WILSON ON THE NERVOUS SYSTEM 
121 
ertcbral 
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 
spine- 
intermediate pieces (b b, cc). f. 
bearing pieces (dd), and on a level with the ligament which stretches between the masses (cc). 
g. 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). 
999 
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. tactile 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 [e e) and ending in the optic 
(d) 
iff) 
obliquely transverse lines, and 
bound, at the free extremity of the bulb, a well-marked space devoid of cones and spotted with 
pigment-points, g. 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. 
Individual eye, highly magnified, a. pigment-cone formed by irregular-shaped nucleated cells 
{b) y 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- 
7 
like ring (e). f. 
layer (g) consisting of polygonal cells (A). 
c. the cornea, bounded externally by a cutieular 
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. 
Ner 
are 
cones to these cells; while other fibres pass longitudinally in the direction of the ambulacral 
cord. 
Tab. XIV. 
Cribella oculata. 
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 (c'). 6. water- vessel, lying 
aboye inferior muscle, g. 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: 56. delicate fibrils from the skin towards the nerve-cells; 
VOL. XXIII. 
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