416 PELMATOZOA. 



oids, however, belong to the Pedunculate division of the group, it is 

 necessary to study the skeleton of these in greater detail. 



A pedunculate Crinoid, such as Pentacrinus ' (fig. 289), consists of 

 a cup-shaped body or "calyx," which encloses the principal viscera, 

 and is furnished with a crown of pinnate "arms," and which is 

 attached to some foreign object by means of a stalk or "column," 

 composed of a number of calcareous pieces or " articulations." In 

 some cases (as in Apiocrinus) the base of the " column " is consid- 

 erably expanded. In other cases the column is simply " rooted by 

 a whorl of terminal cirri in soft mud" (Wyville Thomson). The 

 column may be extremely short, or even wanting in the adult, or 

 may reach the extraordinary length of sixty or seventy feet. The 

 whole column is composed of a series of ring-like or pentagonal 

 joints, which are generally movably articulated with one another, 

 and are furnished with special muscles, the joint -surfaces often 

 having a very elaborate structure, and the entire stem possessing in 

 the living state a larger or smaller amount of flexibility. Very 

 often more or fewer of the column-joints carry lateral jointed pro- 

 cesses or "cirri" (fig. 289). Each joint of the stem, further, is 

 perforated centrally by a canal, which lodges an extension from 

 the " chambered organ " and its fibrillar nerve-sheath. 



As regards its shape, the " column " of the Stalked Crinoids is very 

 commonly round, but it is sometimes oval or elliptical (as in Platyc7-inus, 

 fig. 300), and it is not infrequently pentagonal (as in Extracrinus, fig. 

 290). The separate joints or " articulations " of the column are usually 

 so connected with one another that whilst the amount of movement 

 between any two pieces must be very limited, the entire stem is more or 

 less flexible. The articular surfaces or facets by which contiguous joints 

 are connected, are differently marked in different cases. In many forms, 

 the articulating facets are marked by more or less numerous radiating 

 strise, which run from the central canal of the joint to its margin. In 

 other cases, as in Pentacrinus and Extracrinus (fig. 290), the articular 

 faces are united by crenated ridges arranged in a pentapetalous figure. 

 Though usually articulated movably, the stem-joints are occasionally 

 united here and there by the peculiar mode of union known as " syzygy." 

 By this is understood the immovable union of two originally separate 

 joints by close ligamentous connection and subsequent more or less 

 complete fusion, the primitive line of division usually remaining visible 

 as a line of suture. 



The uppermost joint of the column is often larger than, and differently 

 shaped from, the inferior joints, and may, as in Apiocrinus, enter largely 

 into the formation of the calyx. In many cases, as in Pentacrinus and 

 Extracrmiis (figs. 289 and 290), the column is furnished with more or 

 less numerous auxiliary arms or " side-arms," which represent the " cirri " 

 of the Comatulids. The column generally increases in height by the 

 addition of new joints at its summit, and also by the intercalation of 

 others between those previously formed in the upper, and therefore the 

 youngest, part of the stem ; and the whole series is traversed centrally by 

 a variously shaped tube — the misnamed " alimentary canal " of old writers 



