276 BULLETIN 82, UNITED STATES NATIONAL MUSEUM. 



The general structure of the cirri is the same throughout the group, and may 

 be thus described : The first two segments are very short, very much broader than 

 long, and approximately of equal size, though a close examination always discloses 

 a slight increase in the proportionate length of the second (figs. 312, 313, p. 271). 

 Ordinarily there are only two of these short basal segments; but if the cirri are very 

 long, as in most of the species of Thalassometridas, there may be one or two addi- 

 tional which are somewhat longer than the first two, the outer again being slightly 

 longer than the more proximal (figs. 361, 362, p. 295, and 392, p. 307) ; the third (or 

 fourth or fifth) segment is considerably longer than those preceding, and the following 

 two or three still further increase in length, becoming, on an average, approximately 

 twice as long as broad when viewed laterally ; after four or five more the segments 

 gradually decrease in length, at the same tune becoming compressed laterally, and 

 more and more sharply rounded dorsally, while the distal dorsal edge becomes 

 produced; in the distal part of the cirrus we find the segments ordinarily broader 

 than long, strongly carinate dorsally, with the projection of the distal dorsal edge 

 narrowed to a point, and forming a median or subterminal dorsal spine. 



Typically the distal profile of the cirrus segments when viewed laterally shows 

 a broad S-shaped curve which lies diagonally, running from the ventral distal edge 

 downward and backward to the dorsal distal edge (figs. 312, 313, p. 271) ; the portion of 

 this curve ventral to the transverse ridge is strongly convex and lies at a compara- 

 tively small angle to the longitudinal axis of the segments ; the portion dorsal to the 

 transverse ridge is, less strongly, concave, and makes a much greater angle with 

 the longitudinal axis of the segments. Lateral compression of the segments is 

 accompanied by a straightening of this curve, and by a marked tendency for the 

 straightened ends of the segments to approximate a position at right angles to their 

 longitudinal axes (fig. 397, p. 309). 



The distal end of the cirrus terminates in a sharply pointed more or less curved 

 hooklike process, the terminal claw (figs. 4, p. 63, and 314-318, p. 273) ; in mature 

 cirri this is almost always slightly longer than (occasionally almost twice as long as) 

 the penultimate segment which next precedes it, and it is usually evenly curved 

 (figs. 312, 313, p. 271), the radius of curvature being the same as, or slightly less 

 than, that of the distal portion of the cirrus as a whole in lif e ; it tapers from a 

 rather stout base to a slender and needle-like tip, sometimes evenly, but more com- 

 monly with greater rapidity in the proximal third or half, so that the distal two- 

 thirds or half is comparatively slender; in certain oligophreate forms it is more or 

 less abruptly decurved at the junction between the comparatively stout basal 

 third and the proportionately slender distal two-thirds, the latter being often 

 approximately straight (figs. 317, 318, p. 273). 



The terminal claw is usually well developed, and an important structural and 

 physiological feature of the cirrus; but in species with long, slender, and smooth 

 cirri, living upon sandy, oozy or muddy bottoms devoid of arborescent organic 

 life so that the cirri collectively function merely as a sort of circular snowshoe, 

 by their large numbers forming a broad circular base upon which the animal may 

 rest without danger of sinking into the ooze and becoming mired, the terminal claw 

 often becomes straightened, dwarfed, blunted, and rudimentary, sometimes being 



