374 



COMPARATIVE ANATOMY 



CHAP. 



At definite intervals the stem may carry whorls of so-called 

 cirri. These are jointed processes of the stem, pointed at their tips, 

 and perforated by a longitudinal canal which communicates with the 

 central canal of the stem (Figs. 257, 258, pp. 311, 312). 



The cirri are, as observations on living animals have shown, very mobile. Five 

 cirri, as a rule, belong to one whorl, being inserted 011 the five sides of the nodal 

 ossicle. Between two consecutive nodes there are a varying number of columnals 

 which do not carry cirri. These together form an internode. Whereas in the 

 Iiuiclunnta, ArticiiJutii, and 6''///' /"/" cirri are, as a rule, wanting, or only present 

 at the lower part of the stem, in the CanaJictihtta (Pcntacrinidce) nodes are found 

 along the whole length of the stem between the consecutive internodes. In the 



A B 



FIG. 326. Diagram to elucidate Wachsmuth and Springer's rule. A. Cnnoid with dicyclic 

 base. B, Crinoid with nioiiucyclic base. Fur lettering see i>. ::17. 



recent species of Pcntacrinus and Mi'tacrimis, each nodal ossicle is connected with 

 the next ossicle of the internode below it by a syzygial suture. 



Peculiar relations exist between the stem and the base of the apical capsule, 

 according to the " rule of Wachsmuth and Springer," given in the diagram Fig. 325. 

 In Crinoids with dicyclic base (i.f. where the base consists of basals and infrabasals, 

 with pentagonal stem and five-rayed central canal, the five edges or angles are interradi- 

 ally arranged, while the five rays of the central canal and the five cirri of each whorl 

 are radially arranged. In Crinoids with monocyclic base (i.e. where the base consists 

 exclusively of the basals, Fig. 325 B) the reverse is the case. In those Crinoids which 

 possess cirri, and in which the stem and central canal are not round, the character 

 (monocyclic or dicyclic) of the base of the calyx can be determined apparently with 

 great certainty from an examination of the stem. This is of importance in forms 

 in which the infrabasals are very small, or, being covered by the uppermost joint 

 of the stem, are hidden, or when they occur only in a young stage. Such forms are 

 said to be constructed on a dicyclic plan, and have been called " pseudo-monocyclic. " 

 It is possible that certain genera in which Wachsmuth and Springer's rule appears 

 to be violated may eventually be proved pseudo-monocyclic. Meanwhile, however, 

 the rule is not absolutely universal. 



The lower part of the Crinoid stem is called the root. It serves, 

 in various ways, to attach the body to the sea floor. If the latter 



