180 ECHINODERMATA— PELMATOZOA phylum iv 



Crinoids lodge the functional portion of the genital organs. When two or 

 more arm-joints meet transversely by a rigid suture, and only the uj)per one is 

 pinnule-bearing, those joiiits form a syzygy, whether their apposed faces are 

 striated, dotted or smooth. The lower Joint bearing no pinnule is called the 

 hypozygal Joint, the upper one the epizygal ; and the two together constitute 

 physiologically but a single segment, as is shown by the unaltered alterna- 

 tion of the pinnules. 



The ciliated ambulacral furrows of the arms enter by the arm-openings 

 into the tegmen, and all converge to the mouth. Food-particles, consisting 

 chiefiy of diatoms, infusorians and microscopic crustaceans, are propelled along 

 the furrows and into the body by the action of the cilia. 



In all Recent and in numerous fossil Crinoids the brachials and pinnulars 

 are perforated by a single, or in some cases by a duplicate, canal {central canal) 

 containing the dorsal nerve cords, which give off four delicate branches 

 within each segment. The dorsal canal extends also into the radials and 

 basals, perforating the plates when they are thick, and running in a shallow 

 groove on the inside when thin. So far as has been observed, the axial 

 canals begin uniformly in the basals, where they divide dichotomously ; but 

 in the radials the branches generally reunite to form the so-called ring canal 

 (Fig. 329). 



3. The Column. — The stem or column attains in some forms (Pentacrinus) a 

 length of a number of metres ; but in others it is much abbreviated, or 

 even atrophied, so that the calyx is either directly adherent by the base 

 (Gyathidium, Holopus), or is destitute of all means of attachment {Agassizocrinus, 

 Uintacrinus, Marsupites, the Comatulids). The stem is composed of usually 

 short Segments, having either circular^ elliptical or angular (especially penta- 

 gonal) cross-sections, and being sometimes of uniform and sometimes of variable 

 proportions. Lateral appendages, called cirri, are present in numerous forms, 

 being given off either singly or in whorls at regulär intervals along the 

 periphery. The larger and all cirrus-bearing segments are called nodals, and 

 those interposed between them the internodals. The distal end of the stalk 

 may taper gradually to an apex, in which vicinity fine radicular cirri are 

 commonly developed, or it may be thickened at the extremity so as to form a 

 bulbous or branching root, or a heavy, solid, terminal stem plate, or dorso- 

 central. Growth is accomplished by the insertion of new joints at the 

 proximal end of the stem, either just beneath the calyx, or both here and 

 between the earlier formed joints, the earlier segments becoming at the same 

 time gradually enlarged. The last-formed joints are commonly of smaller 

 size than those situated more remotely from the calyx. 



Like the brachials and pinnulars, all the joints of the stem and cirri are 

 pierced by a (usually central) longitudinal canal which is circular, oval or 

 pentagonal in cross - section, and communicates with the peculiar dorsal 

 chambered organ which in the Comatulids is situated within the centrodorsal, 

 and in the stalked forms within the calyx just above the summit of the 

 stem. The outer walls of the chambered organ are composed of nervous 

 tissue, and form the central organ of the dorsal nervous System which 

 innervates all the dorsal structures ; within the chambered organ is divided 

 by partitions into five sections, and is continued ventrally as a thick tube 

 of uncertain function, known as the central plexus, to near the inner surface 

 of the disk, where it ends blindly. 



