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



one row crossing behind the other, so as to enclose the eye-hulb and the tactile organ in 

 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 Haeekel. 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 f. 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 are, how- 

 ever, in every case, directed, as Haeekel 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 |1 . This more or less transparent part of the 

 bulb gives it at first sight, especially when examined du.ring the motion of the animal, a 

 bifid appearance. Haeekel remarked an absence of pigment-cones in the same position 

 in Asteracanthion glacialis. There is often an indentation of the bulb at this transparent 

 free end, so as to make it heart-shaped^. 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 liigher 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 {d). In the Solaster, the cones being large and their l3ases wide, 

 the lenses present a very beautiful appearance, like transparent ova, each in 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 Haeekel to an iris. He also mentions that the pig3nent- 



* Plate Xni. fig. 4. t Ibid. fig. 8. J Ibid. fig. 10. § Ibid. fig. !). 



II Ibid. figs. 4, 5 & 6. t Ibid. fig. 6. ** Ibid. fig. 7. 



