MONOGRAPH OF THE EXISTING CBINOIDS. 



133 



outer genital pinnules, like the distal pinnules, lie horizontally in a plane at right 

 angles to the dorsoventral plane of the arm (fig. 1071, pi. 16). 



The genital and distal pinnules can be folded inward until they almost or 

 quite meet and then laid back along the arm so that they form a covering over 

 the soft ventral surface. 



Pinnules always occur on the opposite sides of succeeding brachials, so that 

 they alternate regularly all along the arm (fig. 1030, pi. 12). They never occur on 

 the hypozygals of syzygial or of 

 sj'narthrial pairs, nor upon 

 axillaries. 



Usually the pinnules taper 

 gradually, slowly, and evenly 

 from the base to the tip, but in 

 certain species in which the 

 adambulacral plating is highly 

 developed, as in the species of 

 Ptilometrina? and Calometridse, 

 they may end abruptly with 

 a few small segments much as 

 the arms do in the same types 

 (figs. 351-593, pp. 235-303). 



In viewing a single arm of 

 almost any 10-armed species 

 with the pinnules extended, 

 the elongate oral pinnules ai-e 

 seen to be very conspicuous, and 

 the change from these to the 

 short lower genital pinnules is 

 very abrupt. The following 

 genital pinnules slowly increase 

 in length until the lower distal 

 pinnules are reached, after 

 which the same length is usually 



maintained until near the end fig. 201. — lateral view of specimen of calometua separata. 

 of the arm, when the length de- 

 creases with more or less rapidity, so that the distal end of the arm is more or less 

 broadly rounded. The slenderness and delicacy of the pinnules and their very con- 

 siderable number give the arms much the appearance of delicate feathers (figs. 

 262-287, pp. 207, 213. 215). 



As a general rule, the pinnules of the oligophreate species (figs. 262-280, pp. 

 207. 213; 283-285. p. 215) are shorter and stouter than those of the species of the 

 Macrophreata (figs. 281, 282. 286, p. 215). 



Proportionately with the increase in the number of arms the length of the 

 pinnules decreases, so that in the highly multibrachiate forms they are usually 

 relatively very short (figs. 270-273, p. 207; 274-277, p. 213). 



