NO. 7 SEA-LILIES AND FEATHER-STARS CLARK 7 



gonads, and in ending in a terminal brachial bearing a pair of pinnules 

 instead of in a growing tip as do normal arms. 



From the second onward the arm segments bear on alternate sides 

 long slender jointed processes called pinnules (figs, i, 44, 48), which, 

 in the comatulids, are of three types. At the base of the arms and on 

 the second ossicle of division series composed of four elements are 

 the so-called oral pinnules which lack the ambulacral groove and are 

 variously modified into very sensitive tactile organs (figs. 47, 48, 51) 

 or into stout spines (fig. 52) extending over and protecting the disc; 

 following these and more or less abruptly differentiated from them 

 are the shorter genital pinnules (figs, i, 51, 52), with, or, more rarely, 

 without (fig. 36, left) an ambulacral groove, on which the gonads are 

 developed ; there are usually between six and twenty of these on each 

 side of the arm, and distally they pass gradually into the longer and 

 usually very slender distal pinnules (figs, i, 51, 52) which serve 

 purely as food gatherers. In a few types one or more of the earlier 

 pinnules are lacking (fig. 43), while in the Comasteridse (figs. 15, 43) 

 the oral pinnules bear curious comb-like structures recalling the 

 pectinate antennae of certain insects. In the crinoids other than the 

 comatulids the pinnules are much more nearly uniform in structure 

 and in function; in some types {Metacrinns, Hypalocrinus, Comas- 

 trocrimts [fig. 41] and Proisocrinus [fig. 40]) they are rudimentary 

 or even quite lacking on the terminal portion of the arms. In one 

 comatulid {C omatulella hrachiolata) many of the pinnules are modi- 

 fied into stout organs resembling cirri wliich assist the animal in 

 clinging to arborescent marine organisms. 



The various ossicles which together form the crinoidal skeleton 

 are tied together by more or less closely packed bundles of fibrillse 

 the ends of which take the form of loops within the calcareous sub- 

 stance. Between most of the brachials or ossicles of the arms there 

 is found in addition to the ligaments a pair of ventral muscle bundles 

 and between the ossicles of the pinnules there are sometimes a few 

 muscle fibers or a small muscle bundle. Excepting in Holopus more 

 or fewer of the brachials are united in pairs by ligament fibrillse only ; 

 such unions, known as syzygies, are extremely close, at right angles 

 to the axis of the arm, and with the joint faces marked with radiating 

 ridges ; two brachials so united usually appear as a single one with a 

 thin dotted line across it (fig. 54). Crinoids seem to have the power 

 of severing the syzygies at will, and arm fracture almost invariably 

 takes place at these unions. As a rule syzygial pairs are regularly 

 distributed throughout the crinoid arm, the first being composed of 

 the third and fourth brachials ; their number decreases with specializa- 

 tion and with increase in the number of arms. 



