Growth-changes in Brittle Stars. 109 



which has 40 arm-segments, the side arm-plates are still distinctly in contact 

 even in the first basal segment. In this particular character, then, Amphi- 

 pholis is more primitive or youthful than Ophiactis. 



ARM-SPINES AND TENTACLE-SCALES. 



One of the most peculiar features of the development of Amphipholis 

 is the fact that it is not the lowest arm-spine which appears first, but the 

 next to the lowest. So unwilling was I to believe this that it was only after 

 repeated observations, on many specimens, that I satisfied myself there 

 is no doubt about the fact. Development at the tip of the arm is so rapid 

 that each side arm-plate of the newest segment usually bears two spines 

 before a new segment begins to form ; this first segment may also have the 

 rudiment of an under arm-plate. If one examines this distalmost segment 

 when the side arm-plates have just met in the midventral line, the rudiment 

 of an arm-spine will be found on the aboral margin of each side arm-plate. 

 Almost immediately a second rudiment appears helow (ventral to) the first, 

 so that there are two rudimentary spines on each side. They remain of 

 about equal length for some time, but the upper is the stouter and more 

 thorny (pi. 3, fig. 11). On the third segment they are of about equal 

 length, but on the fourth the lower is longer. By the time 8 segments are 

 formed a third spine appears on each side arm-plate above the other two. 

 This rapidly becomes the longest of the three, while the middle one is the 

 stoutest and roughest. By the time 25 segments are complete a fourth spine 

 dorsal to the third may arise, but it does not become noticeably longer than 

 the third and is generally shorter. The development of the "second" 

 spine before the lowest is so contrary to a priori expectations that some 

 explanation is necessary. It is true that in its early appearance (pi. 3, 

 fig. 11), it resembles the lowest arm-spine of Ophiactis (pi. 3, fig. 13) and it is 

 possible to consider the two homologous, since each is the first spine formed 

 on the segment. But in that case the lowest arm-spine of Amphipholis 

 requires explanation, for there is nothing homologous to it in Ophiactis. 

 A preferable explanation seems to me to be the suggestion that we have 

 here an extraordinary case of acceleration in development. We have 

 already seen in Ophiactis that after the "second" spine is formed it grows 

 more rapidly than the lowest and soon exceeds it in size. Now, the whole 

 appearance of young Amphipholis shows that its development within the 

 mother's body has led to an acceleration which is most noticeable at the tips 

 of the arms, although marked on the disk. Is it not possible that accelera- 

 tion in the development of the arm-spines has led the more-rapidly growing 

 "second" spine to appear before the lowest? Such a suggestion seems to 

 me well within the bounds of possibility. In any case, however, neither the 

 lowest nor the second arm-spine develops any unusual hooks or teeth or 

 assumes any particularly characteristic form. 



Although the basal joints of the arm show two tentacle-scales on each 

 side, one associated with the under arm-plate and one with the side arm- 



