ME, P. H. CAEPENTER OX THE GENUS ACTINOMETEA. 97 



(iv) The basal bridge {b.b), consisting of two calcareous bars, that represent the 

 unabsorbed peripheral margins of the embryonic basal on which two first radials rested. 

 They extend towards one another from the outer ends of the small radial processes, until 

 they meet at a point that represents the apex of the embryonic basal, and is situated 

 on the dorsal side of the peripheral end of the interradial process (o), developed from the 

 secondary or ventral layer, which becomes united with the basal bridge. 



(v) The ray (*S') of the basal star, which is joined to the interradial process and to the 

 basal bridge, along the line of union of the two primary bars constituting the latter with 

 one another and with the secondary interradial process, i. e. at the apex of the embryonic 

 basal. The development of this ray is quite different from that of either the primary or 

 the secondary portions of the compound basal. It is really a tertiary structure, being 

 nothing more than a deposition of calcareous material in the substance of the connective 

 tissue of the synostosis between the centrodorsal piece and the radial pentagon. 



(vi) At the sides of the interradial process (o), bounded laterally by the radial pro- 

 cesses {p'), and externally by the bars of the basal bridge {b.b), are two large apertures, 

 ^'ij '^'2) y\, y-i-, &c., in each compound basal. Through these apertures pass the secondary 

 basal cords, X^, X, Zi, Y^_, &c. (PI. VIII. fig. 3), which result from the bifurcation of the 

 primary cords, X, Y, Z, proceeding from the angles of the quinquelocular organ. The 

 two secondary cords lie in the depressions on the dorsal surface of the compound basal, 

 between the central ends of its radial and interradial processes. They then pass outwards 

 through the apertures (.t'l, a\, &c.) beneath the bars of the basal bridge, and enter the 

 adjacent openings (x, a/, &c.) on the internal faces of the two contiguous first radials', 

 which contribute to form the dorsal interradial furrow occupied by the single fusiform 

 ray (S) of the corresponding basal. 



The ventral surface of each of these rays of a compound basal (PI. V. fig. 8a; PI. VI. 

 figs. G, 18, 22 a) is not flat, like the dorsal surface, but occupied by a prominent median 

 ridge, so that the ray is triangular in section. Tliis ridge does not extend quite to the 

 central end of the ray, which is occupied by a considerable depression (s), forming the 

 peripheral end of the groove contained in the spout-like interradial process (o). In the 

 natural condition, when the basals are in siiii and in connexion with the radial pen- 

 tagon, the inflected edges of this process unite Avith those of the axial interradial furrow 

 to form an axial interradial canal. This terminates on the dorsal surface of the radial 

 pentagon by a small opening situated at the central end of the dorsal interradial furroAv 

 (PI. V. fig. 12, II), in which furrow the tertiary element (^S*) of the corresponding com- 

 pound basal is received. The depression (s) at the central end of the ray (PI. V. fig. 8 a ; 

 PL VI. figs. 6, IS, 22 a) lies over this opening, and thus forms a blind end to the axial 

 interradial canal (PI. VIII. figs. 3, 5, 7, a.i.c ; fig. 8, s) — precisely in the same manner 

 as the depressions (q) on the ventral surface of the centrodorsal piece of AnL rosacea 

 (PI. IV. fig. 15) receive the blind ends of the axial radial canals which open on the 

 dorsal surface of the radial pentagon by the five large openings, Q (PI. IV. fig. 16). 



A view of a single compound basal does not, of course, show one of the large and 



' In PI. Y. fig. 13, four of these openings are seen on tlie internal faces of tlic two contiguous first radials, 

 viz. .^•', y, y', z. 



