1912.] The Locomotor Function of the Lantern in Echinus. 99 



lantern (fig. 7). Too gentle a slope will not give the desired result, and in 

 my experiments it was only with an incline of 25° or over that a record 

 could be obtained. 



The third method consisted in setting the urchins to travel over a tightly 

 stretched surface of fine tow-netting silk placed in a glass vessel, the bottom 

 of which was thin and smooth enough to allow observation through it. One 

 then watched what happened, by means of a simple arrangement of reflecting 

 mirrors. The meshes of the silk interfere with the action of the suckers at 

 the ends of the podia, and the movements of the spines are, to some extent, 

 hindered through the tips sticking in the meshes, while at the same time the 

 silk is sufficiently transparent to allow the teeth to be distinctly seen when 

 they are protruded and brought up hard against it. Under these circum- 

 stances, both when loaded and unloaded, urchins may be observed to use the 

 lantern freely and with effect. 



Partial Immersion. — Urchins that are only partially immersed under water 

 will be found to use their lanterns for locomotion even over horizontal 

 surfaces, in a degree varying inversely with the depth to which they are 

 immersed. One may evoke stronger or weaker efforts on the part of the 

 lantern simply by siphoning water out of, or into, the vessel in which the 

 urchins are contained. Fig. 8 shows a photograph of the track left on 

 plasticene by an urchin which was just a little more than half immersed (see 

 explanation appended to the figure). 



The employment of the lantern, as described above, may at times be of the 

 greatest service to urchins in the lower tidal zones, by enabling them to 

 escape back into deeper water should the outflowing tide threaten to leave 

 them in an unfavourable position. 



Under water, unless when heavily loaded, even large urchins will leave no 

 impressions of their teeth when travelling over a frond of Laminaria. The 

 bearing of this on the question of feeding is referred to later (p. 101). 



The lower half of an urchin placed mouth downwards may travel for short 

 distances under water, but, like the uninjured urchin, it will not, except 

 when loaded, leave impressions of its teeth on a plasticene surface. 



In nature, urchins may have to travel under water over horizontal 

 surfaces {e.g., sand, mud, rocks, etc., covered by growths of fine algae, etc., or 

 powdered with sand or mud), where the sucker feet cannot come into action, 

 and where it may be of great advantage that the action of the spines should 

 be reinforced by that of the lantern. This is the more important since 

 there is evidence that urchins {Echinus esculentus), whether normally or- 

 pathologically, not infrequently shed all but the smallest of their spines, and 

 as spines are structures of somewhat slow growth, were it not for the lantern 



