124 Papers from the Marine Biological Laboratory at Tortugas. 



20. The podia arise as smooth, paired outgrowths of the radial water- 



vessel just proximal to the terminal plate. Each one is associated 

 with a side arm-plate. They ordinarily remain smooth throughout 

 life, but in some brittle-stars they are rough, minutely or conspicu- 

 ously papillose. Such changes are obviously specializations and 

 such podia are often less retractile than usual. 



21. The number of arm-segments included in the disk increases with age 



and such segments are commonly more or less modified by their 

 position. 



22. The number of arms is determined when the metamorphosis occurs or 



when the typical ophiuran form is assumed. New arms never 

 arise subsequently, except in schizogonous species, when they 

 develop in connection with the new disk formation. A 5-rayed 

 individual never becomes 6-rayed after its 5 arms are fully formed 

 nor does a 6-rayed individual ever become 5-rayed by autotomy 

 and resorption, as has sometimes been supposed. A 5-rayed or 

 6-rayed young brittle-star will remain such throughout life unless 

 schizogony occurs. 



Museum of Comparative Zoology, 

 Cambridge, Mass., March, IQ13. 



LITERATURE CITED. 



Bell, F. J. 1892. A contribution to the classification of Ophiuroids, etc. Proc. Zool. 



Soc, London, pp. 175-183. 

 Fewkes, J. W. 1887. On the development of the calcareous plates of Amphiura. 



Bull. M. C. Z., vol. 13, pp. 107-150, 

 Grave, C. 1898. Embryology of Ophiocoma echinata Agassiz. Prelim. Note. Johns 



Hopkins Univ. Circ, vol. 18, pp. 6, 7. 



1900. Ophiura brevispina. Mem. Biol. Lab., Johns Hopkins Univ., vol, 4, 



No. 5, and also in Mem. Nat. Acad., Washington, vol. 8, pp. 83-100. 

 Jackson, R, T, 1899. Localized stages in development in plants and animals. Mem. 



Boston Soc. Nat. Hist., vol. 5, pp. 89-153. 

 LuDWiG, H. 1878. Beitrage zur Anatomic der Ophiuren. Zeits. f. w, Zool., vol. 31, 



pp. 346-394- 



1879. Das Mundskelet der Asterien und Ophiuren, etc. Zeits. f. w. Zool., vol. 



32, pp. 672-688. 



1880. Neue Beitrage zur Anatomic der Ophiuren. Zeits. f. w. Zool., vol. 34, 



PP- 333-365. 



1881. Zur Entwicklungsgeschichte des Ophiurenskelettes. Zeits. f. w. Zool., 



vol. 36, pp. 181-200. 



1899, Jugendformen von Ophiuren. Sitz. d. K. Preuss. Akad. der Wiss. Berlin, 



vol. 14, pp. 210-235, 



Lyman, T. 1882. Challenger Ophiuroidea. Reports on the Zoology of H. M. S. Chal- 

 lenger, vol. 5, pt, 14, pp. 1-386, 



Mortensen, T. 1912, Uber Asteronyx loveni M. Tr. Zeits. f. w. Zool., vol. loi, pp. 

 264-289. 



Simroth, H, 1876-77. Anatomic und Schizogonie der Op/izadM OT>e»5 Sars, Zeits. f. w. 

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Sollas, L B. J., and W. J. 1912. Lapworthura: A typical brittle-star of the Silurian Age, 

 etc, Phil. Trans. (B), vol, 202, pp. 213-232. 



