Growth-changes in Brittle Stars. 119 



smaller) arise in double and then in triple series ventral to the 

 teeth. The torus expands correspondingly, and in maturity the 

 lower dental papillae may be in irregular quadruple or quintuple 

 series, though the lowest series are triple or only double, like the 

 highest. Resorption of the torus occurs back of the largest dental 

 papillae just as it does back of the teeth. 



6. The vertebrae are remarkable for the high, narrow protapophysis 



anteriorly and the large zygantrum and very small epanapophysis 

 posteriorly. The chief articulation between two vertebrae is 

 formed by the protapophysis of one and the zygantrum of its distal 

 fellow. Other peculiarities of the mature vertebrae are a pair of 

 zygocceles above the zygosphene and a small, round knob, the 

 zygophore, in the zygotreme. The zygapophyses and adoral 

 hypapophyses are well developed. 



7. The side arm-plates are separated throughout the whole length of the 



arm, even in the first segment, by the under arm-plates, so that 

 ventrally the two side arm-plates of any given segment are never 

 in contact. Dorsally those of the first 15 or 20 segments do meet 

 each other in the mid-dorsal line, but thereafter the upper arm- 

 plates keep them apart. The spine-bearing ridges of the side arm- 

 plates are quite prominent, especially on the older segments. 



8. The lowest arm-spine, the first to arise, begins as a straight, slender 



rod, but rapidly develops into a characteristic 3-toothed hook. 

 At the base of the arm, by a process of senescence, it is gradually 

 transformed into a nearly straight, rough, fenestrated spine like 

 those dorsal to it. 



9. There are only 4 (distally fewer) arm-spines on the first 40 segments and 



the full number for the species (6 or 7) does not appear until 60 

 or 75 segments are complete. 

 10. The tentacle-scales are reduced to mere spinules associated with the 

 basal part of the side arm-plate. They do not appear until after 

 6 or 7 segments are formed and they never seem to be of any 

 functional importance. 



SUMMARY AND CONCLUSIONS. 



Although references to changes in appearance, or in details of structure, 

 during the transition from young to adult brittle-star are to be found in 

 many of the writings of Lyman, Lutken, Verrill, Grieg, Koehler, de Loriol, 

 and other less well-known workers, no attention to the sequence or signifi- 

 cance of these changes has been paid, up to the present time, save by 

 Ludwig and Mortensen. And although it has been a generally accepted 

 and well-known fact that the terminal segments of the adult ophiuran arm 

 are the youngest, no use has been made of the important corollary that these 

 segments reveal the developmental stages through which the arm has 

 passed. The present report has endeavored to make full use of this corollary 

 in studying the development of Ophiactis, Amphipholis, and Ophiothrix. Its 

 great value lies, of course, in the fact that even where only a single specimen 

 of a species is available, and that specimen adult, if a single arm is complete 

 to its terminal plate the development of that species throughout its post- 

 larval growth, so far as the arms are concerned, can be read almost as 



