Fig. 251.—Dried shell of Schizaster, showing peripetalous fasciole. ant.amb, Anterior ambulacrum; fasc, peripetalous fasciole; g.p, genital pores, × 1. (After Agassiz.)
Fossil Echinoidea.—Echinoidea are well represented in the geological record, and form a characteristic element in many fossil faunas. They appear in the Ordovician formation, but the first representatives of an existing family (Cidaridae) only appear in the Permian.
Space will only permit us to treat of the extinct members of the group very briefly. Leaving out of sight the representatives of families still living, the fossil Echinoidea may be divided into two great groups, viz.:—
(a) Palaeozoic forms, which in some points serve to connect the Endocyclica with the primitive Asteroidea.
(b) Mesozoic forms, which serve to connect the Clypeastroidea and Spatangoidea with the Endocyclica.
Fig. 252.—A, Bothriocidaris. × 1. B, Palaeoechinus. × 1. amb, Ambulacral plates; inter, interambulacral plates. (After Zittel.)
The Palaeozoic forms are often called Palaeoechinoidea, and they are above all distinguished by the fact that the number of vertical bands of plates composing the corona is variable, in a word, that the corona has not yet acquired a fixed definite constitution. One genus (Echinocystites) has the anus outside the apical system. It has four rows of pore-plates in each radius, and numerous rows of plates each with a single spine in the interradii. Another (Palaeodiscus) has been shown by Sollas[[504]] to be in many respects the missing link between Asteroidea and Echinoidea. Inside the plates of the corona there is a series of ambulacral plates like those of Asteroidea. The tube-feet in the oral portion of the radii seem to have issued between the (outer) ambulacral plates. No anus has been detected. All the rest are Endocyclic. The oldest known form, Bothriocidaris (Fig. 252, A), from the Ordovician, has only one row of interambulacral plates and two of ambulacral; no peristome is distinguishable from the corona. The Archaeocidaridae appear in the Devonian. They have narrow ambulacra of two rows of pore-plates as in the Cidaridae, but the interambulacra consist of many rows, the members of which overlap, and therefore were probably slightly movable, as in the Echinothuriidae; the primary tubercles are large, and there is only one on each plate. The Melonitidae (Fig. 252, B) appear in the Carboniferous. Each interambulacral plate, of which there may be five rows in each interradius, bears numerous small tubercles, and there may be four or more vertical rows of pore-plates, though in the genus figured, Palaeoechinus, there are only two. The Tiarechinidae are represented by one genus, Tiarechinus, with an enormous apical system, from the Triassic of the Tyrol. The interambulacra consist of one plate bordering the mouth, three, side by side, forming the interradial area of the corona, and one large genital plate; the ambulacra, of two rows of pore-plates. This family consists of dwarfed forms which probably inhabited the land-locked seas and salt lagoons of the Triassic epoch.
When we recollect that some of the oldest Asteroidea known to us had very narrow arms and interradial areas edged by large square marginals, it does not require a very great effort to imagine how these marginals could be converted into the vertical rows of the interambulacra, and the pointed narrow arms becoming recurved, could have formed the ambulacra. The physiological advantage of this will be discussed in the chapter on development.
True Cidaridae occur in the Permian, and are abundant in all the younger formations. One Cretaceous genus, Tetracidaris, has four rows of interambulacral plates near the mouth, diminishing to two at the apex. This circumstance renders it probable that the Cidaridae are the direct descendants of the Archaeocidaridae. The Saleniidae, Echinothuriidae, and Diadematidae appear in the Jurassic, the Echinidae in the Cretaceous, and the Arbaciidae only in the Tertiary epoch.