DR. WILSON ON THE NERVOUS SYSTEM OF THE ASTERID^E. 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 t . 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 much 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" %. 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 during life, I observed, as in the Cribellu, 
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 eve and tactile organ is well marked in the Uroster. 
The elements of the skeleton which enter into its formation are, first, the lateral calcareous 
masses already 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 ray. They enclose the eye-bulb t f . The next pair (b) 
also consisting of three 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. X 
II Plate XIV. fie. 10. dd. H Ibid, fig- 1 1. ** * bid - fi S' 7 ' e '' tT 
