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.
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.
On the under-side of each ray of a star-fish, a central groove or furrow extends throughout its whole length, and the semi-calcareous flexible membrane which covers the back and rays not only bends down round the sides of the rays, but borders both edges of the grooves. Upon these edges ridges of small calcareous plates beset with spines are placed transversely: they are larger near the mouth, and gradually decrease in size as they approach the point of the ray.
Interior to the spines, these ridges are pierced by alternate rows of minute holes for the long rows of feet, which diminish in size to the end of the ray. The feet are contractile muscular tubes communicating through the holes with internal muscular sacs, which are regarded as their bases. The sacs are full of a liquid, and when the animal compresses them the liquid is forced through the holes into the tubular feet, and stretches them out; and when the muscular walls of the hollow feet are contracted, the liquid is forced back again into the sacs, and the feet are drawn in. The liquid is furnished by a circle of small vascular tentacles, or sacs, surrounding the mouth, which are both locomotive and prehensile. From these a canal extends through the centre of each ray, which in its course sends off lateral branches to the bases of the feet to supply them with liquid. The whole of this system of vessels and feet are lined with vibratile cilia, which maintain a perpetual circulation in the liquid.
The toothless mouth on the under-side of the disk dilates so as to admit large mollusca with their shells. The short gullet and stomach are everted, protruded through the mouth, and applied round the object to be swallowed, which is then drawn in, digested, and the shell is discharged by the mouth. However, in three orders of this family there is a short intestine and vent. From the large stomach, which occupies the central part of the disk of the star-fish, a couple of tubes extend to the extremity of each ray, where they secrete a substance essential for digestion: the stomach is in fact a radiating organ, partaking the form of the animal it sustains.