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



to determiue if they go to tlie suckers. On examining, however, tlie feet while the animal 

 is in motion, a delicate streak, more transparent than the surrounding structure, may be 

 seen, which, probably, is the nerve-filament given to it. 



In addition to the filaments already enumerated, very delicate fibres may be observed 

 passing from th& mass of cells towards the integument. I have not yet succeeded in 

 tracing these to a cell ; but should they prove more than connective tissue, the fact would 

 greatly support Professor Goodsir's hypothesis, " that the so-called unipolar nerve-cells 

 are in reality multipolar, of which one pole is an ordinary nerve-fibre, while the others 

 consist of much more deheate and slender fibrils *." There is no doubt that the skin 

 of the Solaster is exceedingly sensitive — 'SO much so, that gentle irritation of any part will 

 cause the suckers to be extended and withdrawn in a rapid and irregular manner, whereas 

 the slightest wound of the cord itself causes their instantaneous contraction. 



When the animal adducts its ambulacral plates, the integument covering the groove 

 becomes folded into a ridge and carries some of the nerve-cells along with it, so that these 

 appear to lie in the fold so formed. The ridge formed by the folded integument has a 

 wavy outline given to it by the row of suckers on either side, and would indicate that the 

 skin is not contractile t- By reflecting a portion of this fold, and spreading it carefully on a 

 glass slide before examining it with the microscope, bipolar nerve-cells may often be seen. 

 I have not as yet detected any in a vertical section. Their filaments run either obliquely 

 outwards or in the direction of the cord. In a longitudinal section of the arm, made along 

 the centre of the nerve-cord, we occasionally succeed in obtaining a view of the longitudi- 

 nal arrangement of the cells. In such instances we often notice that they ckister somewhat 

 more thickly opposite the intervertebral spaces, much in the same manner as the linearly 

 arranged ganglia of some of the Articulata. Nerve-cells may be traced along the whole 

 length of the cord, as far as the upper part of the bulbous enlargement of the eye-spot. 



The eye of the Solaster is situated on the under siu'face of a well-marked peduncle, which 

 is connected by its root, not only to the cord, but also slightly to the base of the tactile organ, 

 from which it projects %. It is very mobile, and may be seen, if the animal is examined 

 with a lens whilst moving, to be, to a great extent, the means of turning the eye laterally, 

 upwards, or downwards. These movements may be carefully watched in the Solaster, as 

 its eye-bulb is comparatively large, free of protective spines, and well elevated on its 

 peduncle. On turning up the ray so as to expose its ambulacral or ventral surface, and 

 removing, with the assistance of a dissecting-microscope, all the suckers around the eye, 

 the latter appears more or less cylindrical, with its longest diameter in the longitudinal 

 direction §. 



The bulb rests partly on the inferior ligament, bvit mainly on two calcareous masses {e e) 

 set side by side, being probably a coalescent condition of the ultimate vertebrse {c c), thus 

 united for protective purposes. These masses bear on their ventral surfaces and distal 

 extremities oblique rows of spines, corresponding serially with those already described in 

 connexion with the vertebral plates. These spines can be turned inwards by the animal, 



* MS. Notes of Lectures on Comparative Anatomy, by Prof. Goodsir. Summer of 1858. 



t This fold is well seen in the transverse section of the arm of CriheUa oculata in Plate XIV. fig. 1. 



X Plate XIII. fig. 4. § Plate XIII. fig. 5. 



