It is known as the odontophore, and appears to be homologous with the oral plate of many other Echinoderms. See the general account of the Phylum. A single row of minute adambulacral ossicles articulating immediately with the outer ends of adjoining ambulacrals can be made out in nearly the whole length of the arm; as well as the transverse rows of five intermediate ossicles, which unite them with a longitudinal row of inferior marginal ossicles bordering the ventral aspect of each arm. The median intermediate ossicle of the five is enlarged and spine-bearing, and is connected to the intermediate ossicle in front and behind. In some Starfish, e. g. Astrogonium, there is a series of well-developed dorsal superior marginal ossicles.
The perisoma or integument consists of two layers, an outer and an inner, between which exists a system of irregular channels which are ultimately continuous with the system of perihaemal spaces. The calcareous ossicles, spines and pedi-cellariae, belong to the outer layer with the single exception of the ambulacral ossicles which belong to the inner one. The outer layer passes across the ambulacral groove from side to side, inclosing a space between itself and the ossicles. In this space are lodged (proceeding from the dorsal to the ventral wall) the water-vascular radial canal, the transverse ambulacral muscles, the right and left-perihaemal space with the bloodvessels, and the nerve-forming ectoderm.
The outer surface of the perisoma is covered by a ciliated epithelium (ectoderm) which possesses a distinct cuticle pierced by pores for the cilia. The constituent cells are (1) supporting cells, (2) gland cells, (3) sense-cells. At the base of the epithelium is a network of nerve-fibrils with ganglion cells. In the ambulacral nerve and circum-oral nerve-ring the supporting and sense-cells are of great length, and the nerve-fibre layer much thicker, and the fibres parallel to one another. These structures are continued on to the terminal feeler or tentacle, the first formed tube-foot of the arm, as well as on to the paired feet; but in the latter the nerve-fibres are arranged in longitudinal bundles, and gland-cells occur plentifully on the terminal sucking disc. The eye-speck is placed on the ventral surface of the feeler and is composed of several eyes, the number increasing with age. Each eye consists of a conical depression in the epithelium over which the cuticle passes uninterruptedly. The walls of the depression are formed of sense-cells, some of which contain pigment, and its central cavity is filled with a clear liquid.
If the radial nerves are divided close to the ring, the animal loses the power of coordinating the movements of the several arms: and if the eye-specks are removed, it ceases to react to light.
The pedicellariae in the Asteroidea possess only two blades with the exception of Luidia, where they possess three as in Echinoidea. But the stalk of the Asteroid, unlike that of the Echinoid pedicellaria, is formed entirely of soft structures. There are in the Asteroidea, which possess four rows of ambulacral feet, two kinds of pedi-cellariae. Both agree in having three calcareous pieces, a basilar piece bearing two blades opened and shut by muscles. But in one kind which occurs chiefly isolated the blades are articulated opposite to one another, in the other the blades cross one another at a certain point like the blades of a pair of scissors. The latter are termed crossed pedicellariae. They occur in rings or semi-rings upon the spines. In other Asteroidea there is a form of pedicellaria termed 'valvulate,' in which the blades are broader than they are long. The pedicellariae are used to take hold of objects such as algae until the feet can be applied.
The apical system of plates is not traceable in this nor in the majority of adult Starfish. It is disguised by the formation of new plates and ossicles during growth. See the general account of the Echinodermata and Asteroidea. The ossicles are developed from a calcareous network as is usual in Echinodermata, and the calcareous matter is chiefly Calcium carbonate.
The series of ambulacral ossicles end at the tip of each arm with a terminal plate which supports the tentacle on its ventral surface, and is formed in the young Starfish at an early period. All new ossicles and plates are added between it and already existing ossicles and plates. A series of plates on the dorsal surface connects it to a radial plate of the apical system in the young Starfish and some adults. And when the radial becomes indistinguishable, this median row of plates may remain conspicuous. In Asterias it is often well marked, but the ossicles are similar to the other ossicles of the general perisoma which are arranged in linear series.
According to Viguier, ten muscles correspond to each pair of ambulacral ossicles: four vertical muscles (two on each side) uniting the ambulacral and adam-bulacral ossicles: four longitudinal (two on each side, one superior and one inferior), and two transverse muscles. One of the transverse muscles is ventral and deepens the groove when it contracts: the other dorsal and antagonistic to the ventral.
The dermal branchiae of the dorsal surface are delicate contractile tubular processes of the perisoma.
