CRINOIDEA 11 



great variation in regard to size, number and shape of the joints 

 or segments. The last cirrus-segment is transformed into a hook 

 or claw, and the penultimate segment is very often produced on 

 its distal, dorsal edge into a smaller spine, the opposing spine. 

 In many forms the distal segments have the distal, dorsal edge 

 produced into a small spine {e.g. Fig. 12, 1). As a rule the cirri 

 curve more or less downwards, in accordance with their 

 function as a means of attachment. It may be pointed out that 

 the concave side of the cirrus is the dorsal side. In descriptions 

 of the Comatulids the number of the cirri is given in Roman, the 

 number of the cirrus segments in Arabic figures. 



The body has the shape of a cup, the calyx, the outer, or dorsal, 

 side of which is formed by strong, regularly arranged plates. 

 These j^lates are of great importance for the classification of 

 sea -lilies. Their special designations are given when dealing with 

 their develojoment. 



Along the dorsal side of the arms there is a series of strong, 

 calcareous joints, the brachials, resembling a vertebral column ; 

 they are connected with one another by means of ligaments and 

 strong muscles. At regular intervals joints occur which are 

 connected by ligaments alone ; such joints, which are termed 

 syzygies,^ are easily distinguished, as they are separated only 

 by a faint transverse furrow, while the other joints are 

 separated by a broader furrow, in which the dorsal ligament 

 is seen. 



The number of the arms is originally five ; in most sea-lilies, 

 however, the arms bifurcate at the second joint, the axillary, 

 which has two oblique articular surfaces on its outer, distal end. 

 In numerous, mainly tropical forms the arms bifurcate again 

 farther out, once or several times, so that the number of arms 

 may be very large. Along the arms are found side branches, 

 pinnules, which arise alternately from each side and are articulated 

 to the outer edge of the arm joints. Each arm joint thus, as a rule, 

 carries a pinnule, excepting the lower joint in a syzygial pair. 

 Like the arms, the pinnules have along the dorsal side a series 

 of calcareous ossicles, only much thinner and more slender than 

 those of the arm ; the outer (distal) joints are often rather 

 strongly spiny. The lowermost pinnules are usually more or 



^ There are, in reality, two kinds of non-muscular articulation of the 

 arm- joints, synarthry and syzygy. There is, however, no necessity for 

 going into such details here, as they are of no material importance for the 

 identification of the Crinoids known from, or to be expected to occur in, the 

 British seas. Likewise, several other technical details unnecessary for the 

 identification of the N.-E. Atlantic Crinoids are omitted. 



