This order comprises the ordinary Star-fishes, and is defined by the following characters:
Fig. 97. - The common Star-fish (Uraster rubens), natural size, viewed from above.
The body (fig. 97) is star-shaped or pentagonal, and consists of a central body or "disc" surrounded by five or more lobes or "arms" which radiate from the body, are hollow, and contain prolongations of the viscera. The body is not enclosed in an immovable box, as in the Echinoidea, but the integument ("peri-some") is coriaceous, and is strengthened by irregular calcareous plates, or studded by calcareous spines. No dental apparatus is present. The mouth is inferior, and central in position ; the anus either absent or dorsal. The ambulacral tube-feet are protruded from grooves on the under surface of the rays. The larva is vermiform, and has no pseudembryonic skeleton.
The skeleton of the Asteroidea is composed of a vast number of small calcareous plates, or ossicula, united together by the coriaceous perisome, so as to form a species of chain-armour. Besides these, the integument is abundantly supplied with spines, tubercles, and "pedicellariae." Lastly, the radiating ambulacral vessels run underneath a species of internal skeleton, occupying the axis of each arm, and composed of a great number of bilateral "vertebral ossicles" or calcareous plates, which are movably articulated to one another, and are provided with special muscles by which they can be brought together or drawn apart. The upper surface of a star-fish corresponds to the combined inter-ambulacral areas of an Echinus, and exhibits the aperture of the anus (when present), and the "madreporiform tubercle," which is situated near the angle between two rays. The inferior or ventral surface corresponds to the ambulacral areas of an Echinus, and exhibits the mouth and ambulacral grooves.
The mouth is central in position, and is not provided with teeth; it leads, by a short gullet, into a large stomach, from which a pair of sacculated diverticula are prolonged into each ray. A distinct intestine and anus may, or may not, be present; but the anus is sometimes wanting (in the genera, Astro-pecten, Ctenodiscus, and Luidia).
The ambulacral system is essentially the same as in the Echinoidea, and is connected with the exterior by means of the "madreporiform tubercle," or " nucleus," two, three, or more of these being occasionally present. The conical or cylindrical ambulacral tube-feet are arranged in two or four rows, along grooves in the under surface of the arms (fig. 98). Each ambulacral groove is continued along the lower surface of one of the arms, tapering gradually towards the extremity of the latter. The floor of each groove is constituted by a double row of minute calcareous pieces - the "ambulacral ossicles" - which are movably articulated to one another at their inner ends. At the bottom of each groove is lodged one of the radiating canals of the water-vascular system or ambulacral system, from which are given off the rows of suctorial feet, or " tube-feet."
It follows from this that the radiating vessels of the ambulacral system are outside the chain of ambulacral ossicles, so that these latter are to be regarded as an internal skeleton, and they do not correspond with any part of the skeleton of Echinoids* - at least they do not correspond with the perforated ambulacral plates of the Sea-urchins. The ambulacral ossicles, however, of the Star-fishes are of such a form that by their apposition an aperture or pore is formed between each pair. By means of these pores (fig. 98, a) the tube-feet communicate with a series of little bladders or "ampullae," placed above the chain of ossicles. These perforations, however, do not correspond with the perforated plates of the Echinoid test, and the tube-feet of the Star-fishes pass through no "poriferous " plates on their way to the exterior.
