The members of this order are Echino-dermata, in which the body is fixed, during the whole or a portion of the existence of the animal, to the sea-bottom by means of a longer or shorter, jointed, and flexible stalk. The body is distinct, composed of articulated calcareous plates, bursiform, or cup-shaped, and provided with slender arms, which are typically five or ten in number, and are grooved on their upper surfaces for the ambulacra. (The position of the body being reversed, the upper surface is ventral; whilst the dorsal surface is inferior, and gives origin to the pedicle.) The tubular processes, however, which are given off from the radiating ambulacral canals of the Crinoidea, unlike those of the Echi-noidea and Asteroidea, are not used in locomotion, but have probably a respiratory function. The mouth is central, and looks upwards, an anal aperture being sometimes present sometimes absent. The ovaries are situated beneath the skin in the grooves on the ventral surface of the arms or pinnules, as are also the ambulacral or respiratory tubes. The arms are furnished with numerous lateral branches or "pin-nulae." The embryo is "free and ciliated, and develops within itself a second larval form, which becomes fixed by a peduncle" (Huxley).
If we take such a living Crinoid as Rhizocrinus (fig. 101), we shall be able to arrive at a comprehension of the leading characters of this order. Rhizocrinus is one of those Crinoids which is permanently rooted to some foreign object by the base of a stalk which is composed of a number of calcareous pieces or articulations. In some cases (as in Apiocrinus) the base of the stem or "column" is considerably expanded. In other cases the column is simply " rooted by a whorl of terminal cirri in soft mud" (Wyville Thomson). The joints of the column are movably articulated to one another, the joint-surfaces often having a very elaborate structure, so that the entire stem possesses in the living state a greater or lesser amount of flexibility. Each joint is perforated centrally by a canal, which by the old writers was very inappropriately termed the " alimentary canal," but which in truth has nothing to do with the digestive system of the animal. At the summit of the stem is placed the body, which is termed the " calyx," and which is usually more or less cup-shaped, pyriform, bursiform, or dis-coidal. The calyx exhibits two surfaces, a dorsal and a ventral, of which the dorsal is composed, wholly or in part, of calcareous plates articulated by their margins, whilst the former is composed of a more or less leathery integument, strengthened by the deposition in it of numerous small plates of carbonate of lime. The ventral surface exhibits the aperture of the mouth, which may be subcentral or may be very excentric, and which in many extinct forms is wholly concealed from view. The ventral surface also exhibits the aperture of the anus, which is usually placed excentrically in one of the spaces between the arms, and which is often carried at the end of a longer or shorter tubular eminence or process, which is called the "proboscis." Owing to the animal being supported on a stalk, it is evident that the "ventral" surface is turned upwards, and the "dorsal" surface downwards. The column springs from the centre of the dorsal surface; and a stalked Cri-noid may therefore be compared to a Star-fish turned upside down, with its lower or ambulacral surface superior, and its dorsal surface looking downwards. The calyx contains the digestive canal, and the central portions of the nervous and water-vascular (ambulacral) system; but it does not contain the reproductive organs, as is the case with the visceral cavity of the other Echinoderms.
From the margins of the calyx, where the dorsal and ventral surfaces join one an-? other, arises a series of longer or shorter flexible processes, which are composed of a great number of small calcareous articulations, and which are termed the "arms" (fig. 102). The arms are usually primarily five in number, but they generally divide almost immediately into two branches, each of which may again subdivide; the branches thus produced perhaps again dividing, until a crown of delicate graceful filaments is formed. The arms carry smaller lateral branches or "pinnulae" on both sides; and they contain the so-called "coeliac" and "subtentacular" canals, these being tubular extensions of the cavity of the body. The upper surface of the arms and pinnulae is covered with a soft membrane, and below this are placed the reproductive organs. The generative organs are therefore not placed within the calyx, and it follows of necessity that there is no generative opening or "ovarian aperture" in the walls of the calyx. The ventral surfaces of the arms and pinnulae are furnished with grooves, which in the living species are seen to be covered with vibratile cilia. The brachial grooves coalesce till they constitute five primary grooves, which are continued from the bases of the arms to the mouth. The action of the cilia gives rise to a constant current of sea-water, bearing organic matter in suspension; and this current proceeds from the brachial grooves to the mouth. In this way the animal obtains its food. As the bases of the arms are separated from the mouth by an intervening space, it follows that the brachial grooves are continued over the ventral surface of the calyx, till they reach the oral opening.
Fig. 101. - Crinoidea. Rhizocrinus Lofotensis, a living Crinoid (after Wyville Thomson), four times the natural size. a Stem; b Calyx; c c Arms.
There is no doubt that it is by the above arrangement that the living Crinoids obtain their food, and the mechanism seems to have been essentially the same in many extinct species. In the Palaeozoic Crinoids, however, there seems to have been a modification of this arrangement. In these forms, the arms have much the structure of those of the recent Crinoids, and are deeply grooved on their ventral surfaces. The ventral surface of the calyx, however, exhibits no central aperture, but only a proboscidiform tube, which arises from one of the inter-radial spaces (i. e., one of the intervals between two of the arms). This tube is almost certainly anal, but good observers regard it as discharging the functions of both mouth and anus. However this may be, the brachial grooves are certainly not continued over the ventral surface of the calyx, but stop short at the bases of the arms. Hence they are continued as covered passages or tunnels to a central point in the ventral surface of the disc. Here is placed the mouth, concealed by the calcareous plates of the perisome.