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.
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.
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.
The backs of all the star-fishes are covered with most minute movable spines, and with microscopic organs like minute pincers, called pedicellariæ, which are diffused generally over the surface, and form dense groups round the spines. They have a slender, contractile, calcareous stem, and a head formed of two blades, which they continually open and shut, the whole being coated with a soft external tissue. They grasp anything very firmly, and are supposed to be used to free the star-fish from parasites. In some species of Goniaster the pedicellariæ resemble the vane of an arrow, and are so numerous as to give a villous appearance to the skin of the back.
