Grctbau — Biserial Arm in Certain Crinoids. 289 



Akt. XXYIII. — Notes on the DemlopmentoftTie Biserial Arm 

 in Certain Crinoids^ ; by Amadeijs W. Gkabau. 



In 1898, while working in Professor K. T. Jackson's labora- 

 tory at Harvard, I noticed that in a perfect arm of JEncri- 

 nios liliiformis the apical portion was uniserial. This feature 

 had never been stated to be the case in any text-books or works 

 on crinoids that were available, and figures of this species of 

 Encrinns always show the condition of imperfect specimens, 

 with the apices of the arms broken away and the biserial 

 arrangement unchanged to the end. It is, of course, well 

 recognized that crinoids the adults of which have biserial 

 arms are uniserial when young, the change from uniserial to 

 biserial beginning at the tip of the arm and passing gradually 

 proximal- wards. But that, after the animal has reached the 

 biserial condition, the new plates added at the top begin uni- 

 serially and only later on become biserial, has never been explic- 

 itly stated, so far as I am aware, though it may have been 

 recognized. Wachsmuth and Springer, indeed, after speaking 

 of the arms of the young Platycrinus as strictly uniserial, go 

 on to say : " In somewhat older specimens, the plates at the 

 tips gradually interlock, and new ones still forming at the dis- 

 tal end are strictly hiserialA With advancing maturity the 

 interlocking gradually extends to the proximal ends, nntil 

 finally in the adult Platycrinus ^^A^ icholearm hecomes hisejnal^^ 

 except perhaps as to a few plates near the calyx, which perma- 

 nently retain their larval condition. ";}: This, as shown beyond, 

 is not the case in any perfect specimen of Platycrinus w^iich I 

 have examined, not even in P. huntsvillcB^ which is used as an 

 illustration of their statement by Wachsmuth and Springer. 



The facts here discussed have apparently been recognized 

 by Bather, for he writes : " The change from uniserial to 



biserial begins in both ontogeny and pln-logeny at 



the growing tip of the arm, and proceeds gradually proximal- 

 wards."§ 



*This paper was originally read before the Boston Society of Natural 

 History, Nov, 7, 1900. It was again read at the Albany meeting of the Geo- 

 logical Society of America, January, 1901. Its publication has been deferred 

 in the hope of finding more illustrative material, but this has been only par- 

 tially realized. A summary was published by Jackson in his memoir on 

 localized stages in development, I am under great obligation to Professor 

 Jackson for loan of specimens from his laboratory, to Mr. Schuchert for loan 

 of material from the National Museum, and to Mr. Edwin Kirk, student in 

 Columbia University, for access to his collection of crinoids. 



f Italics are mine. 



:|: Wachsmuth and Springer, North American Crinoidea Camerata, vol. 1, 

 p. 79. * . . 



§ Bather, F, A., The Echinoderma, Part III, of Lankaster's Treatise on 

 Zoology, p, 116, 1900. 



