154 buij:.etin 82, united states national museum. 



same manner that the transition from the oral to the genital pinnules takes place. 

 The first indication is an elongation of the distal segments and an increase in 

 their number, due jjartly to the addition of new segments and partly to the with- 

 drawal of the gonad toward the base of the pinnule, which leaves the outer 

 segments free. The segments supporting the gonads are not altered until the 

 gonads are withdrawn from them. In the outermost genital pinnules, therefore, 

 where the gonad rests only upon the third and fourth segments, these are exactly 

 as in tlie preceding pinnules, even though all of the segments beyond them are of 

 the elongate type characteristic of the distal pinnules. 



Owing to their comparatively uniform structure in all comatulids the genital 

 pinnules are generally of but slight systematic importance. They are useful in 

 differentiating genera or species in groups in which they are more or less swollen 

 to form a covering for the gonads, or where they are much elongated, but other^vise 

 their systematic significance is small. 



In life the lower genital pinnules stand up in planes parallel to the plane 

 passing through the dorsoventral axis of the arm, but farther out they gradually 

 lean outward so that the outermost genital pinnules are normally in a plane almost 

 or quite at right angles to that of the earlier ones and extend horizontally outward 

 from the arms. 



Distal pinnnleti. 



Whereas the oral pinnules vary widely in the amount and nature of their 

 specialization in the various comatulid groups, and often to a very considerable 

 extent among the several species of the same group, there is comparatively little 

 variation in the genital pinnules and very much less in the distal (figs. 339, 

 342-350, p. 229). 



The oral pinnules are specialized as organs of touch or of defense, or as 

 both combined, and the kind and amount of specialization varies according to 

 whether the calyx is broad and open or closed and compressed, and in accordance 

 with numerous other more obscure correlations. 



The genital pinnules serve to contain the genital products and have the same 

 functions everywhere. Their degree of differentiation is therefore comparatively 

 slight, being almost entirely confined to a variation in the lateral diameter of the 

 third and following segments. 



The distal pinnules serve to gather the food particles, a function which may 

 to a great extent or lesser degree be assumed by the more distal genital pinnules. 

 In all groups the interception and capture of food particles involve similar 

 mechanical and physiological problems, requiring a maximum of length com- 

 bined with a maximum of exposed ambulacral surface ; and the fulfillment of these 

 two conditions, both of the utmost importance in the economy of the animals, 

 necessarily leaves but very little scope for variation from a general type, and thus 

 it is that the distal pinnules are very nearly uniform throughout all the comatulid 

 groups. 



The oral pinnules stand up parallel to the dorsoventral plane of the arm. 

 distally bending forward more or le.ss over the disk, more rarelv outward toward 



