ME. P. H. CARPENTER ON THE GENUS ACTINOMETRA. 121 



central mass (A 7 ) enveloping the quinqiielocular organ. Proceeding outwards from the 

 centre are seen five dark rays (Si), which represent the closely fibrillar organic basis of the 

 five rays of the basal star. In the upper part of the figure two of them are very short, only 

 their central ends being visible, as the section has here passed above the level of their outer 

 ends through the substance of three first radials (;•]). 



Fig. 2. In this section only two of the basal rays (SJ are visible, as the greater part of it has passed 

 above the level of the synostosis (/) between the first radials and the centrodorsal piece. In 

 the centre are seen the chambers of the quinquelocular organ (c/i), with their ventral openings 

 into the vessels contained within the axial prolongation. At the right of the figure are seen 

 the lower ends of the axial canals, both radial (a:r.c.) and interradial (a.i.c.) ; their cavities 

 are generally crossed by transverse septa, which divide them up into two or three intercom- 

 municating smaller ones. The interval between every two of these canals is occupied by one 

 of the secondary basal cords (Y. 2 , Z u Z 2 , &c.), produced by the bifurcation of the short primary 

 cords (V, Y, &c.) proceeding from the angles of the quinquelocular organ. They are connected 

 with one another laterally by interradial and intraradial commissures (Leo. & r.co.) and 

 enter the central canals of the first radials (r x ) in successive pairs, so that the axial nervous 

 cord (») of each radius is composed of fibres derived from two primary basal cords (Y„,Z 1) 

 &c), just as in Antedon. 



Fig. 3. A vertical longitudinal section through a decalcified calyx, passing on the right through the 

 synostosis of two first radials (A, B) and the fibrillar basis (<S 2 ) of one of the basal rays, and 

 on the left through the segments (j-j, r 2 , r.a) of radius D. The first of these is united to 

 the centrodorsal piece by connective-tissue fibrils (/) similar to, but less abundant than, 

 those in the interradial portion of the section, around which, in the natural condition, the cal- 

 careous material forming one of the basal rays is deposited. The passage of the ventral 

 furrows (cr.f.,v.i.f.) into the axial canals (a.r.c, a.i.c.) is also well seen in this section. 

 Its centre is occupied by the quinquelocular organ, from the ventral portion of which the 

 axial prolongation (a.p.) rises into the circumvisceral ccelom (c.v.c), which, together with 

 the lower end of the wide axial ccelom (a.c), occupies the space between the ventral surface 

 of the skeleton and the lower or dorsal wall of the convoluted alimentary canal. It is 

 traversed by numerous connective-tissue septa, which divide it up into a system of spaces, 

 communicating freely with those both of the intervisceral and of the axial ccelom (ice, a.c). 



Fig. 4. Transverse section through the synostosis of two first radials (A, B) near the peripheral 

 margin of the centrodorsal piece (cd.) , showing the radial (/) and the interradial (S„) fibres 

 which effect the synostosis between it and the united first radials. The latter form the 

 organic basis of one of the rays of the basal star. 



Figs. 5-8. Four vertical sections, selected from a series, through a decalcified calyx of Act. pectinata. 



5. Section through the adjacent inner ends of two first radials (A,B), showing the axial inter- 

 radial canal (a.i.c.) between them, and the open outer ends of the radial spaces (r.s.) 

 between their dorsal surfaces and the ventral surface of the centrodorsal piece. The central 

 ends of their axial nervous cords (//.) are cut very obliquely. 



6. A section rather nearer the centre, showing the closed central ends of the radial spaces (r.s.) 

 of the same two radii (A, B) and their axial canals (a.r.c) ; also the four secondary basal 

 cords which unite in successive pairs (Z. 2> V\ and V 2 , W-j) to form their axial cords, cut 

 obliquely. 



7. A section from a little the other side of the centre, through the outer end of one of the 

 chambers (ch) of the quinquelocular organ, corresponding to radius D. The radial spaces 

 (r.s) of C and E arc cut almost longitudinally ; and above them, in the interior of the 

 radials, are seen the axial nervous cords, with one of the two secondary basal cords 



