86 



MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 



with the slow creeping movements of a starfish or sea-urchin, the difference being due to the 

 employment of different locomotor mechanisms in the two cases; the starfish and sea-urchin 

 depending entirely upon their tube feet and spines while in the ophiurids, the arms themselves 

 are the efficient locomotor organs, they being used much as we use our arms in swimming. 



The arm of an ophiurid consists of a large number of segments, each of which contains a 

 central calcarious ossicle. The calcarious ossicles of adjacent segments articulate with each other 

 like the vertebne of the spinal column, and are joined together by two pairs of muscles in such 

 a manner that motion is possible in all directions. This mechanism is aided in producing the 

 locomotion of the creature not only by the arm. spines, where they are present, but by the foot 

 tentacles. These latter organs, which are the homologues of the tube feet of other echinoderms, 

 have been previously regarded as having given up their locomotor function entirely, but 1 shall 

 show further on that this is not true in the genus Ophiura. 



The experiments I carried on last summer on the movements of ophiurans resulted in little 

 that is new, but on account of the confirmation my notes and photographs give to PREYKRS' work 

 (11) on the same subject, it has been thought advisable to publish them. 



In the usual method of progression one arm precedes, it taking no other part, apparently, than 

 to point out the way; the two arms adjacent to and behind the anterior arm make the stroke; the 

 remaining arms are dragged behind, acting as a rudder. 



FIG. 1. 



FIG. -j. 



FIG. 4. 



FIG. 5. 



No preference as to which arm should precede could be found in an adult ophiurau, each arm 

 being equally capable of going before, making the stroke, or following behind. 



If greater speed is needed, for example, to get away from a strong stimulus, the arm which 

 precedes may also take part in the stroke, its contractions being made simultaneously with those 

 of the side arms. This added force, if produced repeatedly on one side, would soon change the 

 course of progression, but this-difficnlty is overcome by an alternation of the stroke of the preced- 

 ing arm, first on one side, then on the other (text tig. 2). 



In a third method of normal locomotion the arms are arranged as is seen in text fig. 5, in 

 which only one arm follows, acting as the rudder. This leaves two pairs of arms for the stroke, 

 but the anterior pair is usually most vigorous in its contractions. 



Since no physiological differentiation into anterior, posterior, or lateral parts is to be found in 

 ophiurids, the creatures are under no necessity of turning the body when a change in the direction 

 of progression is to be made. The arm which finds itself pointing in the new direction to be 

 traveled takes the lead, although it may have been either lateral or posterior in position in the 

 previous movements. 



As has been mentioned before, the foot tentacles aid in making the strokes of the arms efficient 

 in propelling the body. After a stroke has been made, while the arms are being drawn forward 

 and extended for a new stroke, the tentacles can be seen moving actively about, but as the arms 

 come to rest for the backward movement the tentacles are thrust down against the substratum 

 and cease to move. The tentacles thus fit themselves into the inequalities of the surface and 

 afford fixed points for the arms to pull against. The tentacles of the posterior arms act in the 

 same way, and are efficient in preventing the force of the stroke being lost in side motion. 



In ophiurans with long arm spines these latter structures may perform the function just 

 described for the foot tentacles, but in the genus Ophiura the arm spines are very minute and 

 closely applied to the sides of the arms. 



