The Rotifers are male and female, but, like the greater number of Infusoria, the males are only produced at intervals. The female Rotifers have their perfect form when they leave the egg: they even come out of the egg while it is attached to the tail of the mother, as in the Brachionus pala (fig. 136). The males, when hatched, have neither spines nor mouth, yet, during their short lives, their motions are very fleet on account of the vibrations of long cilia round their front. Some Rotifers are remarkably fertile. Professor Ehrenberg estimated that, in the course of twenty-four days, the offspring of a single individual of the genus Hydatina might amount to seventeen millions. Female eggs laid in autumn are collected in heaps and covered with a gelatinous substance, which protects them from the cold in winter, though the Rotifers themselves are sufficiently protected by their great tenacity of life. They revive after being frozen; they may be dried for an unlimited time, but, as soon as they meet with warmth, moisture, and food, they resume their vitality. SECTION VI. ECHINODERMATA. THIS class consists of five orders, all of which are marine. They are, with one exception, creeping animals, and the whole class is remarkable for having most of their members and general structure either in fives or multiples of five. Their skin is hardened by calcareous deposits, sometimes of beautiful microscopic structure: they have a digestive cavity, a vascular fluid system, and some distinct respiratory organs, so that they are comparatively of a high grade. _Echinodermata Asteroïdea._ The Asteroïdea, or Star-Fishes, which are the highest order, form two natural families, the Stelleridæ and Ophiuridæ, which comprise twenty-two genera. The simplest form of the Stelleridæ is the common star-fish, with its flat regularly five-sided disk. A tough membrane, strengthened by reticulated calcareous matter, covers the back, and bends down along the sides, while the under-side of the body or disk, on which the animal creeps, is soft and leathery, with the mouth in its centre. In the other genera, although the body is still a flat, five, equal-sided disk, the angles are extended into long arms, broad whence they diverge from the disk, but decreasing rapidly in width to their extremities, so that the animal is exactly like a star with five long, equal, and flexible rays.