102 ME. P. II. CAEPE]STER ON THE GENUS ACTINOMETRA. 



ateral ones (S^), iu whioli tlie fibres have the same direction as those occupying the radial 

 areas ; and a large median mass, in which the fibres ascend vertically for some distance 

 (S.) and then diverge to the two sides (Si), where they pass into tlie protoplasmic basis 

 of the radials ; and the horizontal fibres (i) which pass between the radials fill up the 

 open angle caused by the divergence of the ascending fibres. There is thus a much greater 

 dcvolopment of connective-tissue fibres, elTecting the synostosis of the centrodorsal piece 

 with the radial pentagon, in the interradial than in the radial planes. This is well seen 

 in PL VIII. fig. 3, which represents a longitudinal section through the calyx of Act.poly- 

 morpha. On the right side it is interradial, passing through the synostosis of the first 

 and second radials of the two radii, A, B ; and the connective-tissue fibres (63) connectiug 

 the centrodorsal piece with the edges of these two radii are longer and more abundant 

 than those on the left side {I), passing between the centrodorsal piece and the first radial 

 of radius D, which is cut longitudinally. 



This is also seen in PL VIII. figs. 5-8, Avhich represent portions of four out of a series 

 of sections through a decalcified calyx ofAci. j^ectinata. These are in the same plane as 

 the section of the calyx of! Act. polymorpha represented in fig. 4, i.e. transverse to the synos- 

 tosis of the radii A, B on the one side of the centre, and to the radius D on the other. 



Fig. 5 represents a section, rather nearer the centre than fig. 4, passing verticaliy 

 along the axial interradial canal {a.i.c) ; beneath the dorsal end are seen the vertical 

 ascending fibres {S-i), which liave a much deeper origin in the substance of the centro- 

 dorsal piece of this species than in Act. poUimorpha. The diverging fibres are not seen, 

 as they give rise by their calcification to the long basal ray (PL V. fig. 8, S) ; and this 

 section passes through the depression at the central end of its ventral surface (PL V. 

 fig. 8 a, s) in which the axial interradial canal terminates. 



Fig. G is somewhat nearer the centre, but still shows the long vertical fibres (.S*,.) in the 

 interradial plane, together with a portion of the central calcareous network and the 

 axial radial canals (a.r.c) corresponding to the two radii A, B. 



Fig. 7 is just beyond the centre, i. e. across the inner end of the first radial of D, so 

 that no vertical fibres are visible, as they are only interradial in position. Two sets of 

 them, liowcver, are seen in fig. 8, Avliich shows the first radial of B cut transversely 

 rather further from the centre, so that the fibres [l) eff'ccting its synostosis w'ith the 

 adjacent radials of C and B are cut obliquely ; beneath these are seen the interradial 

 ascending fibres [S.,), which diverge slightly at their u])per extremities (^1). 



These diverging fibres and the upper ends of the vertical ones are the basis around 

 which the calcareous material forming the rays of the basal star is deposited. As the ven- 

 tral surface of the centrodorsal piece on which these rays rest is much higher at the centre 

 than at the circumference, it is impossible to obtain horizontal sections in which these 

 five rays are seen at all complete. Oblique sections, liowever, may be obtained in which 

 one or more of them are cut along the greater part of their length. Two such sections, 

 seen from their dorsal side, are represented in PL VIII. figs. 1 & 2 ; their lower left-hand 

 portions lie nearer the dorsal surface of the calyx than the upper right-hand portions. 



The centre of fig. 1 is occupied by the fibrous envelope V of the quinquelocular organ, 

 from the dorsal portion of which cords [u.c) proceed to the cirrhi {cir). At the top and 



