1912.] The Locomotor Function of the Lantern in Echinus. 95 



rotate through two or three right angles without shifting its place for more 

 than an inch or so. Again, if it does travel actively and rotate at the same 

 time, the rotation may he very slight (10°) or may amount to a full turn of 

 the circle. At the same time, rotation and progression probably affect one 

 another in determining the character of most of the tracks which come under 

 group (b) as well as of some which fall under group (a). Taking the 

 latter first, we find that there are instances in which each forward step is 

 accompanied by a slight twist to one side or the other, while in the next 

 lurch there occurs a similar twist, but in the opposite direction. Or several 

 successive twists may be to one side, and these may next be rectified by a 

 (roughly) corresponding number in the opposite direction. The line of 

 progress here shows wavy deviations from side to side, and at the end of the 

 journey little or no rotation has been effected. Such instances, however, only 

 form a small proportion of group (a). As will be seen from the accompanying 

 table the average amount of rotation in this group is still distinctly high. 



It is in the cases coming under group (b) that an association between 

 rotation and progression is definitely indicated. This group is marked by 

 continuous curving of the track to one side or the other. From the table it 

 will be seen that in the great majority of instances rotation in the same 

 direction also occurs. 



The factors that produce rotation seem to be twofold. First, there occurs 

 a screwing action transmitted through the lantern, and caused by unequal 

 contraction on the part of the single muscles making up each pair of 

 protractors and retractors, or by unequal contraction on the part of the 

 protractors and retractors at opposite sides. When the teeth are opened and 

 pressed hard against a surface, sufficient power and purchase is thus obtained 

 to produce slight rotation of the whole urchin, and this movement may be aided 

 by pushing on the part of the spines. ISTo doubt the cases where there was 

 rotation through two right angles or more, without sensible progression, are 

 to be explained by successive small twists of this character. It is not 

 improbable that the habit may be of use in feeding by enabling the urchin to 

 dig in its teeth with greater effect, and to repeat the bites many times without 

 travelling far away from ground which it may have found to be suitable. 

 Possibly also the action is reminiscent of a former boring habit, or related to 

 this habit in other urchins (p. 101). Should the action occur while the 

 urchin is travelling, the track will tend to curve in the direction of rotation. 

 There will obviously be no definite limit to the total amount of deflection 

 and rotation thus produced. 



The second element in the production of rotation has reference to the 

 spines alone. These have always to play their part in travelling, chiefly as 



