DR. WILSON ON THE NERVOUS SYSTEM OF THE ASTERIM. Ill 
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one row crossing behind the other, so as to enclose the eye-bulb and the tactile o: 
separate chambers. Owing to their being inclined towards the ray-point, they in a manner 
rest against their distal neighbour, and thus conceal from view the organs they protect. 
In profile, the part of the bulb covered with pigment presents a convex outline, the 
convexity being greatest towards its free end, so as to give it the appearance of an irregu- 
larly convex cylinder. The bulb is, in addition, slightly bent upon itself at the root of 
the peduncle, so as to face towards the mouth *. 
When examined microscopically, these eyes have, in many respects, the same structure 
as described in other genera by Haeckel. They are compound, — the individual eyes con- 
sisting of pigment-cones whose apices are directed towards the centre of the bull), whilst 
their bases rest against the transparent cornea. Their arrangement on the surface of the 
bulb varies, the animal seeming to have the power of separating them more or less from 
each other, and thus, to a certain extent, altering their relative positions. Even while 
the Solaster is moving, we may observe these cones now having a concentric arrangement, 
and now placed in obliquely transverse lines diverging from a mesial one. This curious 
fact seems to depend on their being placed in lines radiating from a central equator, at 
more or less equal distances, not only from the contiguous eyes in the same radius, but 
also from the corresponding ones in the neighbouring radii t. Accordingly, if the animal 
decreases the distances between the radii, then the individual eyes appear as if arranged 
transversely | ; but, on the contrary, if all the eyes in the different radii be separated from 
one another, the convex linear arrangement becomes evident §. All the apices arc, how- 
ever, in every case, directed, as Haeckel remarked, towards a mesial longitudinal line. 
The whole surface of the bulb is not equally furnished with individual eyes ; for at its 
distal or free extremity there is a triangular central portion quite free from these, being 
only sparingly studded with pigment-points || . This more or less transparent part of the 
bulb gives it at first sight, especially when examined during the motion of the animal, a 
bifid appearance. Haeckel remarked an absence of pigment-cones in the same position 
in Asteracanthion glacialls. There is often an indentation of the bulb at this transparent 
free end, so as to make it heart-shapedf . The individual eyes extend laterally quite round 
the free extremity of the bulb, and may be found even on the upper surface, which rests 
against the tactile organ. From what has been said it is evident that the eye has a bila- 
teral arrangement ; and this greatly favours the supposition that the nerve-cord is also 
bilateral or double, though apparently single. 
The number of individual eyes varies with the age or size of the animal. If a single 
eye be examined with a higher power, it is found to consist of a cone formed of minute 
nucleated pigment- cells, vermilion in colour **. Near its base is imbedded a highly re- 
fractive globular lens (4). In the Solaster, the cones being large and their bases wide., 
the lenses present a very beautiful appearance, like transparent ova, each m its own scar- 
let nest. Those cells forming the border of the base of the cone curve inwards, so as to 
form a ring (*), well seen in a front view, which rests upon and screens the margin of the 
lens. This is admirably likened by Haeckel to an iris. He also mentions that the pigment- 
Plate XIII. fig. 4. f Ibid- % 8. t Ibid- fig- 10- § «**' fi S- 9 ' 
II Ibid. figs. 4, 5 & 6. « Ibid. fig. 6. ** "» d - fig- > ■ 
