A pulsatory vessel near the gullet propels the yellow blood into a system of fine tubes, that are spread over the walls of the stomach and its rays. Through these walls the blood receives a nutritious liquid, which it carries with it into a network of capillary vessels, widely extended throughout the body, being propelled by the contractile powers of the vessels themselves, and after having supplied the tissues with nourishment, it is carried by tubes to the point from whence it started, to begin a new course. The capillary network passes immediately under a portion of the skin of the star-fish, through which an exchange of the respiratory gases takes place. Besides, the star-fishes breathe the sea-water through numerous conical tubes, that project in patches from the back. Through these tubes, which can be opened and shut, the water is readily admitted into the cavity containing the digestive organs, with which they are in communication. The star-fish slowly distends itself with water, and then gives out a portion of it, but at no regular time. The cavity is never empty of water, and as its lining is densely bristled with cilia, their vibrations keep the vascular surface of the digestive organs perpetually bathed with the respiratory medium.
The star-fishes have a radiating system of nerves suited to their form. A ring of slender nerve-cords surrounds the mouth, from whence three nerves are sent off at the commencement of each ray: two of these, which are filaments, go to the organs in the central disk, while the middle one, which is a great trunk, passes through the centre of the rays, and terminates in a nerve-centre, or ganglion, placed under a coloured eye-speck at their extremity. The structure of the rays, the eye-specks, and the nerve-centres below them, are so similar, that they are merely repetitions of one another; hence no nerve-centre can control the others, but they are all connected by the ring encircling the mouth, which is a common bond of communication. How far the movements of these animals indicate sensation we have not the power to determine, but they feel acutely, for the mouth, the feet, and especially the pedicellariæ, are highly sensitive, and shrink on the least touch. The eye-specks are probably sensitive to light, and as the star-fishes often feed on putrid matter, they are supposed to be endowed with the sense of smell.
The family of the Ophiuridæ, or Snake Stars, are widely distributed in the ocean. The genus Euryales with branching rays, and that of Ophiura with simple rays, comprising the Brittle and Sand Stars, are abundant in the British seas. In the sand stars there are cavities full of sand at the points from whence the rays diverge, which appear like warts on the surface of the disk. Their rays are exceedingly long, thin, and flexible; they have no central groove nor feet, but they are employed as organs of locomotion and prehension, for by their alternate strokes the sand stars can elevate or depress themselves in the water, creep on the bottom, and by twisting them round objects they can fix themselves, firmly aided by spines or bristles on their edges. The Ophionyx has the addition of movable hooks beneath bristled spines. The rays are bent by the contraction of internal muscles, and extended again by the elasticity of the external leathery coat. The Ophiuridæ, like the Luidia fragilissima belonging to the preceding order, cast off a ray if touched, and even all the five if rudely handled; but they can replace them with as much ease. If only a fragment of a disk remains attached to a ray the whole animal may be reproduced.
The Ophiuridæ have an internal calcareous skeleton or framework, in the form of spicules, scattered in their tissues. They have a capacious mouth with tentacles and ten small chisel-shaped teeth, five on each side, which meet and close the mouth. The mouth is separated from the stomach by a circular muscle that opens and shuts the passage, but no canal diverges from the stomach through the rays. The nervous system and the circulation of the blood are similar to those in the Stelleridæ; and respiratory organs, in the form of from two to four plates, or lamellæ, project from each of the spaces between the bases of the rays into the central cavity, by which sea-water has free access to bathe the digestive organs and aërate the blood.
The colour of the star-fishes, as well as of other marine invertebrate animals, seems to be independent of light. The Ophiuridæ that had been living at a depth of 1,260 fathoms in the North Atlantic were coloured, though not a ray of light could reach their dark home, and those dredged up from 100 to 300 fathoms on the coast of Norway were of brilliant hues—red, vermilion, white, and yellow. In general, both plants and animals of the lower kinds become of a sickly white when kept in darkness.
The Stelleridæ are male and female, and form fertilized eggs of an orange or red colour. These eggs are first converted into a mass of cells and then into larvæ, not radiating symmetrically like their parents, but of a bilateral form, the two sides being perfectly alike and bordered by a ciliated fringe nearly throughout their whole length. These two fringes are united by a superior and inferior transverse ciliated band, and between the two the mouth is placed. A stomach, intestine, and vent are formed; the creatures can provide for themselves, and swim about as independent zooids. A young star-fish is gradually developed by a succession of internal growths, part of the original zooid is retained, and the rest is either thrown off or absorbed; then the star-fishes having lost the power of swimming, crawl slowly away and acquire their full size. There is great diversity in the external form of the zooids of the different genera, as well as in the portion of them retained in the adult star-fish.
Fossil star-fishes have a very wide range. They are found among the earliest Silurian organic forms, but they scarcely bear any resemblance to existing genera. The Ophiuridæ, fished up from the bottom of the North Atlantic, come nearest to them. Five genera are found in the Oolitic formation, all extinct; three genera range from the Lias to the present seas; and five genera belonging to the Cretaceous period are represented by living species.
Echinodermata Crinoïdea.
The Crinoid Echinoderms, or Stone-Lilies, are like a tulip or lily on an upright stem, which is firmly fixed to a substance at the bottom of the sea. During the Jurassic period, miniature forests of these beautiful animals flourished on the surface of the Oolite strata, then under the ocean. Myriads of their fossil remains are entombed in the seas, and extensive strata of marble are chiefly composed of them. Their hollow joints are known in several parts of England as wheel stones, and as St. Cuthbert’s beads on the Northumbrian coast, in honour of the patron saint of Holy Island, where they abound. The Crinoïdea are of two kinds: the Encrinites, which chiefly flourished in the Palæozoic period and are now represented by a minute species (Rhizocrinus Lofotensis) lately discovered on the coast of Norway by Professor Sars, have a smooth, cylindrical, jointed stem; and the Pentacrinites, which began at the Lias, and have a five-sided jointed stem, the present representative of which is the Pentacrinus caput-Medusæ, found in the West Indian seas.
The hollow, five-sided, calcareous, jointed stem of the living Pentacrinite is filled with a spongy substance, and supports a cup on its summit, containing the digestive organs, mouth, and tentacles of the animal. The cup is formed of a series of calcareous plates, and from its margin five long many-jointed rays diverge, each of which is divided into two-jointed branches. Groups of curled filaments, called cirri, are placed at regular distances from the bottom of the stem to the extremity of the rays, while, on the opposite side of the rays, there are groups of feathery objects called pinnæ at each joint. Food is caught by the tentacles and digested by the stomach and viscera at the bottom of the cup, from whence vessels diverge through a system of canals in the axes of the rays, pinnæ, and down the stem, all of which convey sea-water mixed with nutritious liquid, for the nourishment of the animal.