132 BULLETIN 82, UNITED STATES NATIONAL MUSEUM. 



and in the species of those genera of Thalassometrinse in which the arms are 

 relatively short, as for instance Stenometra and Daidalometra. 



The abruptness of the distal termination of the arms seems to bear a more or 

 less definite relation to their relative length, and to have no other significance. 

 Species with very long and slender arms have the arm tips tapering very slowly 

 and the pinnules decreasing in length very gradually, but in species with short and 

 robust arms the arms terminate very abruptly and the terminal pinnules decrease in 

 length very rapidly. A considerable difference in this respect is noticeable be- 

 tween Antedon adriatica and A. petasus within the same genus. 



Even in the Ptilometrinse and Calometridae where the abruptness of the 

 termination of the arms reaches an extreme it is a character assumed at a relatively 

 late stage and is never indicated in the 10-armed young, or in the young of 

 10-armed species before they are at least half grown. 



The posterior arms of the Comasteridse when ungrooved are peculiar in 

 ending definitely in an axillary ossicle which bears two distal pinnules, one on 

 either side (fig. 1034, pi. 12, and part 1, fig. 47, p. 81). 



PINNULES. 



General features. 



The pinnules of the comatulids are of three types, each type occupying a 

 definite location on the arm and being, in its perfected form, entirely distinct 

 from the other two (figs. 281, 282, p. 215, and part 1, fig. 1, p. 60). The first 

 type is usually somewhat sharply differentiated from the second, though in many 

 of the higher forms there is more or less intergradation. The second type always 

 changes more or less gradually over into the third. 



These three types are: 



(1) The Oral or Proximal Pinnules; these occur on the first two to twelve 

 brachials (figs. 2G2-273, p. 207; 274, 277-280, p. 213; figs. 281-287, p. 215), and their 

 function is to serve as tactile organs (fig. 277, p. 213), or to protect the disk (figs. 

 270, 271, 273, p. 207). 



(2) The Genital or Middle Pinnules; these follow the oral pinnules and bear 

 the gonads (figs. 281 282 p. 215). 



(3) The Distal Pinnules; these occupy the outer part of the arm and serve 

 chiefly as food collecting organs (figs. 281, 282, p. 215). 



In life the oral pinnules, when flexible, are bent forward and downward, so 

 that they form a sensitive network over the disk (fig. 1044, pi. 12). If rigid they 

 extend inward and more or less upward so that the disk is roofed over by a strong 

 network from which sharp points protrude. The oral pinnules on each arm are 

 always parallel to each other, and only move in a plane parallel to the dorso- 

 ventral plane of the arm. 



The lower genital pinnules on either side of the arm are in parallel planes 

 and extend outward more or less at right angles to the arm with their distal 

 portion usually curved broadly upward toward the arm tip, but the succeeding 

 genital pinnules begin to lean outward, and this step by step increases until the 



