MONOGRAPH OF THE EXISTING CRINOIDS. 167 



The outer or distal element of a syz3'gial pair, which bears on its distal border 

 a muscular articulation and a pinnule socket, is called an epizygal, and the proximal 

 element, carrying on its outer end only the syzygial joint face, is Imown as a 

 hypozygal. 



Syzygies occur in the postradial series at more or less regular intervals from 

 the tip of the arm down to the third and fourth brachials, which together always 

 form the first syzygial pair. If division series be present consisting of four or 

 of three ossicles the two outermost always form a syzygial pair, which, as has 

 already been explained, is in reality a reduplication of the first syzygial pair of 

 the free arms. 



There are very few exceptions to this rule. In the Zygometridse a syzygy 

 replaces the synarthry normally found between the elements of the IBr series, but 

 the following division series, when present, are of the usual type and the first two 

 brachials are united by synarthry. In Comatella and N eocomatella, and sometimes 

 in Comatula, on the free arms arising from the outer side of the axillaries, the first 

 four ossicles are usually miited in two syzygial pairs. In most of the species of 

 the genus Comaster all of the division series except the second consist of two ossicles 

 united by syzygy, and the second consists of two syzygial pairs ; but the IIBr series 

 are sometimes 4(3+4), and in one species all of the division series are of the 

 usual type. 



The details of the distribution of the brachial syzygies have already been dis- 

 cussed (see p. 128, last paragraph, and p. 131, middle). 



There is a very considerable difference in the readiness with which comatulids 

 belonging to different groups lose their arms, and the relative brittleness of the 

 brachial syzygies is always correlated with the brittleness of the syzygies between 

 the first cirrus segments and the centrodorsal, and with the tenacity of the unions 

 between the radials. As the arms of the species of Macrophreata are always more 

 brittle than those of the species of Oligophrcata, their cirri more easily detached, 

 and their radials more readily dissociated, a possible correlation is observable be- 

 tween the brittleness of the animal as a whole and the size of the central ner- 

 vous organ. 



The cirri and the arms are always in the same degree fragile. In a species 

 which loses its arms very readily the cirri are also found to be readily lost ; but in 

 a species in which the arms are rarely broken the cirri will be found also to be 

 unusually tough. 



The only exceptions to this interesting correlation occur in the species of the 

 comasterid genera Comaster^ C omateUa^ and Neocomatella, in which one or more of 

 the synarthries in the proximal portion of the post-radial series are replaced by 

 pecidiarly brittle S3'zygies. 



Another point of agreement between the arms and the cirri is that, whereas in 

 the multibrachiate species the 10 larval arms are frequently discarded at the first 

 brachial syzygy, in most forms during growth the oldest cirri, nearest the tip of the 

 centrodorsal, are continuously discarded at the sj'zygy joining the cirri to the 

 centrodorsal. 



142140— 21— Bull. 82 13 



