1912.] The Locomotor Function of the Lantern in Echinus. 97" 



II. Under Water. 



In ordinary circumstances the lantern of Aristotle is of little importance as 

 regards locomotion under water. Even large urchins will travel mouth 

 downwards over a smooth plasticene surface without leaving the marks of 

 their teeth. The sucker feet are now in action and, in addition, the spines are 

 able to play their part not merely as passive supports, but, by means of the 

 muscular collars at their bases, as active agents in pushing the urchin along, 

 now that its weight is so greatly lessened. Indeed, under water a vigorous 

 urchin can move with no little freedom over a horizontal surface without 

 using anything but its spines. Accordingly, although a rhythmic swing of 

 the lantern in the direction of progression may still occur, the teeth are not 

 protruded so far or so vigorously at any time as to press hard against the 

 supporting surface. 



Again, in ordinary movements on a vertical surface the sucker feet and 

 spines seem to suffice. But when the urchin is travelling quickly upwards on 

 such a surface it may use its lantern with powerful effect to aid the 

 progress. If one observes this through the wall of a thin glass vessel, one can 

 hear the grating of the teeth against the glass and see them slipping in the 

 middle of the push, or even twice or thrice in each push, owing to the 

 smoothness of the surface. I noted this particularly in some urchins brought 

 into sea- water which had first been boiled, to expel the air, and then cooled 

 down to ordinary tank temperature (p. 106). Xo doubt, in nature, the lantern 

 will be of no little use in quick upward progression on any vertical surface 

 which is sufficiently rough to allow the teeth to push without slipping, and 

 yet smooth enough to give attachment to the sucker feet. 



However, even on a horizontal, or practically horizontal, surface, I have 

 been able by three methods to compel the use of the lantern for locomotion 

 under water, and by two of these methods to obtain a record on plasticene of 

 the marks left by the teeth, while in using the third I had to be content with 

 ocular observation of the action. Of the first two methods, one consisted 

 simply in loading the urchin with a weight of half a pound or more and setting 

 it to travel on a horizontal plasticene surface (fig. 6). In the second method 

 I left the urchins unloaded, but induced them to travel up an inclined 

 plasticene surface by first setting them on such a surface and then 

 irritating the downward side. The sucker feet are able to come into action 

 against a smooth plasticene surface, but not so effectively as, for example, 

 against glass, and this circumstance, combined with the fact that the weight 

 of the shell had to be lifted up the slope, apparently made it necessary that 

 the action of the feet and the spines should be reinforced by that of the 



VOL. LXXXV. — B. H 



