In Thaumatocrinus the arms like the radials are five in number, but in other Comatididae and stalked living Crinoids they are ten or more, owing to the branching of the five rays. The arms themselves, their branches and pinnules, like the stem with its cirri, are made up of a single series of calcareous joints. The series is rarely double (certain sp. of Encrinus, some Palaeocrinoided). The calcareous matter is deposited in the form of fasciculated bundles. The joints of the arms are termed brachials. The first two joints, or the first four or six joints in Meta-crinus, are the second, third, etc. radials. The third radial is as a rule an axillary. If the arms branch twice the joints between the first and second places of division are known as distichals; if thrice, the joints between the second and third places of division are designated palmars.
The mode in which the calcareous parts of the skeleton unite varies. Union occurs (i) by suture of the edges of the plates in the apical system; (2) by striated muscle bundles in addition to ligaments between the radials (first radials so-called) of the apical system and the second radials; between a pinnule-bearing brachial, its successor and the pinnule; and in many instances between every two pinnule joints; (3) by bundles of ligamentous fibres solely between the joints of the stem and cirri; between brachials and the joints of pinnules not united by muscles. The lines of union between plates of the calyx may be obliterated by anchylosis, e. g. basals of Bathycrinus. The ligamentous connections may become very close, and the external line of union may disappear. Two joints thus connected are termed a syzygy, e.g. internodal joints of the stem in some instances, second and third radials very commonly, and brachials not connected by muscles. The bundles of ligaments uniting the joints of the cirri and the dorsal inter-articular ligaments between brachials not united by syzygy appear to be contractile. The cirri can lay hold of neighbouring objects, and may be tetanically curved by stimulation of their axial nerve cord.
Slow bending movements of the stem have been observed.
The mouth is central, the anus excentric and interradial on the disc save in Actinometra (Comatirtidae), in which the mouth is excentric or marginal and the anus subcentral. Thaumatocrinus and Palaeozoic forms have the anal interradius large and the symmetry of the disc thereby destroyed. Four interradial plates stretch from a first interradial towards the anus in the former. In the extinct Cyathocrinidae there is a heavily plated anal sac. If the anal interradius be considered as posterior when the disc is turned upwards, the gut passes from the mouth to the left, then anteriorly and round the right side of the calyx to the posterior interradius, where it turns upwards and forwards to the anus. The mouth is thus in the centre of a spiral coil (endocyclic). In Actinometra there are three additional coils concentric with the anus; the mouth is exocyclic. The digestive tube is ciliated, and divisible into oesophageal, median, and anal sections. The coelome is more or less filled with connective tissue supporting the gut, blood-vessels, and nerves. Calcareous spicules and plates are developed in this connective tissue.
In some species of Antedon there is a well-marked axial and perivisceral cavity in the disc 1. The prolongations of the coelome in the arms and pinnules form two canals - one, the snbtentactdar, occasionally divided into a right and left canal underlying the radial water-vascular vessel, the other, the coeliac, lodged in the ventral (oral) groove of the calcareous brachials and pinnule joints. The two canals communicate at the apices of the arms and pinnules, and a current caused by ciliated cups lodged in the pinnule joints sets up the subten-tacular and down the coeliac canal. A smooth peristome surrounds the mouth, and from it radiate the five ambulacral or food grooves which divide and ascend the arms, their branches and the pinnules, with minute extensions on the tentacles. The groove with underlying blood-vessel and tentacles is absent from the adoral pair of pinnules in Antedon, and from a variable number of posterior, i.e. adanal, arms in Actinometra. The grooves with their extensions are, unlike the rest of the perisome (body-surface), covered by a columnar ciliated epithelium, among which are probably sensory cells. The cilia waft any particles of food (Radio/aria, Forami-nifera, Diatoms, etc.) to the mouth.
Below this epithelium, and rarely separated from it by a connective tissue lamella, is the ambulacral nerve. These nerves are connected by a circumoral ring, or more probably by a plexus beneath the oesophagus. Irritation of the nerve causes only movements in adjoining pinnules. There is an anti-ambulacral nervous system composed of fibrils and ganglion cells, and both sensory and motor in function, governing the flexion and extension of the arms in swimming. It consists of a central mass lodged within or below (Comatididae) the circle of basals from which an extension passes down the stem with an axial branch to every cirrus. Five axial branches enter the basals, and each of them divides into two. Two adjoining branches enter each radial and pass on, dividing as the arm divides and entering the pinnules. A commissure connects all the ten secondary branches in the first radials, and another commissure the two main branches in the axillary radials. But there is some difference of detail in various Crinoids. The axial nerves are at first lodged in a ventral groove throughout their course, a condition permanent in some Palaeocrinoidea. But in all living Crinoidea the calcareous matter finally surrounds them.