This may be rendered more intelligible by examining a section of the arm of a Star-fish from which the soft parts have been removed (fig. 99). In such a section the ambulacral ossicles (a a) are seen in the centre of the lower surface, united along the middle line by their inner extremities. They are so placed as to form a kind of elongated pent-house, and immediately beneath the line where the ossicles of one side are articulated with those of the other side is placed the ambulacral vessel (b). Superficial to this, again, is a nerve-cord ; so that the whole chain of ambulacral ossicles is placed in the midst of the soft parts of the animal, and is thus clearly an internal skeleton. At their outer extremities the ambulacral ossicles are articulated by the intervention of the "adambulacral plates" (fig. 98, b), with plates belonging to the external or integumentary skeleton. As before said, the shape of the ambulacral ossicles is such that a pore is formed by the apposition of each pair ; and by these apertures each tube-foot communicates with a vesicle placed internal to the chain of ossicles. It will be seen, however, that the tube-feet (indicated by the dotted lines in the figure) do not pass through these apertures, or through any other pores of the skeleton, on their way to the surface. The "poriferous zones" of the Sea-urchins are part of the external skeleton, and are not represented in the Star-fishes. On the other hand, the integumentary skeleton in the Star-fishes is absent along the ambulacral areas, or along the areas occupied by the ambulacral grooves.
Fig. 98. - Diagram of a Star-fish (Goniaster), showing the under surface, with the mouth and ambulacral grooves. a Ambulacral ossicles, with the ambulacral pores between them; b Adambulacral plates, bounding the ambulacral grooves; m Marginal plates, (wanting in many species); o Oral plates, placed at the angles of the mouth.
The circulatory system of the Asteroids is represented by a group of vessels communicating ventrally with an oral ring and dorsally with an anal plexus, from which branches are distributed to the genital glands. There are no distinct respiratory organs, but the surfaces of the viscera are abundantly supplied with cilia, and doubtless subserve respiration; the sea-water being freely admitted into the general body-cavity by means of numerous contractile ciliated tubes, which project from the dorsal surface of the body.
* The structures in the Echinus, which are truly homologous with the ambulacral ossicles of the Asteroidea and Ophiuroidea, are the so-called "auriculae."
Fig. 99. - Section of the ray of Uraster rubens. a a Ambulacral ossicles; b Position of the ambulacral vessel; c c Plates of the external skeleton; n Nerve-cord. The dotted lines show the tube-feet proceeding from the ambulacral vessel.
The nervous system consists of a gangliated cord, surrounding the mouth and sending filaments to each of the rays. At the extremity of each ray is a pigment-spot, corresponding to one of the ocelli of an Echinus, and, like it, supposed to be a rudimentary organ of vision. The eyes are often surrounded by circles of movable spines, called "eyelids."
The generative organs are in the form of ramified tubes, arranged in pairs in each ray, and emitting their products into the surrounding medium by means of efferent ducts which open round the mouth. In their development, the Asteroidea show the same general phenomena as are characteristic of the class ; but the larvae are not provided with any continuous endoskeleton. In some Asteroids the larval forms have side-lappets, and have been described under the name of Bipin-nariae; and in these, as in the Pluteus of the Echinoids, a large portion of the larva is cast off as useless. In Bipinnaria asterigera (Sars) the digestive cavity is a simple sac which sends no prolongations into the rays, and the mouth is inter-radial, instead of being placed in the centre of the ambulacral system. The mouth of the adult is at this stage closed by the soft external skin of the larva. In other Asteroids the larvae have three anterior vermiform processes, and are known as Brachiolariae.
The general shape of the body varies a good deal in different members of the order. In the common Star-fish (Uraster rubens) the disc is small, and is furnished with long, finger-like rays, usually five in number (fig. 97). In the Cribellae the general shape of the body is very much the same. In the Solasters the disc is large and well marked, and the rays are from twelve to fifteen in number, and are narrow and short (about half the length of the diameter of the body). In the Goniasters (fig. 98) the body is in the form of a pentagonal disc, flattened on both sides; the true "disc" and rays being only visible on the under surface of the body. In the singular genus Brisinga, we have in some respects a transitional form between the Asteroids and Ophiuroids, the arms being much longer and more slender than is the case in the typical Asteroids, at the same time that they are much thicker and softer than is the case amongst the latter. In none of the true Starfishes, however, are the arms ever sharply separated from the disc, as in the Ophiuroidea, but they are always an immediate continuation of it.
The principal groups of Asteroidea are the following: