200 BUIAETIN 82, UNITED STATES NATIONAL MUSEUM. 



in the division series and represent the projection into the arm proper of the 

 longitudinal pairs of the arm bases. 



The growth of the crinoid arm may be described as consisting of the for- 

 mation of continuous series of pairs of similar ambulacral plates, the elements 

 of each pair lying side by side; one of these develops rapidly and soon gives rise 

 to another pair, while the other slowly grows into a pinnule. 



Thus it appears that the comatulid brachials are each fundamentally an 

 axillary, comparable to the axillaries in the di\dsion series, and the elongated 

 linear series of brachials in the free arms is in reality a series of axillaries, each 

 of which bears a pinnule and the continuation of the arm (another axillary). 

 Whereas in the division series successive pairs of ossicles, one distal to the other, are 

 produced, of which the outer element always carries a pair of exactly similar 

 elements, both of which may develop into arms, or one into an arm and the other 

 into a pimiule, in the arms the longitudinally arranged pairs are relatively rare, 

 and in the transverse pairs one of the elements always develops into an arm, the 

 other into a pinnule. 



Since the free arms of the Flexibilia are represented in their entiety by the 

 pinnules of the comatulids, and the comatulid brachials are in reality axillaries 

 comparable to the axillaries in the division series, the comatulid arm is composed 

 of a linear series of ossicles, each homologous with the last axillary in the Flexi- 

 bilia and each bearing on one side a similar ossicle and on the other a complete 

 arm of the Flexibilia type. 



In certain species this arrangement has been carried over into the perfected 

 comatulid arm, and we find arising from each IBr axillary a series of division 

 series, each of which bears on one side an arm and on the other another 

 division series, so that a long series of ossicles results, of which every second one 

 gives off an arm on alternate sides (figs. 148, 150, p. 83) or on the same side 

 (figs. 156, p. 83, and 134, p. 79). 



In the echinoids and starfishes the ambulacral plates are always in two 

 columns, which are always sharply separated from each other in the midradial 

 lines which represent the intersegmental boundaries. But in the crinoids the 

 midradial regions carry the arms. 



The position of the anus, water pores (especially when only five in number, 

 as in the young comatulids and in Rhizocrinus), and primary nerve trunks, all 

 interradially situated, shows that in the crinoids the intersegmental dividing lines 

 fall in the middle of the radial areas as in the echinoids and starfishes. 



The chief characteristic of the series of ambulacral ossicles in the crinoids is 

 the doubling of all the elements, usually transversely across the mid- ambulacral 

 line, sometimes longitudinally, and the pairing of the ambulacral appendages on 

 either side of the mid-line. This doubling is especially evident in those crinoids 

 in which the arms are biserial — a condition toward which there is a greater or 

 lesser amount of reversion in the comatulid arm beyond the proximal oblong 

 brachials, where the following brachials for a variable distance are triangular. 



In the comatulids only the pinnules show no indication whatever of a doubling 

 of their component segments. The pinnules represent the arms of the Flexibilia. 



