140 BULLETIN 82, UNITED STATES NATIONAL MUSEUM. 



In Erythrometra of the Perometrinae and in C'yllometra, Colobometra, Pro- 

 metra, and Decametra of the Colobometridse P is always absent. It is always 

 absent in Perometra diomedea:, but not in the other species of that genus. It is 

 frequently absent in the species of Cotylometra and occasionally in certain species 

 of Oligometra. In most cases its absence seems to be related to a close crowding 

 of the arm bases. 



Oral of proximal pinnules. 



The oral pinnules vary from 1 to 12 in number, but most commonly there 

 are from 2 to 6; that is, from 1 to 3 pairs. They occur upon the second and 

 fourth brachials and sometimes upon one or more of those following. When 

 division series of four ossicles arc present (figs. 123, p. 79; 171, p. 86; and 

 part 1, fig. 81, p. 134), or when the second element of the IBr series is not 

 axillary (fig. 189, p. 109, an part 1, fig. 83, p. 136), the pinnule of the second 

 brachial is repeated upon each reduplication of this ossicle. When the division 

 series consist of three ossicles, as in the genera Capillaster (fig. 181, p. 94) and 

 Nemaster, the pinnule of the second brachial is always repeated upon the first 

 of these. 



Apparently the oral pinnules are morphologically associated with the first 

 brachial and first syzygial (second brachial) pairs of ossicles in the free undivided 

 arms, and we find them exclusively so associated in most of the Macrophreata. 

 But in a few of the most highly specialized macrophreate forms (figs. 292-294, 

 301, p. 221) and in most of the Oligophreata (figs. 268-273, p. 207) the pinnules 

 of a few of the brachials immediately following the first syzygial pair have 

 become oral in character. 



The oral pinnules are differentiated from all the other pinnules of the arm 

 by the absence of the ambulacral groove and associated structures, combined with 

 a marked specialization whereby they have become transformed into tactile organs 

 (figs. 277, p. 213, and 1044, pi. 12). or into organs of defense (figs. 270. 271, 273, 

 p. 207). They take no part in the collection of food, nor do they ever bear gonads. 



In the Macrophreata the oral pinnules are almost invariably of a single type 

 (figs. 289, 290, 292-296. 298, p. 221), and usually there are only two of them, 

 P t and P , sometimes only one. Pj (fig. 289, p. 221). They are commonly much 

 elongated with very numerous segments, of which the earlier are more or less 

 beveled on their articular faces, in dorsal view cut away at the corners, thus 

 giving great flexibility, and serve as tactile organs. In life they are bent down- 

 ward, forming a close and sensitive network over the disk. If one of them 

 be touched the arms immediately curl up, protecting the disk still further. 



The extreme specialization of the first pair of pinnules in the direction of 

 purely tactile organs is seen in the genera Hathrometra (fig. 289, p. 221), Tricho- 

 metra, Compsometra (fig. 296. p. 221, and part 1, fig. 107, p. 173), and Antedon 

 (figs. 1040, 1044, pi. 12, and part 1, figs. 103, p. 165; 104, p. 167; 105, p. 169; and 

 106, p. 171), as well as in the oligophreate genus Pontiometra. In these genera 

 they are from two to eight times as long as the following pinnules and exceedingly 

 flexible, with from 20 to nearly 100 segments. Essentially the same conditions 



