DR. WILSON ON THE NERVOUS SYSTEM OF THE ASTERIDiE. 115 
the optic bulb, or contracts, while the slender thread-like suckers are extended forwards 
and lay hold of the object. 
Nerve-filaments are distributed to this organ from the terminal nerve-cells lying above 
the optic bulb. The cornea differs only in thickness from that of S. papposa, and is denser, 
being with more difficulty injured. 
The ganglion at the commencement of the ambulacral cord consists of nerve-cells* ; but 
the relation of the filaments to these requires further observation. In dissecting the 
animal, this ganglion appears much more distinct than in the former genus, and is situated 
at a level lower than that of the ambulacral nerve-cord, as the first vertebral arch, on 
which it rests, projects below the succeeding ones. 
The perioral skeleton is formed from the same elements as in the Solaster-f. Only one 
intermediate piece (c) is found between the vertebral plate (a) and the spine-bearing 
mass (b). The inter ambulacral nerve- cord (<?), well marked in this species, passes on a 
level with the ligament which extends between the opposite intermediate pieces (c, <?') . 
3. Uraster rubens. 
I examined this species in 1857-58, choosing it as being the most easily obtained in this 
locality ; but I have since discovered that it is in many respects more complicated than 
either of the preceding for microscopic investigation. Tor dissection it answers admirably, 
as its nervous system is easily seen in all its extent. 
The ambulacral cord lies along the centre of the groove, having on either side of it 
double rows of alternating suckers J. The vertebral plates (a a), forming the ambulacral 
arches, are comparatively very thin and numerous ; they present the same characters 
as already described in the others. Both muscles are well marked, especially the inferior. 
The fulcrum («*), upon which the plates move, is also pretty extensive. Immediately 
above the tubercles (c, e) of the inferior muscle, an oblique groove id d) extends towards 
the vertical depressions containing the tubes from the vesicles to the feet. These oblique 
grooves are, probably, for the lodgement of the branches from the water-canal to the 
VGSIcIps 
The vertebral plates rest upon irregular-shaped calcareous masses (e e), bearing each 
two long spines. These spines are much sharper than the corresponding ones of the 
other species, and have often at their points slender radiating spinulcs. They have, 
almost always, clustered around them bunches of pedicellaria, which are entirely absent 
in the Solaster and CribelU. Another mass (e ), triangular in shape and separated from 
the first by an intermediate piece (/), gives articulation to three thick, short and bluntly 
acuminated spines. All these pieces are kept in contact with one another by means of 
connective fibrous tissue having the characters already enumerated. Externa ly they are 
covered by the integument, and give also attachment to the suckers, and internally re- 
ceive a peritoneal lining and support the double rows of alternating vesicles. 
The ambulacral cord%) bears the same relation to the inferior muscle and the ^mtegu- 
ment (A) as before stated. The nerve-cells that enter into its ^^ ' re .^"T 
polar, though occasionally we meet with bipolar § . They are, to a great extent, opake, 
+ tuvi fi„ 7 n § Plate XV. fig. I. 
* Plate XIV. fig. 4. d. t Ibid. fig. 4. X Ibid, fig- 7. 9 . » 
