172 BULLETIN 82, UNITED STATES NATIONAL MUSEUM. 



all the other articulations will have separated yet some force is still necessary 

 to break asunder the syzygies. 



In life it is certainly true that fracture of an arm almost invariably occurs at 

 a syzygy. If an animal be exposed to unfavorable conditions, placed in warm 

 or fresh water, or very often if merely exposed to the air, all the arms are com- 

 monly cast off at the first brachial syzygy, and the detached arms continue to 

 break up into smaller and smaller pieces by fracture at succeeding syzygies. 



Reichensperger has shown that syzygial unions are innervated in exactly the 

 same way as muscular unions, and this would seem to indicate that fracture at 

 the syzygies is a voluntary act under the control of the animal. 



In the more brittle types, as in Antedon, if one of the cirri be seized firmly, 

 or if it be cut across, it will almost invariably be cast off at the syzygy between 

 its first segment and the centrodorsal. This action is evidently voluntary. 



Unfavorable conditions when not severe enough to lead to the breaking up 

 of the arms sometimes cause the loss of the pinnules. When this occurs the 

 genital pinnules are the first to fall off, followed by more or fewer of the distal 

 pinnules. The oral pinnules are not affected. When the pinnules break off they 

 always separate at the articulation between the second and third segments 

 that is, at the first articulation composed entirely of very short fibers, and there- 

 fore strictly comparable to the brachial syzygies and the syzygies between the 

 first cirrus segments and the centrodorsal. 



The term " syzygy " was first employed by Johannes Miiller for the type 

 of articulation first described in 1825 by Lansdowne Guilding, who believed 

 that all the articulations in the comatulid arm were of this nature. The appli- 

 cation of the term has in the past been somewhat broad. Two brachials united 

 by syzygy and forming a syzygial pair have been commonly considered as but a 

 single brachial, which was called by synecdoche a syzygy. Thus Carpenter 

 repeatedly speaks of the " third brachial " as a " syzygy." or says " first syzygy in 

 the third brachial." Another meaning is given to the word by Pourtales, who, 

 besides speaking of the "syzygium in the third joint," also calls, in the distal 

 part of the arm, the interval between two syzj'gial unions a syzygium. Thus he 

 describes the arms of a comatulid as having " syzygia composed of three, or seldom 

 four, articulations." 



Miiller and Carpenter believed that when two brachials become united by 

 syzygy the lower (hypozygal) entirely loses its individuality, and the syzj'gial 

 pair, usually in general size and shape as well as in its pinnulation resembling a 

 single brachial, should then be treated as a single brachial. But Carpenter made 

 an exception to this in the case of the IBr series which in the comatulids he always 

 considered as consisting of two separate elements whether they be united by 

 syzygy or not. 



Bather was the first to appreciate the necessity of a more consistent interpre- 

 tation of the conditions, and lie strongly urged that the term " syzygy " be employed 

 solely to designate a syzygial union, and that the two elements of the syzygial pair 

 be considered, as they morphologically are, separate units. In this eminently 



