102 BULLETIN 82, UNITED STATES NATIONAL MUSEUM. 



S. 



Sacculi. Small globular or ovate sacs which occur, often abundantly, along the 

 edges of the ambulacral grooves of the disk, arms and pinnules; in preserved 

 specimens they are usually dark brownish or reddish, and very conspicuous, 

 but sometimes are nearly colorless; they also occur in the interior of the 

 body; they are not found in the species of the family Comasteridse (see figs. 15, 

 16, p. 67, and p. 111). 



Second bracJiial (Br 2 ). (1) The ossicle which bears upon its distal face the first 

 oblique muscular articulation (see fig. 30, p. 71), and normally also P t . 

 (2) The second ossicle of the free undivided arm. 



Second pinnule (P 2 ) . The pinnule borne by the fourth brachial of the free undi- 

 vided arm; it is absent in a number of species belonging to various genera (see 

 fig. 6, p. 63). 



Secondary anteroposterior axis. See Axis Ib. 



Secondary bilateral symmetry. See Symmetry and Axis. 



Secondary cords. The nerve cords after their first division as far as the intraradial 

 commissure (see figs'. 63, p. 89, and 65, p. 91). 



Secondary groove trunks. See Groove trunks 2. 



Secondary interradials. See Interradials 2. 



Secondary skeleton. See Perisomic skeleton. 



Secundibrachs (IIBr). The undivided series of ossicles following the IBr axillary; 

 this series may terminate in an axillary or may remain undivided. In the 

 latter case the term secundibrachs is not now employed, but the ossicles are 

 considered as constituting the free arm (see fig. 29, p. 71). 



Segment. An individual ossicle from a linear series. 



Sense organ. See Sensory organs. 



Sensory organs. Same as Ovoid bodies. 



Separated rays or division series. Rays or division series which diverge sufficiently 

 so that the perisome is readily visible between them (see figs. 41, 42, p. 77, 89, 

 p. 147, and 98, p. 159). 



Side plates. Small, usually squarish or oblong, plates developed along the ambu- 

 lacra of the arms and pinnules just outside of the covering plates; that is, 

 between the covering plates and the ventral edges of the brachials or pinnulars; 

 side plates are always accompanied by covering plates, though the latter often 

 occur alone, as in the genera Nemaster and Comatilia, and in many stalked 

 groups (see figs. 7, p. 63, and 55, p. 81, and p. 112). 



Simple extraneous arm division. Arm division in which all of the branchings are of 

 the extraneous type, as hi Metacrinus. 



Skeleton. Strictly speaking, the entire calcareous framework of the animal, but 

 used by most authors to indicate the calcareous framework or the dorsal 

 surface of the arms, calyx, and pinnules. 



Small mature cirri. The apical and subapical cirri, when differentiated from the 

 peripheral by their smaller size (see figs. 310, 311, p. 269, and pp. 250-251); 

 (see Large mature cirri). 



