Bl i.l l.i i II ED si\i E8 N \TI<>\ \i. \irs: 



THE PINNULE GAP 



The earliest pinnules, appearing al the ends of the arms, arc not formed until the 

 arms have developed a certain Dumber of brachials. The gap between these and the 

 arm bases is filled l>y the successive appearance of 1',. then usually P 8 , and finally P 2 

 and P Gislen pointed out that in the Macrophreata the first pinnules appear on the 

 tenth-twelfth brachials when aboul 15 brachials have developed. Inother comatulids 

 the pinnule gap is possibly in certain cases somewhat smaller, and possibly the form- 

 ation of the proximal pinnules dor- not always take place as regularly as was assumed 

 by Mortcnsen. In full-grown comatulids the pinnule gap is retained in Atelecrinus, 

 in w Inch up to ! .". brachials lack pinnules, and in a reduced degree in ComatUia, Ili/pnln- 

 tiiitni, Clarkometra, Balanometra, ami Pentametrocrinus, as well as in certain Colo- 

 bometridae and Perometrinae. In the two last-mentioned groups, however, only 

 d in rare special cases 1\ arc lacking. 



Gislen said that the author has attempted to explain the cause of this by assuming 

 that the comatulids, before the appearance of the first pinnules, might be considered 

 "a fairly typical representative of the Flexibilia [mpinnata." He remarked that the 

 author also mentioned se\ eral other similarities to this group that support the assump- 

 tion of a closer relai ionship. Regarding that portion of the arm proximal to the second 

 syzygy these assumed similarities are: (1) Short, oblong, wedge-shaped brachials. (2) 

 Absence of syzygies after the first syzygial pair — a corollary, according to Gislen, of 

 the preceding. Gisleh noted that the author said the interval between the first and 

 second Byzygies is the greatest in tic whole arm, but that in reality this concerns only 

 those comatulids that have very short intersyzygial intervals distally. and the second 

 syzygy *t about the tenth brachial. In certain Mariametridae, in PtUome&ra, and in 

 the Notocrinidae, Pentametrocrinidae, and Atelecrinidae, the interval is often very 

 short, and the second syzygy appears as early as about the seventh brachial. (3) 

 Before the Becond syzygy the pinnules are said to "be absent until after the brachials 

 aie completely formed;" Gislen -ays that this sentence becomes explicable only if 

 understood as meaning "until after the brachials that form the second syzygial pair 

 are completely Formed." 'flu- assertion, according to Gislen, does not concern forms 

 in which the Becond Byzygy first appears far out on the arm — certain Oligophreata. 



< risleD -aid that a< rding to the author there often appear among the young of 



comatulids these similarities to the flexibilia Impinnata: (1) All the brachial articula- 

 tion- are of the -a me nature. 'I'll,-, be -ays, is not correct, for when the earliest pinnules 

 begin to be developed muscular and nonmuscular articulations can be distinguished 

 histologically. (2) The flexibility of the arm is ea used by a continuous ventral muscle 

 band which is not subdivided into interbrachial muscles. Gislen remarked it is easy 

 '.. prove by dissecting or sectioning young comatulids that interbrachial muscles really 

 occur. There are al-o other similarities found there, but they are of a too general 

 nature to point directly toward the Flexibilia [mpinnata— Gislen remarked that, 

 except for the paragraphs concerning the pinnule gap and the formation of a centrodor- 

 aal, they might just as well be mentioned as similarities with the [nadunata. 



Gislen -aid that for a long time the pinnule gap was a puzzle to him, until at last 

 through comparison with the condition in Phrynncrinus he began to see the matter 

 more clearly. In PhrynocrinilS the disk extends outward at least as far as the twen- 

 tieth brachial, half burying a number of pinnules in the perisoinc The lirst pinnules 



