100 



Dr. J. F. Gemmill. 



[Jan. 17 



such urchins would be comparatively helpless in surroundings where their 

 sucker feet could not take hold. 



Fig. 8. — Photograph (natural size) of track left by an urchin (3f inches in equatorial 

 shell diameter) which was travelling over a plasticene surface while only 

 rather more than half covered by sea-water. All the sucker feet, which are 

 normally used when a completely immersed urchin is travelling mouth down- 

 wards, were in action in this instance, but nearly half of the bulk of the urchin 

 did not have its weight neuti-alised by submergence. Accordingly, the lantern 

 has been used with effect, and yet has not needed to be protruded in each step 

 with sufficient force to bring the teeth firmly together. Separate dents are 

 thus left by each of the five teeth in each step. It will be remembered that 

 powerful action cf the protractors causes closure of the teeth (p. 88). The 

 direction of the movement was from left to right. When the urchin reached 

 the lower right-hand corner it was irritated on the side looking towards this 

 corner. The crowding of the tooth-marks at this part is a record of those 

 short and comparatively ineffective swings of the lantern which usually 

 initiate a change of direction due to irritation (p. 93). 



III. Relation to Other Activities. 



(«) Feeding. — No doubt this is by far the most important function of 

 the lantern. The movements employed will resemble in many respects 

 those which have just been described, the chief difference being that little or 

 no lateral swing will be required. However, in feeding, as in locomotion, 

 there must occur : — (1) Commencing protrusion with teeth fully opened ; 

 (2) gradual closure of teeth, accompanied by fuller protrusion, giving firm 

 pressure against the surface which is being bitten or cropped; (3) gradual 



