46 BULLETIN 82, UNITED STATES NATIONAL MUSEUM 



THE PINNULE GAP 



The earliest pinnules, appearing at the ends of the arms, are not formed until the 

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

 arm bases is filled by the successive appearance of PI, then usually P 3 , and finally P 2 

 and P 4 . Gislen pointed out that in the Macrophreata the first pinnules appear on the 

 tenth-twelfth brachials when about 15 brachials have developed. In other comatulids 

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

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

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

 in which up to 15 brachials lack pinnules, and in a reduced degree in Comatilia, Hypalo- 

 metra, Clarkometra, Balanometra, and Pentametrocrinus, as well as in certain Colo- 

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

 P a and in rare special cases P! are 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 Impinnata." He remarked that the 

 author also mentioned several other similarities to this group that support the assump- 

 tion of a closer relationship. 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. Gislen noted that the author said the interval between the first and 

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

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

 syzygy at about the tenth brachial. In certain Mariametridae, in Ptilometra, and in 

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

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

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

 are completely formed;" Gisle'n says that this sentence becomes explicable only if 

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

 are completely formed." This assertion, according to Gislen, does not concern forms 

 in which the second syzygy first appears far out on the arm certain Oligophreata. 



Gislen said that according to the author there often appear among the young of 

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

 tions are of the same nature. This, he says, 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 caused by a continuous ventral muscle 

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

 to prove by dissecting or sectioning young comatulids that interbrachial muscles really 

 occur. There are also other similarities found there, but they are of a too general 

 nature to point directly toward the Flexibilia Impinnata Gislen remarked that, 

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

 sal, they might just as well be mentioned as similarities with the Inadunata. 



Gisle'n said that for a long time the pinnule gap was a puzzle to him, until at last 

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

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

 tieth brachial, half burying a number of pinnules in the perisome. The first pinnules 