The madreporite contains tubular ciliated canals radiating conformably with its superficial furrows. These canals send up minute vertical non-ciliated tubes which open in the furrows by ciliated funnels. Of these openings A. rubens has about two hundred. The radiating canals join the stone-canal. This canal has at its dorsal end an ampulla, which is seven-lobed. Its cavity is partially divided by a septum, but in many Asteroidea it is completely broken up into many tubes. To the circumoral water-vascular ring are attached in pairs nine racemose vesicles or Tiedemann's bodies. A tenth is missing, and the stone canal opens where it should be. The bodies consist of branched canals lined by cubical cells. The radial vessels spring from the circumoral. They give off laterally, corresponding to the interval between two ambulacral ossicles, the branches for the feet. These open into the bases of the feet in such a manner that water cannot regurgitate. A branch from the base of every foot passes through a corresponding ambulacral pore and swells into an ampulla on the dorsal aspect of the series of ossicles.
Longitudinal muscles exist in the walls of the ampullae and feet, circular in other parts of the water-vascular system.
The so-called heart or plexiform organ (gland) is composed of a plexus of vessels lying to the inner, i. e. adcentral, side of the stone canal inclosed in a peri-haemal space. At its ventral end it is connected to a circumoral plexus which lies in an incomplete septum dividing the circumoral perihaemal space into an outer and an inner space. The radial vessels are similarly lodged between a right and left space. At its dorsal end the plexiform organ is connected with a peri-anal ring which gives off the ten vessels to the genitalia and two vessels, one on either side of its dorsal end, to the intestine. These vessels are also lodged in spaces. The vascular system contains brown cells, which also occur, but more sparingly, in the water-vascular system. The perihaemal spaces are probably a part of the coelome. They are lined by an epithelium which in the radial spaces forms an interrupted ventral band of columnar cells which were supposed by Lange to be nervous. They are said to communicate both with the coelome and the channels in the perisoma.
The water-vascular oral ring lies to the dorsal and outer side of the two perihaemal spaces, and below or ventral to them is the thickened nerve-ring just as in the arms.
The coelome is lined by ciliated epithelium.
Echinodermata, Encyclopaedia Britannica (ed. ix.), vii. System der Asteriden, Muller und Troschel, Brunswick, 1842. Stellerides du Musee, Perrier, A. Z. Expt. iv. 1875; v. 1876.
Species of genus Asterias, Bell, P. Z. S. 1881.
Skeleton. Gaudry, A. Sc. N. (3), xvi. 1851 (for figure of Asterias rubens, P1. 18, Fig. 1); Agassiz, Memoirs Harvard Museum, v. 1877; Viguier, A. Z. Expt. vii. 1878.
Names of ossicles belonging to ambulacral series. J. Muller, Abhandl. Akad. Berlin (Classis Physica), 1853-54, p. 162; p. 210.
Oral and apical system of Asteroids. Sladen, Q. J. M. xxiv. 1884 (contains general references).
Oral ossicles. Viguier, A. Z. Expt. vii. 1878; Ludwig, Z. W. Z. xxxii. 1879; cf. remarks in Carpenter on 'oral and apical system of Echinoderms,' Q. J. M. xxii. 1882.
Pedicellariae. Perrier, A. Sc. N. (5) xii. 1869. In Echinoids, Id. A. Sc. N. (5), xiii. 1870; Geddes and Beddard, Trans. Roy. Soc. Edinburgh, xxx.; Foettinger, Archives de Biologie, ii. 1881; Sladen, A. N. H. (5) vi. 1880. In Euryalidae, Ludwig, Z. W. Z. xxxi. 1878.
General Minute Anatomy. Beitrage zur Histologic der Echinodermen, Ha-mann, pt. ii. Die Asteriden, Jena, 1885.
Nervous system, eye, perisoma, feet, etc. Hamann, Z. W. Z. xxxix. 1883, p. 170.
Mode of locomotion. Romanes and Ewart, Ph. Tr. 172, 1881, p. 836. Function of eye and pedicellariae. lid. J. L. S. xvii. 1884, p. 131; cf. Romanes, Jelly-fish, Starfish, and Sea-urchins, Internat. Series, 1. 1885.
Fission in Asteroidea and Ophiuroidea. Kowalewsky, Z. W. Z. xxii. 1872; Simroth, Ibid. xxviii. 1877; Haeckel, Ibid. xxx. (suppl.) 1878; E. von Martens, A. N. 32, 1866, p. 68.