CRINOIDEA. 573 



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 (Comatulidae), 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 pro- 

 longations of the coelome in the arms and pinnules form two canals one, 

 the sub tentacular > 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 (Radiolaria, For ami- 

 nifera, Diatoms, &c.) 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. 



1 In consequence of this fact the visceral mass of the disc is easily detached. Hyponeme Sarsii 

 (Loven), a supposed Cystoidean from Torres Straits, is the visceral mass of an Antedon common at 

 Cape York. 



