Growth-changes in Brittle Stars. 103 



one that can be considered complete (pi. i, fig. 6); the third has only a 

 rudiment of the upper arm-plate; the second lacks the upper arm-plate 

 altogether; the first lacks both upper and under arm-plates. It is true that 

 the rapidity with which new segments are formed shows more or less 

 diversity in different individuals. Apparently, in the regenerating tip of 

 an adult arm the segments are formed more rapidly and develop much more 

 slowly than in a young arm. As a consequence, the sixth, tenth, and 

 fifteenth segments of such an arm are all in about the same stage of develop- 

 ment. Simroth (1876) has correctly described how the new "arm-buds" 

 arise on the regenerating half of the disk. As soon as they are visible, the 

 terminal plate appears at the tip just above the water-tube. It grows 

 down and around the tube very rapidly and soon the two sides meet and 

 fuse in the mid-ventral line. By the time this process is complete the 

 first pair of side arm-plates is fully formed (pi. i, fig. 7). The first side arm- 

 plates meet broadly both above and below, the under and upper plates 

 being relatively very small on the young joints; but both grow more rapidly 

 than the side arm-plates and slowly but surely force the latter apart. By 

 the time 20 segments are formed, the side arm-plates are nearly separated 

 dorsally and the separation is completed before 25 segments have developed. 

 Ventrally the process is a trifle slower, but it is completed before 30 segments 

 are formed. On the basal part of the arm, therefore, the side arm-plates 

 are completely separated above and below, while the under arm-plates are 

 in contact with their fellows and the upper plates with theirs. 



As regards their relation to the vertebra which they inclose, in the adult 

 arm-segment it can be easily seen that the side arm-plates are in very close 

 conjunction with the alae, fitting more or less neatly into the area between the 

 alar ridge and the parapophysis. The under arm-plate is in contact with 

 and apparently somewhat attached to the aboral and median hypapophyses. 

 The upper arm-plate rests on and is perhaps attached to the epapophyses 

 and even to the protapophysis. 



ARM-SPINES AND TENTACLE-SCALES. 



The first-formed arm-spine is the lowest and no additional spine ever 

 appears normally between it and the tentacle-scale. It ordinarily appears 

 as soon as the side arm-plate is well formed, but is usually lacking from the 

 first segment (i. e., the segment directly adoral to the terminal plate). 

 On the second segment the lowest arm-spines are well marked; they each 

 originate from a separate center of calcification close to the aboral margin 

 of the side arm-plate near its lower end. The second arm-spine is the 

 next dorsal to the lowest; it may appear almost simultaneously with the 

 lowest or it may be wholly lacking on the second segment. Although 

 appearing later, the second arm-spines grow more rapidly than those first 

 developed and are soon distinctly larger. The third arm-spine of each 

 vertical series generally appears on the sixth or seventh segment from the 

 tip; it lies just dorsal to the second, of course. The fourth spine, just dorsal 



