DR. WILSON ON THE NERVOUS SYSTEM OF THE ASTERID^. 117 



rangement ; for it may not be improbable that each papillseform projection is equivalent 

 to the simple eye of an insect " *. At that time I was too busily engaged with the 

 nervous system, and paid very little attention to the eye-spots. Their true structure we 

 now know from Dr. Haeckel's accurate observations. 



The cones take more generally the obliquely transverse arrangement. The pigment- 

 cells of the individual eyes are nucleated and densely aggregated f. They have a pur- 

 plish-red colour, undergoing the same changes as already specified on the addition of nitric 

 acid. The lens is small and is very mucli covered by the iris-like ring of pigment-cells 

 situated around its margin. The apices of the cones all point towards a common mesial 

 plane. Nerve-filaments can be traced into them ; but the connexion of these to the cells 

 cannot be distinctly made out. Solaster was the only genus in which I observed with 

 certainty a nerve-fibre passing from cell to cone. 



The cornea is quite distinct in a profile view. " As the integument runs along the 

 distal portion of the cord it is continued over the pigment-deposit in a modified condition, 

 becoming perfectly clear and apparently denser" J. The integumentary cuticle is con- 

 tinued over the cornea as a membrane formed of minute polygonal cells. 



The light-purple-coloured Urasters I found the best for the examination of the tactile 

 organ during the animal's movements. The younger the specimen, the longer in proportion 

 is this organ. By carefully watching the animals dm'ing life, I observed, as in the Cribella, 

 that they never used it as a sucker. Those of the rays fronting the direction of progress, 

 were generally the only ones protruded. They resemble, in shape, a slightly tapering 

 cylinder, contractile and very much thicker than the thread-like suckers surrounding the 

 eye- spot §. Their extremities are more pointed than those of the feet. Their colour is 

 similar to that of the integument over the cord ; that is, of a light-brown. That portion; 

 however, against which the eye rests, is in many cases highly tinted with the same 

 pigment as that of the cones. 



Although the optic bulb moves, generally, along with the base of the tactile organ, it 

 has in addition an independent motion in its own peduncle. It is interesting to notice 

 the curious manner in which the pigmented, wing-like lateral halves of the eye can be 

 turned in any direction, so that the profile outline of the bulb is constantly changing, the 

 transverse diameter becoming greatly increased or equally diminished. 



The protective apparatus for the eye and tactile organ is well marked in the IJraster. 

 The elements of the skeleton which enter into its formation are, first, the lateral calcareous 

 masses akeady noticed as coalesced ultimate vertebral arches || ; and secondly, obliquely 

 transverse rows of short, thick, bluntly acuminated spines ^. The latter are serially 

 homologous with the outer row of spines before mentioned in connexion with the ambu- 

 lacral groove **. Those of the first series, three on each side, rest upon the lateral masses, 

 and lie towards the distal end of the i*ay. They enclose the eye-bulb ft- The next pair (6) 

 also consisting of tliree spines each, and springing from the distal extremity of the calca- 

 reous mass, protects the tactile organ and bars it off from the optic bulb. The tentacle 



* The sentences marked by inverted commas are quotations from my Thesis. t Plate XV. fig. 4. % Thesis. 



§ Plate XV. fig. 6. h represents the comparative size of one of the suckers situated round the bulb. 



II Plate XIV. fig. 10. rfrf. 5IIbid. fig. 11. * *= Ibid. fig. 7. e'. ft Ibid. %• 1 1- «. 



