258 BULLETIN 82, UNITED STATES NATIONAL MUSEUM. 



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

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

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

 scattered and somewhat irregular or very closely crowded and regular. The 

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

 number which lie in a single row under each radial. 



The relationship of the chief types of centrodorsal to the larger systematic 

 groups is briefly shown 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 are in columns, three or more to each radial area, 

 but the 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 are in three columns in each radial area. 



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

 sharply differentiated into radial areas, the cirrus sockets arc in two columns in 

 each radial area. 



Comasteridse, Zygometridas, Ilimerometridse, Stephanometridse, Mariametridse, 



Colobometridee, Tropiometridae, Calometridse, Pentametrocrinidae A 



Thalassometridae (greatest emphasis at D) B-D 



Charitometridas (greatest emphasis at B C) A-C 



Antedonidoe (greatest emphasis at A). A-D 



Atelecrinidse C-D 



Cirri. 



The cirri which among the comatulids are organs of the very greatest impor- 

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

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

 ence of the 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 then- 

 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 the 

 systematic standpoint. 



Comatulids living among abundant arborescent growths which are flexible or 

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

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

 amount of dorsoventral flexion (figs. 306, 307, p. 265); such cirri are seen, in a more 

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

 Comactinia, Co-master, Comanthus, Zygometra, Eudiocrinus, Catoptometra, Amphi- 

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

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

 metridae and of Antedoninae, Pentametrocrinus and Atelecrinus. 



