296 BULLETIN 82, UNITED STATES NATIONAL MUSEUM. 



indeterniinato numlxT, sometimes as niiiny as an additional third, which are merely 

 the result of lessened vitality, causing an inliibition of the power to limit further 

 vegetative growth and to develop to maturity instead the segments already formed. 



Care must always be taken in working with the comatuUds to differentiate 

 these more or less rudimentary marginal cirri, wliich are usually longer and more 

 slender than the tme mature cirri and have additional segments, from the cirri 

 which are properly characteristic of the adult. 



These cirri are pccuUar in that they never perform any grasping functions, nor 

 do they appear ever to become curved distally, remaining always nearly or quite 

 straight. Tliey usually extend directly upward between the arms, reaching for 

 some distance beyond the tips of the oral pinnules. They appear to function as 

 tactile organs, assisting the oral pinnules, and in their tactile nature, as well as their 

 tapering build and polyarticular, yet practically undifferentiated, composition, 

 strongly suggest the antennae of insects, a similarity wliich is heightened by the fact 

 that, like antennae, they are developed at the anterior or proximal end of the series 

 of segmented appendages. 



The systematic significance of the cirri varies very greatly in the different 

 groups. One family (Colobometridse) is most easily recognized by the peculiarities 

 of the cirri, many genera find in these organs their most obvious distinguishing 

 characters, while specific determination rests largely upon their proportionate 

 length and comparative structure. In fact, taken as a whole, the cirri are of para- 

 mount importance from a systematic point of view, exceeding in the number, 

 variety and stabiUty of the characters presented even the proximal pinnules whicli, 

 however, are a close second. 



In general the cirri of the Comasteridae, Zygometridae and Mariametridas are 

 more or less strictly comparable to those of the Thalassometridae; the charitometrid 

 type is seen in Endiocrinus (fig. 84, p. 137), Comactinia (figs. 76, p. 129, and 328, 

 p. 281), Catoptometra (fig. 334,p.283), Comatula (figs. 78, p.l31, and 327, p. 281) and 

 Comatulides (fig. 80, p. 133) ; while the thalassometrid tj-jje prevaUs in Leptoncmaster 

 (fig. 325, p. 279), Comissia, Capillaster (fig. 323, p. 277), Nemaster (fig. 324, p. 279), 

 Palseocomatella, Comatella (fig. 321, p. 277), Neocomatella, Comatulella and in 

 nearly all of the species of Comanthus and of Comaster, as well as in Zygo^netra 

 (figs. 332, 333, p. 283), Poniiometra (fig. 341, p. 287), and Epimetra. The cirri of 

 Comatilia and of Microcomatvia are so very slender as to resemble most closely those 

 of the small antedonids, especially Iridometra and Compsometra. 



Usually in the Thalassometridte the production of the distal edges of the cirrus 

 segments as seen in those immediately^ following the transition segment is abrupt 

 and has a smooth sharp outer border, in an end view projecting from the general 

 profile of the segment in the form of a broad and flattened U ; distally this gradually 

 narrows (coincident with the increasing doi-sal carination of the segments), becoming 

 progressively more and more V-shaped, finally resolving itself into a carinate dorsal 

 spine. In the groups now under consideration, however, a slightly different 

 condition exists (fig. 323, p. 277) ; in the earher segments following the transition 

 segment the production of the distal dorsal edge is in dorsal view broadly U- 

 shaped, and in end ^dew appears as a low rounded serrate transverse ridge. The 



