258 BULLETIN 82, UNITED STATES NATIONAL MUSEUM. 



lono' rounded conical; (2) in tho number of the cirrus sockets, which varies from 

 15 to 100 or more; (3) in the proportionate size of tho cutus sockets; and (4) in 

 their mutual arrangement and regularity, whether or not they are more or less 

 scattered and somewhat h-regidar or very closely crowded and regular. The 

 proportionate size of tho cirrus sockets is most conveniently judged from the 

 number which lio in a single row under each radial. 



The relationship of tho chief t^-pes of centrodorsal to the larger systematic 

 groups is briefly showTi in the following table: 



A. The primitive type of centrodorsal. 



B. Thick discoidal or columnar centrodorsals, tending to become more or 

 less conical; the cirrus sockets aro in columns, three or more to each radial area, 

 but tho radial areas are not marked off from each other. 



C. Columnar or conical centrodorsals, with the surface distinctly marked off 

 into radial areas; the cirrus sockets aro in three columns in each radial area. 



D. Columnar or conical centrodorsals, much reduced in size; the surface is 

 sharply differentiatod into radial areas, the cirrus sockets aro in two columns in 

 each radial area. 



Comastcridae, Zygometridse, Himerometridaj, Stophanometridse, Mariametridae, 



Colobometridse, Tropiometridse, Calometridse, Pentametrocrinidae. -A 



ThalassometridjB (greatest emphasis at D) B-D 



Charitometridic (greatest emphasis at B-C) A-C 



Antcdonida?, (greatest emphasis at A) A-D 



Atelecrinidas C-D 



Cirri. 



Tho cirri — which among the comatulids aro organs of the very greatest impor- 

 tance in serxdng to attach the animals to tho sea bottom or to various organisms on 

 the sea bottom, and thus to hold them fast, enabhng them to withstand tho influ- 

 ence of tho motion of the water and of the movement of active animals in the 

 immediate vicinity, such as fish, which would tend to wrench them from their 

 position, and at the same time to keep them in a definite more or less upright 

 attitude, so as to insure a regular supply of food — in this group assume the most 

 extraordinary diversity of form and size, more or less in correlation with wide 

 differences in habit, and furnish data of the very greatest importance from tho 

 systematic standpoint. 



Comatulids living among abundant arborescent growtlis wliicli are flexible or 

 semirigid, such as hydroids and gorgonians, tend to develop short stout curi with 

 comparatively short more or less subequal segments which are capable of a great 

 amount of dorsoventral ilexion (figs. 306, 307, p. 265); such cirri aro sceji, iu a more 

 or less perfected form, in part or all of the species of the genera Comissia, Comatulella, 

 Comactinia, Comaster, Comantlxus, Zygometra, Evdiocrinus, Catoi)tometra, Amphi- 

 metra, Dichrometra, Liparometra, Lamprometra, Cenometra, Cyllometra, Decametra, 

 Prometra, Oligometra, Tropiometra, Neometra, Pectinometra, various genera of Chaiito- 

 metridsB and of Antedoninae, Pentametrocrinus and Atelecnnus. 



