1912.] The Locomotor Function of the Lantern in Echinus. 91 



Climbing. — An urchin of small or medium size travelling actively with the 

 help of the lantern can surmount considerable inequalities of surface. For 

 example, it may be guided by stimulation to hitch itself for some distance up 

 a stair, the successive steps of which, if broad enough, may be as much as a 

 fifth of an inch in height. Such steps would prove an insuperable obstacle 

 were it not for the powerful aid afforded by the lantern. Compare climbing 

 uphill on a plasticene surface under water (p. 97). 



Maries of Teeth on Laminaria. — Urchins of the medium and larger sizes 

 (from 2f inches in diameter upwards) leave the mark of their teeth when 

 travelling over a frond of Laminaria. Each mark exhibits five dents, some 

 little distance apart, showing (what is brought out also by the various other 

 methods of observation) that the teeth are not closed at the time when they 

 first impinge against the supporting surface. The surface of the frond is not, 

 however, broken through at these dents, still less is any part actually bitten 

 out. The smaller urchins have not weight enough, unless when artificially 

 loaded, to leave the marks of their teeth on Laminaria, but down to the 

 smallest size investigated (-£- inch diameter) they will leave such marks, even 

 when unloaded, if made to travel over a surface of plasticene. 



In the paper already referred to (p. 84) Eomanes and Ewart ascribe only 

 a lifting action to the lantern, the push being apparently given by the spines 

 alone. I am convinced that the lantern also pushes strongly, indeed much 

 more strongly than the spines, in active travelling. Observation by means of 

 a simple arrangement of mirrors allows one to watch the action at leisure. 

 The dents and scratches made by the teeth on plasticene (fig. 4) or paraffin 

 wax always tend to take a backward slope. Lastly and most convincingly, 

 small and medium-sized urchins, the oral surface of which has been denuded 

 of spines, can move by almost as large and as quickly repeated lurches as 

 other urchins of corresponding size in which the spines are intact. The 

 observations on urchins travelling under water (p. 97) have also a bearing on 

 this point. 



Progression by Spines only. — It has just been stated that urchins of small 

 and medium size may travel freely by the action of the lantern alone after 

 all the spines on the oral surface have been removed. Can the spines alone 

 enable the urchin to travel ? The difficulty here is to exclude the use of the 

 lantern without causing such injury as would interfere with co-ordination of 

 the spines. One simple method consists in placing the urchin mouth down- 

 wards over a hole in a piece of wood, the hole being just so big that its edges 

 are clear of the tips of the teeth when swinging begins. Too big a hole puts 

 the spines near the mouth out of use, but if the direction of the lantern 

 swing be first determined by irritation, a long narrow hole can be used. The 



