. "fti 



.*^ de"\t;lopment of AKTEDOX (COMATULA, LATVIK.) EOSACETS. 729 



lower part of each being an acute-angled triangle with its apex pointing do^^'nwards, and 

 its upper part an obtuse-angled triangle with its apex directed upwards. The sides of 

 tlae lower triangle arc bordered by a somewhat thickened edge of solid transparent cal- 

 careous substance, the presence of whicli marks that the plate has received its full inci'ease 

 in that direction. Tlie adjacent borders of these plates, however, do not come into 

 absolute contact, a thin lamina of sarcode being interposed between them ; and there is 

 also a passage left at the tnuicated apex of the inverted pyramid formed by their 

 junction, througli which the axial sarcodic cord of the stem is continued into the caljTc. 

 The upper margins of the basal plates have no distinct border, and seem to be still in 

 process of growtli. The frst radiah {r\ r') now form (with the anal, a, intercalated 

 betv>-cen two of them) a nearly complete circle, resting on the basals, and separating 

 them entu-ely from the orals. Their shape is somewhat quadrangular ; two of their 

 angles pointing vertically upwards and downwards, and the other two laterally towards 

 each other. Their lower angles are received between the upper angles of the basals ; 

 whilst on their upper, whicli are somewhat truncated, the narrow second radials (/•-, r") 

 are superimposed. Considerable spaces still exist between the adjacent radial plates, 

 except where the anal plate is intercalated in the series ; and these are filled only by 

 sarcodic substance. The central portion of these first radials is thickened by the endo- 

 genous extension of the calcareous reticulation ; and this extends towards its upper 

 angle, so as to form a kind of articular surface for the support of the second radials ; 

 but it does not extend over the lateral or alar expansions of these plates, which still 

 retain their original condition of cribriform films. The second radials (r^ t^) dift'er 

 completely from the first in shape, being rather rods than plates ; but they are deeply 

 groo-^ed on their oral aspect, that which is subsequently to become a central canal being 

 not yet closed in. The calcareous reticulation of their outer or aboral surface is cribri- 

 form ; but the ingrowth from which they derive their solidity is produced, as Professor 

 Wyville TiiOiisox has shown (p. 541), by the development of fasciculated tissue analo- 

 gous to that of which the stem-joints are composed. The same general description 

 applies also to the third or axillary radials (/■^ ?-^), which, like the preceding, are nearly 

 cylindrical at their proximal extremities, but expand towards their distal ends, so that 

 each presents two articular surfaces, on which are imposed the pair of first brachials. — 

 The oral plates (o, o), which alternate mth the second radials, though somewhat internal 

 to them, now present somewhat of a triangular form, their apices pointing upwards ; 

 their basal angles, however, arc cut off" (as it were) by the encroachment of the first 

 radials. At no part of then- contour have these plates any definite margin like that 

 which borders the two lower sides of the basal plates ; but the calcareous reticulation 

 of which they are composed is continued into the layer of condensed sarcode with which 

 they are invested. Although the form of these plates is generally triangular, yet their 

 surface is not that of either a plane or a spherical triangle, but presents a remarkable 

 unevenness. Near the apex of each there is a deep depression externally, and a coitc- 

 sponding projection internally; and the effect of this projection seems to be that, when 



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