120 Papers from the Marine Biological Laboratory at Tortugas. 



plainly and satisfactorily as if a large series of young specimens were avail- 

 able. Obviously this is not true of the disk. I would emphasize, therefore, 

 as the first conclusion of these studies, this truth of the very great importance 

 of localized stages in the distal part of the ophiuran arm. The other con- 

 clusions which follow are taken directly from Ludwig's very valuable paper 

 "Jugendformen von Ophiuren" (1899), supplemented by Mortensen's 

 observations on Aster onyx (191 2) and by my own work. The following list 

 shows the species of brittle-stars, the growth-changes of which have been 

 more or less fully studied : 



OphiacHs asperula (Philippi). Amphiura magellanica Ljungman. 



kroyeri Liitken. Ophiacantha vivipara Ljungman. 



savignyi (Miiller & Troschel). Ophiothrix angulata (Say). 



Amphipholis patagonica Ljungman. Ophiomyxa vivipara Studer. 



squamata (Delle Chiaje). Asteronyx loveni Miiller & Troschel. 



This list strikingly demonstrates the paucity of facts on which to base 

 a classification of brittle-stars. We have a fair knowledge concerning the 

 growth-changes of 10 of the species (out of approximately 1,100!), but these 

 represent only 7 genera and perhaps 5 families. It may be added that a 

 considerable number of facts are recorded concerning changes during 

 development from young to adult of several euryalids {Euryale, Gorgofio- 

 cephalus), but in no one species has any series of changes been followed. 



The following conclusions, a summary of our knowledge to date, will 

 serve as a basis for further work and may prove useful in future attempts to 

 properly locate doubtful genera and difficult species. Continued study of 

 growth-changes will ultimately lead to a natural classification of ophiurans. 



1 . The terminal segments of the arm are the youngest ; passing from the 



terminal plate towards the disk, each succeeding segment is older. 

 The segments in their individual development pass through the 

 same stages that the arm has passed through in its development and 

 therefore the terminal portion of the arm is made up of localized 

 stages, to use Jackson's (1899) expressive term, which reveal the 

 developmental or ontogenetic stages of the arm as a whole. The 

 study of the terminal portion of the arm in great detail is necessary 

 therefore to the proper understanding of the relationships of any 

 brittle-star. 



2. The original disk-covering in ophiurans consists of a central and 



5 radial plates; interradial plates are a somewhat later addition r 

 the radial shields still later; further development of both radial 

 and interradial plates between the primary plates is a more ad- 

 vanced stage ; the development of spinules and spines in connection 

 with the disk-plates is further evidence of specialization; and the 

 resorption of the disk-plates or their concealment by thick skin 

 or by a coat of granules must be regarded as still more advanced 

 stages in the development of disk-coverings. 



3. The radial shields are originally small and comparable to any of the 



other secondary disk-plates. Their development to a large size 

 is evidence of specialization. The larger the radial shields in 

 proportion to the other disk-plates, the more specialized is the disk- 

 covering, other factors (such as granule formation) being equal. 



