DR. WILSON ON THE NERVOUS SYSTEM OF THE ASTERID^. 121 



and c c. intermediate calcareous pieces lying between the vertebral plates (a a) and the spine- 

 bearing masses [d d) : the latter are modifications of the rounded mass (e) in fig. 1, and sup- 

 port each six spines, e. ganglionic enlargement lying below inferior muscle and between the 

 intermediate pieces (i 6, cc). /". interambulacral cord i-unning immediately above the spine- 

 bearing pieces [d d), and on a level with the ligament which stretches between the masses [cc). 

 ff. integument covering the ganglion and extending laterally over the skeleton. 



Fig. 4. Profile view of eye-spot and tactile organ, a a. optic bulb, irregularly cylindrical and slightly 

 bent upon itself at its root {d), convex in outline (the convexity being greatest at its extremity, c). 

 a ff ff. pigment-cones in profile, each having its base directed towards the cornea and con- 

 taining imbedded in it a lens. Their arrangement is more or less transverse ; and the rows 

 of cones nearest the free extremity enclose a space (c) having no individual eyes and studded 

 with pigment-dots. _/. integument covering ambulacral cord and illustrative of its sudden 

 modification to the clear cornea (e). h. tactde organ. 



Fig. 5. Optic bulb and tactile organ in situ, taken with the point of the ray lying on its dorsal surface. 

 a. ambulacral nerve-cord, resting between the vertebral plates (c c) and ending in the optic 

 bulb {d) : the latter is supported by two calcareous plates (e e), divested of all but their terminal 

 bunches of spines (//) ; the individual eyes are arranged in obliquely transverse lines, and 

 bound, at the free extremity of the bulb, a well-marked space devoid of cones and spotted with 

 pigment-points, ff. tactile organ. 



Fig. 6. Bird's-eye view of eye and tactile organ. The bulb (a) has its free end (c) slightly indented so as 

 to make it somewhat heart-shaped : the cones appear as irregular pigment-masses with the 

 transparent lens in the centre ; the mesial series of cones, from which the others pass off 

 transversely on either side, is well seen. b. ambulacral nerve-cord. d. tactile organ. 



Fig. 7. Individual eye, highly magnified, a. pigment-cone formed by irregular-shaped nucleated cells 

 {b), vermilion colour ; its base is directed towards the cornea (c) and has lying in it a large, 

 highly refractive globular lens {d), the border of which is partly concealed by the curving 

 inwards, towards its centre^ of the marginal pigment-cells of the cone, in the form of an iris- 

 like ring (e). /. nerve-fibre entering its apex. c. the cornea, bounded externally by a cuticular 

 layer (^) consisting of polygonal cells (A). 



Fig. 8. Diagram giving the individual eyes in their probable normal distribution. 



Fig. 9. A second, illustrating why the individual eyes sometimes appear in convergent lines. 



Fig. 10. A third, accounting for the transversely oblique arrangement occasionally seen. 



Fig. 11. Nerve-cells situated above the optic bulb. Some filaments are seen to connect the pigment- 

 cones to these cells ; while other fibres pass longitudinally in the direction of the ambulacral 

 cord. 



Tab. XIV. 



Cribella ocidata. 



Fig. 1. Transverse section of ambulacral groove, a a. ambulacral plates; ax. fulcrum; c c. tubercles, 

 between which the adductor muscle stretches ; d. superior or abductor muscle; e. round calca- 

 reous piece supporting vertebral plates and giving articulation to double row of spines; /. inter- 

 mediate portion, affording attachment to another spine-bearing mass (e'). b. water-vessel, lying 

 above inferior muscle, ff. ambulacral nerve-cord, consisting of nerve-cells, some of whose fila- 

 ments pass through the substance, others by the side of inferior muscle, and extend upwards 

 round water-vessel ; a third set runs transversely outwards, h. integument, folded, and dragging 

 downwards in its fold some nerve-cells, i i. vesicles, k k. suckers. 



Fig. 2. a. nerve-cells of the ambulacral cord ; b b. delicate fibrils from the skin towards the nerve-cells ; 

 VOL. XXIII. R 



