42 



apparently only by help of the muscles imbedded in their walls; not as is usually snpposed 

 by any independent contraction of the ampollse belonging to the water-feet. These ampollse 

 are in any case in the Brisinga so extraordinarily thin-skinned, that they scarcely could have 

 any other destination than to take up the superfluous water as simple reservoirs when the 

 water-feet are contracted, and again to deliver out the water required when the water-feet 

 are extended. 



I have not been able directly to observe any independent movement of the arm- 

 spines; but according to the frequently very different direction which they, especially the 

 long marginal spines, shew in the captured specimens, sometimes standing out from the arm 

 at nearly right angles, sometimes lying close to the sides of the arms, we may conclude that 

 they really have a voluntary movement; an inference that seems also to be warranted by 

 their peculiar articulation to the skeleton, and by the muscular skin surrounding their base. 



Also the whole arm is to a certain extent movable; all the ambulacral vertebrae 

 being, as already mentioned, movably connected with each other by elastic muscular liga- 

 ments. This movement, which is easily observed in the specimens recently captured, can 

 take place both in a horizontal and in a vertical direction. In the last named movement 

 the whole arm can even be curved upward nearly in a complete circle. The first named 

 movement is chiefly observed in the exterior part of the arm only. All these movements of 

 the arm are however far from being effected with the same force and rapidity as in the 

 Ophiurag, but in an extremely slow nearly imperceptible manner; it must however be re- 

 marked that in all probability all the movements, and consequently also these, would, while 

 the animal is creeping on the bottom in its normal state, be effected with greater liveliness 

 than noticed in the captured specimens. The greatest mobility seemed to exist between the 

 exterior arm-joints; for which reason it is also especially this part of the arm which will 

 be found variously bent and twisted, in the captured specimens. Nearer to the base the 

 mobility becomes more and more limited, owing to the much shorter muscular ligaments 

 between the single vertebrae, until, at the junction of the arm with the disc, it is nearly 

 reduced to nothing. I have therefore reason to presume, that in the normal state the basal 

 parts of the arm always retain unaltered their relative positions during the movements of 

 the animal. When the animal is captured, and I think also in another case which will be 

 subsequently noticed, an abnormal convulsive movement of the whole arm takes place, which 

 causes the complete separation of the arm from the disc. Such a separation, always taking 

 place just where the first vertebra of the arm connects itself with the immovable skeleton 

 of the disc, may easily be explained, partly by the movement of the whole arm operating 

 just at that point with the greatest force, and partly by the skin at that part being more 

 easily broken than at any other. 



Any movement of the disc itself is however made impossible by the firm attachment 

 of the calcareous pieces composing the bucal ring. It is only in the softer parts, stretched 

 within the calcareous ring, that phenomena of movement can be observed. Apart from the 



