Growth-changes in Brittle Stars. I23 



15 The under arm-plate arises very soon after the formation of the side 

 arm-plates and may even precede the formation of the vertebral 

 rudiments. It is at first small and lies on the oral side of the arm, 

 in the median line just distal to the side arm-plates. It may grow 

 backward (adorally) sufficiently to completely separate the side 

 arm-plates and it then comes in contact with its fellow on the 

 succeeding segment. In such cases the under arm-plates are more 

 or less clearly in contact throughout the entire length of the arm. 

 There is no doubt that small, widely separated under arm-p ates 

 are the simple, primitive arrangement and large under arm-plates 

 in contact with each other indicate specialization. The absence 

 of under arm-plates or their occurrence in irregular series and of 

 diverse sizes are certainly indicative of specialized conditions. 



16. The upper arm-plates are the last of the arm-plates to arise, but they 



appear usually not long after the under arm-plates and follow a 

 similar line of development. Small, widely separated upper arm- 

 plates are undoubtedly a primitive condition, while large upper 

 arm-plates, in contact with each other and separating the side arm- 

 plates, are certainly specialized. On the other hand, absence of 

 upper arm-plates, if associated with large side arm-plates broadly in 

 contact in the mid-dorsal line, shows a primitive type of structure, 

 while absence of upper arm-plates if associated with widely sepa- 

 rated side arm-plates is probably a highly specialized condition. 



17. The lowest arm-spine is the first to appear, as a rule, though the one 



just above it may accompany or, in very rare cases, slightly pre- 

 cede it. Succeeding spines arise dorsal to the earlier-formed and 

 the number increases slowly with the age of the segment until the 

 number characteristic of the species is attained. This number is 

 commonly to be found on about the tenth segment from the mouth 

 and thence distally for a variable number of segments. On the 

 basal arm-segments the number is always fewer than normal, but 

 whether the influence of the disk has promoted resorption or pre- 

 vented formation of the full number has not been ascertained. 



18 The hook-shaped form of the young lowest arm-spine is not character- 

 istic of any particular family of brittle-stars, but its persistence 

 and perfecting on the mature segments of the middle arm may 

 be a family or at least a generic character. Young spines arise 

 as straight rods, but soon become hooked or give rise to tooth- 

 like projections. They usually acquire a secondary straightness. 

 Young spines are solid, but may become hollow with increasing 

 size. Hollow spines undoubtedly indicate a specialized condition. 



19. The tentacle-scales may arise as early as the under arm-plate and 

 before the lowest arm-spine, or their appearance may be delayed 

 until after several spines have been developed. Each tentacle- 

 scale is at first flat and rounded and is associated with the ventral, 

 distal margin of the side arm-plate. It may be homologous with 

 an arm-spine, but its position ventral to the lowest arm-spine counts 

 against that view. The evidence is as yet inconclusive whether 

 the absence of tentacle-scales is a primitive condition or not. 

 When two tentacle-scales are present, the second one arises in con- 

 nection with the under arm-plate a little later than the first one. 

 Its occurrence is almost certainly a specialized condition and it 

 can not be homologized with an arm-spine. A spine-like tentacle- 

 scale is certainly a modified form. 



