572 THE ANIMAL KINGDOM. 



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 Comatulidae 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 Comatulidae, and it is pro- 

 bably 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. 



In Thaumatocrinus the arms like the radials are five in number, but 

 in other Comatulidae 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, &c. 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 palmar s. 



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 con- 

 nected are termed a syzygy> e.g. internodal joints of the stem in some instances, 

 second and third radials very commonly, and brachials not connected 



