Growth-changes in Brittle Stars. 117 



beside each other, the progress of development in the two genera is quite 

 unlike. There is at no time an "Ophiactis stage" in the development of 

 the Ophiothrix vertebra. No other fact noted has so strongly impressed 

 me with the radical difference between Ophiothrix and the other two genera 

 studied. 



ARM-PLATES. 



In the development of the arm-plates, Ophiothrix shows a striking 

 difference from Ophiactis and Amphipholis. As in those genera, the side 

 arm-plates are the first to appear and are almost immediately followed by 

 the under arm-plate, but in Ophiothrix the growth of the under arm-plate 

 is so rapid and vigorous that the side arm-plates never meet ventrally, but 

 are from the first kept apart by the under arm-plates. The latter are in 

 contact with each other throughout the entire length of the arm. The 

 upper arm-plates, on the contrary, are very small near the tip of the arm 

 and it is not until 15 or 20 segments have been formed that they are suffi- 

 ciently large to separate wholly the side arm-plates. Even at the base of 

 the arm, the basal (adoral) end of the upper arm-plates is so narrow that 

 the side arm-plates of any one segment are not widely separated from each 

 other as they are ventrally. The spine-bearing ridge of the side arm-plates 

 is very well developed, so that the arms appear to be constricted between 

 them, and this appearance is accentuated by the fact that the broader, 

 distal end of each upper arm-plate is somewhat swollen. The side arm- 

 plates, like the vertebrae, are relatively very much longer distally than near 

 the base of the arm. 



ARM-SPINES AND TENTACLE-SCALES. 



In ophiothrix, as in Ophiactis, the lowest arm-spine is the first to appear. 

 It is, on the first segment, a simple, smooth, straight rod with an enlarged 

 base (pi. 3, fig. i), but by the time a second segment has formed it begins 

 to curve at the tip and becomes broader and somewhat flattened (pi. 3, 

 fig. 2). On the third segment it is distinctly curved at the tip and a tooth 

 is evident on the concave side (pi. 3, fig. 3). On the fourth segment this 

 tooth has become conspicuous and another one has arisen below it {i, e., 

 nearer the base of the spine), while the spine-base is notably large and 

 projecting (pi. 3, fig. 4). On the sixth segment these various features are 

 all accentuated (pi. 3, fig. 5) and on succeeding segments the spine may be 

 said to show its characteristic form (pi. 3, fig. 6). As the animal matures, 

 these lowest arm-spines begin to undergo a sort of retrograde change which 

 well illustrates Hyatt's principle of senescence. Thus if the lowest arm- 

 spine on the twentieth or thirtieth segment from the mouth be examined, 

 on an adult arm, it will be found essentially like that on the twentieth 

 segment from the tip. But if one from somewhere about the twelfth basal 

 segment be looked at (pi. 3, fig. 7), it will be found to be obviously different 

 from any of the lowest arm-spines of the middle portion of the arm ; except 

 that it is immensely stouter, it is more like that of the fourth segment 



