MONOGRAPH OF THE EXISTING CKINOIDS. 135 



If the third and following pinnulars have strongly projecting or spinous distal 

 ends or a spinous dorsal surface, or if they are greatly elongated, or greatly attenu- 

 ated, or very short, or even if they are sharply triangular with a knifelike or 

 spinous dorsal keel, the first two pinnule segments always remain practically 

 unchanged. Their correlation is not with the succeeding pinnulars, but with the 

 brachials, and therefore any variation in them is quite independent of the condi- 

 tions in the other segments. 



The third and following pinnulars gradually increase in length, at the same 

 time gaining in individuality, and the pinnule finally terminates in a definite tip 

 (fig. 345, p. 229). As in the case of the brachials, and especially of the cirri, when 

 the outer pinnules are greatly elongated their ends tend to become swollen so that 

 they often develop greatly expanded articulations. 



The distal ends and the dorsal surface of the first two pinnule segments are 

 always smooth, and the same may be true in regard to all the other segments. 

 Almost always, however, the distal edge of the third and following segments is 

 more or less everted, overlapping the bases of the succeeding segments, and finely 

 spinous (fig. 325, p. 227). This character is least marked on the third segment, and 

 gradually increases in prominence distally, as a rule coincidently with the develop- 

 ment of fine spinelets on the dorsal surface of the segments. The extent of the 

 occurrence of spines on the distal ends and on the dorsal surface of the pinnule 

 segments (excepting usually those of the oral pinnules) is strictly correlated with 

 the exactly similar development of spines on the distal edges and on the dorsal 

 surface of the brachials, so that species with smooth arms have smooth pinnules, 

 and species with spinous brachials have spinous pinnule segments. The spinosity 

 of the segments of the oral pinnules is correlated with the condition of the division 

 series and first brachial pair, and not with the condition of the outer brachials. 



The two to four terminal pinnule segments are always armed dorsally with 

 from two to six or eight or more recurved hooks (figs. 351-593, pp. 235-303), and 

 part 1, figs. 48-52, p. 81), no matter how smooth the rest of the pinnule may be. 

 These reach a maximum development in the Comasteridffi, especially in the sub- 

 family Comasterinse (figs. 351-363, p. 235, 364-371, p. 237, 372-385, p. 243, 387-389, 

 397, p. 245, 414-421, p. 255, 422, p. 257, and part 1. figs. 51, 52, p. 81), and give to 

 many of the species in that family, as is well illustrated by Comfinthus sa?noana, 

 and by many of the species of Com/aster, a characteristically rough or dry feeling. 



The stiffening of the pinnules, as seen in the oral pinnules of very many 

 oligophreate species (figs. 270, 271, 273, p. 207), and in all the pinnules in the 

 Calometridse and PtilometrinEe (figs. 280, p. 213, and 285, p. 215), and in most of the 

 ThalassometrinsB (figs. 283, 284, p. 215) and Charitometridse, comes about through 

 the reduction of the bevel on either side of the transverse ridge of the articulations 

 which limits the possible motion. In spinelike pinnules, such as the oral pinnules 

 of Stephanometra (fig. 273, p. 207), the two halves of the joint face on either side 

 of the median ridge lie in the same plane, thus rendering the pinnule rigid, as all 

 intersegmental motion is prevented. Conversely the enormous flexibility seen in 

 the oral pinnules of the Comasteridae and Heliometrinse (figs. 544-546, p. 289), and 



142140 21 Bull. 82 11 



