266 BULLETIN 82, UNITED STATES NATIONAL MUSEUM. 



species with much shorter cirri which are stouter and composed of much shorter 

 segments, Capillaster sentosa, C. multiradiata, Comatula purpurea, Comaster fruti- 

 cosus, Amphimetra producta, Oxymetra finschii and Dichrometra flagellata. 



Gravelly bottoms tend to induce a type of cirrus which is more or less inter- 

 mediate between the rocky and muddy bottom types, and is illustrated by the 

 cirri of the species of Promachocrinus, Heliometra, Anthometra, Florometra and 

 Solanometra. 



As a general rule species living on muddy bottoms have extremely fragile 

 cirri which drop off at the slightest touch; the cirri of the species living on gravel 

 bottoms are almost as delicate; but the cirri of the species which live attached 

 to inorganic masses or to the inorganic rigid skeletons of marine growths, and 

 especially the cirri of the species which live attached to flexible marine growths, 

 are very tenacious. 



On the basis of a broad average it may be stated that the littoral species have 

 the most tenacious cirri, while the cirri of the deep-water forms are the most fragile. 



Though the cirri are ordinarily employed solely as organs of prehension, they 

 are capable of use as swimming organs, for the young of Iridometra nana has been 

 observed to float through the water with motionless extended arms, propelled by 

 the very rapid movements of the cirri. 



The Innatantes, being pelagic and not having developed stems, never possess 

 cirri at any stage. In the Oligophreata and in the Macroplireata, however, cirri 

 are invariably present, in the latter always throughout life, and hi the former 

 usually throughout life but invariably hi the young, the family Comasteridse 

 only containing species lacking cirri when adult, though the majority of its species 

 are provided with them. In the genus Capillaster alone of the nine genera of the 

 Capillasterinse a species is found which loses its cirri when adult, these organs being 

 very highly developed in the other six species included in that genus; in the Com- 

 actiniinse ComatuleUa, Comatulides and Comactinia always have strongly developed 

 cirri, but four of the nine species of Comatula have no cirri when fully grown, while 

 they are normally greatly reduced ha number in one, and occasionally quite absent 

 hi very large specimens of another, of the remaining four. In young examples of 

 these four forms which more or less normally lack the cirri, however, they are 

 comparatively large and stout. In both the genera of the Comasterinas the cirri 

 are frequently absent, either as a specific character or through individual variation, 

 and in some of the species they appear to be lost at a very early age. Ah" grada- 

 tions are observable between such forms as Comaster typica and Comantheria polyc- 

 nemis in which the centrodorsal is typically exceedingly reduced and sharply 

 stellate, countersunk to or even below the level of the radials, with never the 

 slightest trace of cirri, and such forms as Comaster multibrachiata and ComantJius 

 bennetti in which the cirri are extraordinarily large, stout, numerous, and well 

 developed; some species, like Comanthus annulata, usually possessing cirri but 

 occasionally being found without them ; others, like Comanthina schlegelii or Comaster 

 'belli, usually lacking cirri but sometimes occurring with from one or two to as many 

 as twenty, which are large and show no trace whatever of degeneration, still 

 remaining. 



