its anterior extremity; but it has no special small bells. The large ones differ from each other in size, form, and position. The largest is nearly cylindrical, its mouth is turned upwards, and its rim is elevated at one part into two stiff organs like the blinkers that are put over horses’ eyes: besides these, it has six salient points, which nearly close the mouth of the bell at each contraction of the muscular iris that lines the margin of the cavity. The small bell, which goes first in swimming, is thicker and shorter, and its side rises in a hump, upon which the closed end of the large bell rests, and in the cavity between the two the anterior extremity of the filiform body is fixed. Each of the groups of polypites, with their tentacles, lies immediately under its spathe-shaped protecting plate. The polypites are very contractile, and on their protuberant part, containing the digestive cavity, there is a large circular space, which, as well as the whole tissue of the polypite and the stinging capsules at the extremities of the tentacles, are of an orange colour, and are akin in structure to those described.