14 BULLETIN 88, UNITED STATES NATIONAL MUSEUM. 



Arcs. 



See Adl. 

 Axil. 



The angle formed by the junction of the rays. Also known as arcs. In primitive 

 forms each axil is occupied by a single plate, the axillary inframarginal. In 

 others this plate is crowded orally and its place occupied by two prominent 

 ■proximal inframarginals. In forms with large disks having well-developed 

 marginals these are termed disk marginals. Also see Ambital and Interbrachial. 

 Bivium. 



The two rays of a starfish that inclose between them the madreporite. When 

 oriented toward the student, the ray on the left is numbered 1 and the other on 

 the right is numbered 5. A line drawn through the madreporite and the center 

 of the upward directed ray (ray 3) divides the animal into symmetrical halves 

 (=left and right sides). The upper left-hand ray is numbered 2 and the one 

 opposite is 4. Rays 2, 3, and 4 form the trivium. 

 Body-wall. 



The outer surface of both disk and rays, made up of movable calcareous plates 

 which are either closely adjoining or form a more or less loose network of ossicles. 

 These are held together in the living animal by connective tissue and muscular 

 fibers, which after death soon decompose, freeing the plates. It is for this reason 

 that fossil starfishes are so rarely found entire. Imbedded in the body-wall 

 everywhere are blunt, short, calcareous spines, arranged in a more or less 

 definite order, and often movable upon the underlying plates. 

 Covering the whole surface of the body, including the spines and pedicellariae, 

 is a delicate membrane or skin, clothed externally with closely placed vibratile 

 cilia, which keep the integument clean. The calcareous skeleton of starfishes 

 is therefore integumentary. 

 Cxcal pores. 



The openings between reticular plates for the protrusion of the respiratory caeca! 

 processes. These pores are either restricted to the abactinal area or may be 

 distributed over the entire body. The processes are also known as papulx. 

 Central disk. 



See Disk. 

 Central disk plate. 



A prominent plate, most conspicuous in primitive forms, which often appears in 

 the center of the disk. It holds the same position as the centro-dorsal plate in 

 embryonic crinids of the genus Antedon. Also called the abactinal-central plate 

 or simply central plate. 

 Centro-dorsal plate. 



See Central disk plate. 

 Disk. 



The central portion of a starfish. In this work, the term generally applies only to 

 the abactinal central area formed either by the union of the rays, or by the rays 

 plus the axillary areas. The mouth is in the center of the disk on the under or 

 actinal side. Disk plates are the small plates over this area on the abactinal side. 

 Central disk plate refers to a larger primary plate in the center of the disk. There 

 may be other large primary plates, but these are more properly referred to the 

 rays. See Radial, Interradial, Marginal, and Ambital. 

 Inframarginal. 



See Marginal plates. 

 Interbrachial. 



The actinal and marginal areas between the rays . These spaces have plates derived 

 from various regions. In primitive forms, but a single plate is present, the 

 axillary interbrachial, derived by crowding orally the axillary inframarginal. 

 In other genera, inframarginals are crowded in pairs orally and form interbrachial 



