DR. WILSON ON THE NERVOUS SYSTEM OF THE ASTERID^. 115 



the optic bulb, or contracts, while the slender thread-like suckers are extended forwards 

 and lay hold of the object. 



Nerve-filaments are distribvited to this organ from the terminal nerve-cells lying above 

 the optic bulb. The cornea differs only in thickness from that of S. pap^iosa, and is denser, 

 being with more difficulty injured. 



Tlae 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 (5). The interambulacral nerve- cord (e), well marked in this species, passes on a 

 level with the ligament which extends between the opposite intermediate pieces (c, c'). 



3. TJraster ruhens. 



I examined this species in 1857-58, choosing it as being the most easily obtained in this 

 locality ; but I have since discovered tliat it is in many respects more complicated than 

 either of the preceding for microscopic investigation. For 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 ict 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 {ax), upon which the plates move, is also pretty extensive. Immediately 

 above the tubercles (c, e) of the inferior muscle, an oblique groove (c' 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 

 vesicles. 



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 spinules. They have, 

 almost always, clustered around them bunches of pedicellaria, which are entirely absent 

 in the Solaster and Cribella. 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. Externally tliey are 

 covered by the integument, and give also attachment to the suckers, and internally re- 

 ceive a peritoneal lining and support the dotible rows of alternating vesicles. 



The ambulacral cord {g) bears the same relation to the inferior muscle and the integu- 

 ment (A) as before stated. The nerve-cells that enter into its formation are chiefly uni- 

 polar, though occasionally we meet with bipolar §. They are, to a great extent, opake, 



* Plate XIV. fig. 4. d. t Ibid. fig. 4. J Ibid. fig. 7. ff. § Plate XV. fig. ]. 



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