1912.] The Locomotor Function of the Lantern in Echinus. 87 



come together, but are still more than half open when they first strike 

 against the supporting surface. Protrusion now continues, so that perforce 

 the whole urchin begins to be slightly lifted up. Since at the commencement 

 of the pushing stroke the tips of the teeth are in advance of the centre of 

 gravity of the urchin, one finds that the lifting up chiefly affects the anterior 

 half of the shell, which accordingly becomes distinctly tilted. Meantime the 

 tips of the teeth have come firmly together and are pressing hard against the 

 supporting surface. The whole urchin is now pushed slowly forwards, partly 

 by the action of the spines, but chiefly by that of the lantern itself, through 

 its muscles. 



The lantern and its muscles act like a lever of the third order, the tips of 

 the teeth being at the fulcrum, and the weight being centred somewhere 

 inside the shell. In this manner the weight is gradually moved till it comes 

 to lie in front of the fulcrum, and then the urchin tumbles forward under the 

 influence of gravity. The anterior edge is no longer tilted up, but rather 

 becomes depressed as compared with the posterior one, which in its turn is 

 propped up for a time by the teeth. The latter are still protruded strongly, 

 but now point backwards, and, by pushing to their furthest limit against the 

 supporting surface, increase the distance travelled by the urchin in its 

 forward lurch. 



It is, however, only when the urchins have, so to speak, got " into their 

 stride " that one sees matters proceeding exactly in the above manner. At 

 first, when the steps are shorter and the direction less definite, the alternate 

 tilting up of the anterior and posterior edges is not well marked. Gravity 

 accordingly plays a smaller part in the lurches, each of which is rather a 

 push forward over the tips of the spines than a rapid stumble or lurch. This 

 applies at all times to the movements of the larger sized urchins, i.e. those in 

 which the equatorial shell diameter is greater than 4 inches. The lantern is 

 here smaller in proportion to the size of the whole shell, and the amplitude 

 of forward or backward swing of which it is capable is not great enough to 

 bring the tips of the teeth very sensibly in front of, or behind, the centre of 

 gravity and thus produce the tilting above referred to. Plasticene records 

 and other methods of demonstration show, however, that the lantern is still 

 used in exactly the same manner as before, and that the " steps " are still 

 definite, though each is accomplished more slowly and tends to exhibit 

 minor hitches, which depend on the varying amounts of forward move- 

 ment possible over the particular spines on which the urchin is from time to 

 time supported. 



Muscles involved. — On the whole, the anterior* and posterior protractor 

 * See note on p. 85. 



