MONOGRAPH OF THE EXISTING CRINOIDS. 



125 



In Heterometra and Craspedometra the phylogenetic sequence of the arms 

 appears to be reversed, and this coincides with the frequent occurrence in these 

 types of additional division series on the inner derivatives from each IIBr series. 



While each of the arms in a comatulid is an exact counterpart of another arm 

 on the same ray, and the arms are therefore paired appendages in the strictest 

 sense of the word, they are not paired, except for the two arising from the IBr 

 series, with the arms on the same axillaries. 



Ontoyenetwal, phylogenetical. and morphological sequence of the first brochial pairs and tlieir redupli- 

 cations. 



The simplest arm structure found among the comatulids occurs in the genera 

 Thaumatocrinus, Pentametrocrinus, and Atopocrinus, in which the first brachial 

 pair immediately follows the radials and the 

 succeeding brachials are arranged in a linear 

 series. 



Though this arm structure is theoreti- 

 cally the simplest, it is probably not the most 

 primitive, for in no other genera of the 

 Articulata does it recur, though many of 

 these genera are undeniably more primitive 

 than Thaumatocrimis, Pentametrocrinus^ and 

 Atopocrinus in all other ways. We have 

 here apparently a good example of simplifi- 

 cation through specialization. 



The most primitive arm structure is 

 probably that of the 10-armed comatulids, 

 or more properly of those species in which 

 a IBr series is present, which is characteristic 

 not only of the majority of all the Articulata 

 but also of the young of all those which are 

 multibrachiate when adult. The funda- 

 mental significance of the presence of the 

 IBr series is further emphasized by the fre- 

 quency of its occurrence in the fossil types 

 not represented in the recent seas, especially 



,, IT,, i-i- FIG. 197. LATERAL VIEW OF TIPE SPECIMEN OP 



in the *lexibllia. MARIAMETRA MARGAEITIFEEA. 



From the standpoint of the crinoids as 



a whole, or of the Articulata as a whole, the occurrence of additional division series 

 of itself represents no phylogenetical advance, whatever may be its significance 

 within limited groups; the presence of one division series is as fundamental as 

 the presence of many. But the presence of additional division series is correlated 

 with the extreme atrophy of the calyx plates and the consequent relative increase 

 in the size of the visceral mass, both of which have an important phylogenetical 

 bearing. 



