1912.] The Locomotor Function of the Lantern in Echinus. 89 



Eetraction during the forward swing will be effected chiefly by the joint 

 action of the four anterior pairs of retractor muscles. The retractor muscles 

 tend to pull apart the tips of the teeth, and this circumstance accounts for the 

 separation of the teeth occurring during the return swing, and also for a 

 feature frequently noted, that the posterior tooth, or pair of teeth, is now less 

 divergent than the others, since the posterior retractor muscles are in a 

 condition of, at least partial, relaxation in order to permit the advance of the 

 tips of the teeth. 



Recording Surfaces. — The best means I could hit upon for getting an 

 accurate record of the length of each step was to set the urchin to travel over 

 a thin, smooth layer of plasticene, spread on a suitable backing, e.g. wood, 

 glass, or stout paper. Eecords are quite easy to obtain by this method, and a 

 photograph (natural size) of one such record is given in fig. 4. It was made 



Fig. 4. — Photograph (natural size) of track left on plasticene out of water by an urchin of 

 3| inches equatorial shell diameter. The direction of movement was from left 

 to right along the track. The pushing action of the teeth is indicated by the 

 slope of the tooth -marks, particularly in the later steps, where the plasticene 

 happened to be more thickly spread. The time occupied was 7 mins. 



by an urchin of 3f inches equatorial shell diameter, and shows not only the 

 marks of the teeth, but also those of the spines. The best kind of surfaces 

 for urchins to travel over are those which are hard enough to resist the 

 protrusion of the teeth, and at the same time not so smooth as to make the 

 teeth liable to slip during the push. Glass is accordingly not one of the best ; 



