Echinodermata which are fixed permanently or temporarily by an aboral stem, generally jointed and containing a neuro-vascular apparatus. The apical system incloses or supports the visceral mass. The oral system may form a tegmen calycis or be resorbed partially or completely. The water-vascular ring does not open direct to the exterior. Mouth and anus on the actinal surface.
Pelmatozoa with arms more or less branched, borne upon the radials and in most instances furnished with pinnules. There is an ambidacral and anti-ambulacral nervous system. The lateral extensions of the radial water-vascular vessels form tentacles (=tube feet) which are disposed in groups. Water-tubes and water-pores lead into the coelome; no stone-canal.
The oral or actinal surface forms a disc, the aboral or abactinal a supporting calyx. The latter consists either -solely of the plates of the apical system or may include also the basal joints of the arms, i. e. second and third radials, etc, and the lowest pinnules. The dorso-central plate retains its normal position in the few genera, all extinct, in which no stem is developed, e. g. Marsupites. But in the stalked Crinoid it forms the base of attachment, and is separated from the other plates of the apical system by the joints of the stem l. These joints may be few, or exceedingly numerous, e. g. in fossil species with stems seventy feet long, and are usually provided with well-formed surfaces of union. The Comahdidae break away from the stem at a certain stage of growth. The topmost joint of that structure then constitutes the central element of the calyx, and is termed the centro-dorsal plate. Under-basals are found in some fossil Palaeocrinoidea and Neocrinoidea. The basals vary from five to three in number, but all living species possess five except Hyocrinus which has three. In living Comatididae except Thaumatocrinus and Atelecrinus they unite into a small rosette plate which is concealed from without by the centro-dorsal and radials.
The first radials are five in number in all Neocrinoidea; other numbers, however, occur abnormally. They are concealed from view by the centro-dorsal and second radials in living Comatu-lidae except Thaumatocrinus and Atelecrinus, In many extinct forms especially Palaeozoic, the adjoining edges of the first radial plates are partially separated by the circle of first interradials, and in the living Thaumatocrinus, and the extinct Palaeozoic family Rhodocrinidae, an interradial alternates with a radial and touches the basals. Higher orders of interradials are found in many Palaeozoic and some Mesozoic species, but in the latter, as well as the living forms, the interradials, when present, are as a rule numerous and irregular.
1 Holopus is attached by an 'irregular encrusting calcareous expansion.'
The oral system is represented by a circle of five orals surrounding the mouth in Thaumatocrinus, Holopus, Hyocrinus and Rhizocrinus. Present in the pentacrinoid, i. e. stalked Antedon, they are resorbed during growth. They are large, and their circle is broken by the anal tube in many Palaeozoic genera, in many of which an oro-central is present, and together with the circle of orals forms a vault, dome, or tegmen calycis arching over the mouth. The extinct Actinocrinus has in addition circles of dome radials and interradials, but as a rule such plates are small and irregular. In all these forms the ambulacral grooves ascend the dome, and enter the oral vestibule between the bases of the orals.
The surface of the disc between the ambulacra and their divisions, as well as its continuation upon the arms, is sometimes naked, though calcareous spicules are present at the edges of the ambulacra. It is, however, as a rule protected by irregular anambulacral plates, continuous at the edges of the disc with the interradials. The edges of the ambulacra themselves are generally guarded on the disc, arms and pinnules by a series of covering plates (adambulacrals) which in some Comatulidae are represented by soft lobes. Whether plates or lobes, they alternate inter se on opposite sides of the grooves over which they can close. They are often borne upon the edges of a single series of side plates.
The plates of the calyx, disc and ambulacra develope as cribriform calcareous films, and are thickened by addition of new films.
To the radials are attached the arms, and to certain joints of the stem, the cirri. The arms generally branch dichotomously, and the joint known as axillary, which bears the branches, has two equal oblique and terminal facets, one for each branch. The growing point of the arm or its branches forks at short intervals, and one division of the fork, alternately the right and left, remains relatively short and small constituting a pinnule1. In the living Hyocrinus, however, and some fossil forms, the pinnule is about as long as the part of the arm distal to its point of origin. Such pinnules resemble the branches of an arm, but the facets upon the joints by which they are borne are somewhat laterally placed, whilst those for the continuation of the arm are sub-terminal. The cirri are borne in whorls upon certain joints of the stem, hence termed nodal, which are separated one from the other by a series of inter nodal joints. The centro-dorsal of the Comatididae bears several whorls of cirri, but in development it has a first whorl of five cirri placed radially. In Rhizocrinus the upper part of the stem is bare, the lower part bears branching cirri, and may itself be resolved into branches.
The pentacrinoid Antedon and Holopus have no cirri, nor have some fossil forms in which the stem tapers to its apex, and was probably coiled round some foreign object when the animal fixed itself. The stem is normally attached by the terminal joint or dorso-central plate. In some individuals growing in muddy bottoms, this attachment is either lost or not developed, and the stem ends by a rounded nodal joint. The animal in this case anchors itself by its terminal cirri as do the Comatididae, and it is probably free to move about as they do by alternate flexions and extensions of the arms. Or else the tips of the cirri are permanently attached to stones or rocks as they are in Rhizocrinus.
1 Pinnules are absent from every axillary joint, and from the proximal of two joints united by ligament or by syzygy. The lowest brachials of Atelecrinus (Comatulidae) and of Bathycrinus and Rhizocrinus are also devoid of them, and in Antedon it has been observed that the pinnules of the lowest brachials (3-7) are developed at a late period. Hence it appears that pinnules may be formed independently of the growth of the tip of the arm. + Oo