The dorsal perisoma, except in Brisinga, developes numerous minute and delicate processes. These are tubular and contractile. They contain an extension of the coelome, and are known as dermal branchiae. The oral nerve ring with its radial trunks lies at the base of the ectoderm cells, which are large and columnar in these tracts, and contain numerous sense cells. They are merely an exaggerated development of the plexus of fibres and ganglion cells present between the ectoderm of the perisoma. The fibres, however, are more numerous and run all in the same direction. There is an eye speck present on the ventral side of the base of the terminal arm-tentacle except in Brisinga. It consists of an aggregation of minute invaginations of ectoderm cells, many of which develop pigment. The blood-vascular system consists of an oral ring with five radial vessels: of an aboral ring which is circum-anal and gives off ten genital vessels in pairs and two intestinal vessels, one on either side of the dorsal extremity of the plexiform organ. These rings and vessels may themselves be plexi-form. The plexiform organ or heart so-called connects the two rings and lies to the inner, i. e. adcentral side of the stone-canal. It is attached dorsally on the central side of the madreporite.
A right and left perihaemal space surround the radial vessels; an outer and inner circular space the oral ring, of which the latter is connected to a perihaemal space inclosing the plexiform organ and stone-canal. Other spaces surround the aboral ring and its vessels. None of these spaces are separated inter se by complete partitions, and it is stated that the inner oral space and the spaces of the genital vessels are connected to the lacunar canals in the perisoma. Feeble contractions have been observed in the intestinal vessels and plexiform organ. The latter contains numerous amoeboid brown cells which also occur in the vascular system generally; in the water-vascular system, especially in Tiedemann's vesicles, and sparingly in the coelome. They are probably respiratory like the similar cells of Echinoidea. The water-VasCular ring bears four pairs of Tiedemann's vesicles on its inner aspect interradially. The fifth interradius contains one vesicle and the origin of the stone-canal. This canal has Calcareous plates in its walls, is lined with ciliated epithelium, and its cavity is sometimes simple, sometimes broken up into many tubes by internal septa. There is an ampulla at its dorsal end close to the madreporite. This structure is interradially placed, and is sometimes multiple.
It is sometimes in connection with one of the basals of the apical system, but is not so at first, nor in some adults, e.g. Zoroaster fulgens, etc. The radial water-vascular vessels are placed in the axis of the ambulacral groove, immediately ventral to the inner ends of the (sub)-ambulacral ossicles. The tube-feet are arranged in a single row on either side of the vessels. Each foot sends off at its base a canal which, passing through the corresponding pore inclosed by two adjacent (sub)-ambu-lacrals, expands on the dorsal side into an ampulla. These ampullae are absent in Brisinga. The feet are terminated by sucking discs, except in the Astropectinidae, where they are pointed. They are richly supplied by nerves, and gland cells have been detected on their discs. The terminal tube-foot is azygos and the first of the series developed. It forms the so-called tentacle, and has no disc.
The ciliated digestive tract consists of an oesophageal (cardiac) division with short radial pouches, which are eversible and are retracted by special muscles; of a stomach (pyloric division), from which caeca radiate into the arms, and a tubular intestine which opens by an anus placed dorsally and a little excentrically. The anus is absent, however, in all the Astropec-tinidae (Astropecten, Ctenodiscus, Luidia, Leptychaster) the genus Archaster excepted. The stomachal caeca commence as five narrow tubes, each of which divides once, and the ten branches thus formed bear numerous lateral branches with ampullae. They are suspended to the dorsal perisoma of the arms by double mesenteries. The intestine has close to the anus two interradial ampullae - sometimes five, each of which may divide once (Culcita). The anus in Zoroaster is placed interradially within the circle of under-basals. The digestive tract is attached by a dorsal mesentery to the body walls. The coelome is ciliated and extends into the arms. These extensions are narrow in Brisinga, in all others large. The generative glands are generally ten in number, and are placed interradially.
They consist of branched caeca with a common duct, which, as a rule, opens dorsally by one or, more rarely, by many pores on a calcareous plate placed at the base of the arm interradially, and called in the latter case a sieve-plate. When mature the glands extend into the cavities of the arms. There is sometimes a greater or less number of genital glands extending up each side of the arms and opening by separate ducts, a condition most perfectly developed in Brisinga. A blood space surrounds the glands, and it is possible that an infertile rhachis may extend to the aboral blood-ring, as in Ophiuroidea.
The free swimming larva is known as Bipinnaria or Brachiolaria.
Most Asteroidea live in shallow water, but there are a number of deep-water genera which possess primitive or peculiar characters (Zoroaster, Brisinga, &c). The group appears in the lower Silurian.
The Palaeozoic Asteroidea are classified as Encrinasteriae, characterised by the alternate arrangement of the ambulacral ossicles. In all living Asteroidea they are opposite. Viguier proposes (A. Z. Expt. vii. 1878) to group the latter as
I. Asteriae ambulacrariae, with the mouth plates ambulacral, the pedicellariae pedunculate, straight or crossed, and the ambulacral pores as a rule arranged in zigzag, so that there are apparently two rows of feet. Asteriadae, Heliasteridae, Brisingidae.
2. Asteriae adambulacrariae, with mouth-plates adambulacral, sessile pedicellariae, and the ambulacral pores as a rule in a straight line. Other Asteroidea.
See lit. pp. 194, 196.
Brisingidae of the 'Talisman,' Perrier, C. R. 101, 1885.
Haematoporphyrin in integument, MacMunn, Journal of Physiology, vii. (3), 1886.