MONOGRAPH OF THE EXISTING CKINOIDS. 113 



on the outer (rarely also on the mner) side of from three to thirty of their terminal 

 segments with long, thin, triangular processes, forming a peculiar and characteristic 

 terminal comb (see figs. 56-58, p. 83, 59, 60, p. 85, and 76, p. 129). These terminal 

 combs occasionally extend outward on the arm over the proximal genital pinnules, 

 or may even (in the genus Comaster) occur on some of the distal pinnules. 



Following the oral pinnules there comes a row of usuailj' shorter, but propor- 

 tionately stouter pinnules, which may be more or less expanded lateraUy; they 

 frequently lack the ambulacral grooves, though typically they are provided with 

 them. These pinnules carry the genital products, and for that reason are known 

 as Genital pinnules (see figs. 1, p. 60, 6, 8, p. 63, 100, p. 162, 107, p. 173, and 113, 

 p. 181), though on accoimt of their position in the arm they are often called Middle 

 pinnules. 



Toward the end of the arm the pinnules gradually elongate (shortening only 

 in the family Tropiometrida?) and become more slender, the gonad dwindling in 

 size and finally disappearing altogether; the long slender pinnules found bcj-ond 

 the genital pinnules are known as Distal pinnules (see figs. 1, p. 60, 86, p. 141, 107, 

 p. 173, and 113, p. 181). The distal pinnules are always supplied witli ambulacral 

 grooves, unless the ambulacra are absent from the entire arm upon which they are 

 borne, as is frequently the case with the posterior arms in many of the species of 

 Comasteridae. 



The articulations binding together the elements of the division series and the 

 brachials are of two types, each type having two subdivisions. The only articula- 

 tion of importance in systematic study and in identification is the Syzygy (see figs. 

 6, p. 63, and 30, 34, p. 71), a remarkably close ligamentous union of two adjacent 

 ossicles the articular faces of which are (in the comatulids) approximately flat and 

 marked with radiating ridges. Externally the syzygy is usually readily recognizable, 

 appearing as a yctj fine or dotted line traversing the arm exactly at right angles 

 to the longitudinal axis. The lower or proximal component of a syzygial pair 

 (that is two ossicles united by syzygy) is kno^\'n as the Hypozygal, the iipper or 

 distal as the Epizygal. 



In the IIBr and following division series syzygies occur between the two outer 

 ossicles when these are three or four in number, but they are not always easy to 

 distmguish on account of the closeness of all the articulations. 



S}"zygies never occur between the two components of the fii-st division series; 

 but here, as well as elsewhere in the division scries and as far out on the arm as the 

 second brachial, an articulation called the Pseudosyzygy (see figs. 37-40, p. 75), and 

 another known as the Cryptosynarthry (see fig. 36, p. 75), are sometuncs found 

 (in the Zygometridae, and in the genera Comatula and Comaster) which are exactly 

 like the syzygy in outward, and the first also very nearly so in internal, appearance. 

 They are, however, of very different origin. 



In the comatulids there are several internal features which must be taken 

 into account m systematic work, and which therefore merit consideration here. 



The digestive tube, which is long and tubular, usually makes one complete 

 coil between the central mouth and the submarginal anus (see fig. 20, p. 69); 

 but in the majority of the species of Comasteridae it makes four coils, the anus 



