86 



Dr. J. F. Gemmill. 



[Jan. 17, 



lantern points downwards and backwards, and the tips of the teeth, which 

 are firmly closed and strongly protruded, lie a little behind the centre of 

 gravity of the whole urchin, the posterior edge of which is slightly tilted up. 

 The lantern is next retracted so that the teeth are lifted from contact with 

 the supporting surface. The tilting up of the posterior edge now disappears, 

 and the lantern is then moved pendulum-wise till it comes to point down- 

 wards and forwards, and the tips of the teeth are slightly in advance of the 



Fig. 1. — Record of track ( x obtained by setting urchin on a large sheet of paper, 

 marking a particular spine, and placing a small circle on the paper below this 

 spine at the end of each lurch. The lines connecting the various dots indicate 

 approximately the path taken by the spine. The total distance travelled by 

 the central part of the urchin amounted to about 14 inches, the number of 

 lurches was 37, and the urchin itself measured about 3| inches in equatorial 

 shell diameter. The track is less regular than the usual type. (Drawing- 

 made by Miss Abel.) 



centre of gravity of the whole urchin. This is what we have called the 

 return swing. Early in it, the tips of the teeth begin to diverge slowly from 

 one another. The separation increases up till the end of the swing, and 

 reaches its maximum immediately after the commencement of the second 

 part of the whole movement, i.e. the part which we have called the pushing, 

 or poling stroke. This part is initiated by protrusion downwards and 

 forwards of the very widely separated tips of the teeth. The latter begin to 



