MONOGRAPH OF THE EXISTING CKINOIDS. 



191 



series of short oblong proximal brachials is very rarely to be distinguished from 

 those following. All the pinnules beyond the oral are of practically the same 

 length and very similar, and all arise at the same stage in the formation of the 

 brachial which bears them. The syzygial pairs are less frequent and much more 

 widely separated, and may even be almost entirely lacking; the long interval be- 

 tween the first and second syzygial pairs so conspicuous in 10-armed species is not 

 evident. 



During development the first pinnules to appear are always the lowest of the 

 elongated distal types and the next the oral pinnules; the intermediate pinnules 

 on the wedge-shaped proximal brachials are the 

 last formed. In Antedon the oral pinnules ap- 

 pear soon after the first formation of the distal 

 pinnules, but in Hathrometra many of the distal 

 pinnules are present before the oral pinnules are 

 formed. 



The comatitlids and the Flexibilm Impin- 

 nata. The comatulid arm proximal to the second 

 syzygy is characterized by (1) the short oblong 

 or wedge-shaped brachials, (2) the absence of 

 syzygies beyond the first syzygial pair (the inter- 

 syzygial interval between the first and second 

 syzygial pairs is the longest on the arm), and (3) 

 the absence of pinnules until after the brachials 

 are completely formed. 



The developing comatulid at the stage when 

 the arms have developed as far as the hypozygal 

 of the second syzygial pair, just beyond which the 

 first pinnules appear (see figs. 1208, 1214, pi. 34, 

 and 1217, pi. 35), presents the following features: 



(1) The brachials are oblong, with their ends at 

 right angles to the longitudinal axis of the arm ; 



(2) all the brachial articulations are of the same 

 nature; (3) arm flexure is effected by the con- FlG " 

 traction of a ventral muscular band continuous 



with that about the mouth, there being no interbrachial muscles; (4) there are no 

 pinnules; (5) the orals are well developed; (6) the radianal has just been excluded 

 from the radial circlet, and the two posterior radials are not yet in contact; (7) 

 there are commonly five interradials ; (8) the basals are large, forming a complete 

 and conspicuous circlet; and (9) the topmost columnal has increased in size. 



In the young comatulid before the appearance of the first pinnules and the 

 cirri there is no difficulty in recognizing a fairly typical representative of the Flexi- 

 bilia Impinnata. 



From this stage onward the course of development of the comatulid suddenly 

 changes: (1) Arm growth increases in rapidity; (2) pinnules develop on the distal- 



