1895.] ME. r, A. BAtSER oir trrNTAORiinjs. 991 



In the present paper, at any rate, each brachial is treated as a 

 unit, and the term sjzjgy is confined to its original meaning, viz., 

 an immovable sutural union between two ossicles. 



In the fixed region of the arm we may assume that the distri- 

 bution of the syzygies is governed by the law of pinnulation above 

 stated, that is to say, the non-pinnulate secundibrachs are taken 

 to be hypozygals. For U. ivestfallcus this assumption is definitely 

 confirmed by the observations of Schlueter (4), which show that 

 the ossicles in question have a radiate upper joint-surface. In 

 U. socialis this has not been seen, but in external appearance the 

 supposed syzygial sutures are closer and finer than the rest. 

 For IIBr the distribution of the syzygies has been worked out in 



16 arms belonging to 5 individuals, and the results are given in the 

 accompanying diagrams (pages 992 & 993). It appears from these 

 that the syzygial sutures occur at successive average intervals of 

 3, 4, 4, 5, 7, 5, 7, and 8 sutures. The persistent syzygial nature of 

 the 26th suture is remarkable, and is one of those facts that would 

 be obscured by the usual method of counting. The regularity of 

 the syzygies seems to decrease in the more distal regions ; that is 

 to say, although in a single arm the intervals are fairly constant, 

 yet there is considerable difference between individuals, possibly 

 between the several arms of one individual. There is some reason 

 to suppose that the interval between the syzygies is usually less in 

 the proximal portion of the arm than it is in the median portion, 

 bat the evidence is insufficient. A long arm-fragment of medium 

 width has syzygies at intervals of 10-15 sutures ; but another 

 much narrower arm-fragment of over 130 brachials has syzygies 

 at intervals of 5-8 sutures, the usual intervals being 6 and 7. 



The large number of the syzygies and their regular distribution 

 throughout the whole length of the arms, doubtless bear some 

 relation to the animal's mode of life. It is natural to suppose 

 that syzygies have been developed to afford points of easy fracture, 

 such as could never be presented by the muscular and ligamentary 

 articulations. Thus, when an arm is entangled or is caught by 

 some enemy, the crinoid merely breaks it off and swims or crawls 

 away, happy in the knowledge that a new arm will soon grow 

 from the stump. This explanation has been confirmed by the 

 experiments of J. Walther on Antedon rosacea ^ A Uintacrinus, 

 with its sociable disposition, must often have found its long 

 flexible arms inextricably interlaced with those of its fellows. Only 

 by the syzygies can it have escaped from the too close embrace. 



In the fixed portion of the arm the hypozygals and epizygals do 

 not materially differ in height or appearance from the other fixed 

 brachials. It would be preposterous to count IlBr„„ ,,IlBr„„ 



^ ^ 3&4' 6&7» 



or even HBr^^^^, as single morphological units, as would be done 

 on the usual system of counting. After this, the hypozygals and 



1 " Unter3uchimgen ueber den Bau der Orinoiden," Palfflontographica, xxxii. 

 p. 180. (Oct. 1886.) 



63* 



