DEVELOPMENT OF ANTEDON (COMATULA, LAMK.) EOSACEUS. 707 



apparatus from the central to the peripheral portion of its convex surfice, which is 

 effected by the successional development of new cirrhi at the growing margin, Avhilst 

 those which were originally implanted around the surface of articulation with tlie next 

 joint of the stem appear to be cast off. 



23. The upper or ventral surface of the Centro-dorsal plate (Tlatc XXXIII. fig. G) 

 which is formed principally by the thick wall of the basin, but partly also by its inturncd 

 lip, is nearly flat; being slightly elevated between the angles of its external and its 

 internal pentagons, and somewhat depressed in the intervening spaces. The elevations 

 correspond with the lines of junction of the First Eadials that rest upon it; and the 

 depressions with the convexities of their dorsal surfaces (fig. 2). Its adhesion to the 

 under side of the annulus formed by the First Radials is so close, that the line of 

 junction is not readily distinguishable in a vertical section of the " centro-dorsal piece " 

 compounded of both (Plate XXXV. fig. 1, a, a) ; and it is generally more easy to break 

 away the centro-dorsal plate piecemeal, than to detach it as a whole from the annulus 

 of first radials, — unless the composite piece has been boiled in a solution of caustic 

 alkali, which dissolves the organic substance whereby they are cemented together. 

 Kound the margins of the internal pentagon, we commonly find five shallow depressions 

 (Plate XXXIII. fig. 6, a, a) which correspond with the extremities of the elongated 

 spout-like processes of the "rosette" (^ 35); these, however, are seldom as strongly 

 marked as in the figure, and are sometimes wanting altogether. 



24. The internal surface of the wall that bounds the cavity of the basin is marked by 

 minute punetations (Plate XXXIII. fig. 0), which are the internal orifices of canals that 

 proceed from the interior cavity of the Centro-dorsal plate to the centres of the tubercles 

 in the sockets on its convex or dorsal surface. The course of these canals, Avhose diameter 

 averages -^^ of an inch, is shown in sections of the basin taken either perpendicular 

 or parallel to its upper flattened surface (Plate XXXV. figs. l,4,I>,b). In such sections, 

 w^hen sufficiently magnified, it is also to be observed that the calcareous network which 

 forms the basis of each socket is more solid than that of the general substance of the 

 plate ; its meshes being closer, and arranged with a regularity not observed elsewhere 

 (fig. 4, c, c). This is conformable to what is seen in the test and spines of Echinus; the 

 articular tubercles of the former and the basal cups of the latter being composed of a 

 peculiarly close calcareous reticulation'. When this centro-dorsal plate is decalcified, 

 the animal basis is found particularly firm at these spots, each socket having a membranous 

 disk of its own, which is traversed by very fine ligamentous fibres having a radiating 

 arrangement ; and by the convergence of these fibres is formed the internrticular liga 

 ment which binds the first joint of the dorsal cirrhus to its articular socket. 



25. Dorsal Cirrhi. — The dorsal surface of the calyx of Antedon is ordinarily in great 

 degree concealed by the cluster of Dorsal Cirrhi, which radiate from the convex surface 

 of the centro-dorsal plate (Plate XXXII. fig. 4), and which extend, when straightened 



' See Professor Valextin's jruuograpli 'Anatomic du genre Echinus,' jip. 20, 31, figs. lO, 17, 30, 38. — iTy 

 own preparations of these structures fully bear out Professor Yalesxin's descriptions. 



