544 INVERTEBRATE MORPHOLOGY. 



These are in reality continuations of the radial series of 

 plates ; in fact, in some forms certain of the radials appear to 

 enter into the formation of the arm. In most forms, however, 

 a series of arm-plates arises from each facet of the five axillary 

 plates, so that the arms are ten in number a condition which 

 finds an exception in the remarkable genus Tliaumatocrinus, 

 which possesses but five. In some forms these ten arms 

 branch dichotomously ; the plates intervening between the 

 axillaries and the first branching are termed brackials, those 

 between the first and second branchings distichals, and those 

 between the second and third branchings palmars terms 

 which are useful in systematic descriptions. These various 

 plates are united together by ligaments and muscles, or else 

 by ligaments alone (this last form of union being known as a 

 syzygy), the movements of the arms noted above being thus 

 rendered possible. The pinnules repeat the arm in their 

 structure, though usually on a much-reduced scale. They 

 are situated on the joints separating consecutive plates of the 

 arms, and are placed alternately on the right and left sides 

 of the arm which bears them. They appear at first to have 

 been produced by lateral budding from the joints, but closer 

 examination indicates that in reality they represent a branch- 

 ing, one of the branches remaining small, while the other in- 

 creases in size and places itself in the direction of the axis of 

 the arm. The whole arrangement is comparable to that form 

 of inflorescence termed by botanists a scorpioid cyme, the 

 pinnules representing the flower-pedicels. Owing to the 

 pinnules being in reality one of the branches of a dichotomy, 

 it is evident why, in those forms in which the arms branch, 

 there is no pinnule at the joint where the branching occurs ; 

 in addition, however, pinnules are also lacking on syzygial 

 joints, so that their regular succession may be somewhat dis- 

 turbed. 



As regards the oral system of plates an oro-central is found 

 in some fossil forms, but is unrepresented in recent genera. 

 A circle of five interradial oral plates is found in Holopus, 

 RJiizocrinus, Ilyocrinus, Thaumatocrinus, and Calamocrinns, 

 and in the stalked larva of Antedon, but in the adults of this 

 hitter form and in other genera than those mentioned these 



