1912.] The Locomotor Function of the Lantern in Echinus. 93 



considerable time elapses before the change becomes obvious through actual 

 movement of the whole shell. Much earlier, however, as careful watching 

 shows, the lantern of Aristotle has begun to change the direction of its 

 rhythmic swing, and a large part of the delay is due to the fact that the teeth 

 cannot at once get themselves into proper position for the commencement 

 of a lurch. One or more ineffectual swings have usually to occur before the 

 first step is actually taken. These swings tend to be intermediate between 

 the old and the new directions, and work round gradually to the latter (see 

 fig. 8). 



Inversion. — As was pointed out by Eomaues and Ewart, if an urchin be 

 turned upside down out of water, the lantern will exhibit rhythmic swinging 

 movements accompanied by protrusion, retraction, closing and opening of the 

 teeth. These movements are just those of the locomotor swing. In vigorous 

 specimens they are even slightly greater in extent, as well as quicker in 

 rhythm, than when actually employed for movement. A very interesting 

 point is that the direction of swing of the lantern in an inverted urchin can 

 be made to change by stimulation, and that the new direction adopted is 

 always the one that would lead the urchin away from the stimulus were it 

 free to travel. As noted for the normal position, in the previous paragraph, 

 the change of swing of the lantern takes some little time to manifest itself, 

 and is usually initiated by one or more short intermediate oscillations. 



Equatorial Section. — If an urchin shell be divided equatorially, and the 

 lower half, still full of ccelomic fluid, be set mouth downwards on a suitable 

 surface, it will still in some instances progress, though never freely or far, 

 both when free of load and when carrying moderate loads. Under these 

 conditions it is possible to observe from the inside the swinging of the 

 lantern during the course of each lurch, and I have obtained records on 

 plasticene demonstrating the corresponding action of the teeth. At best, 

 however, no doubt owing to the shock caused by the division of' the shell, the 

 movements are very far from being regular or persistent. 



General Facts regarding Progression. — If a number of apparently healthy 

 urchins be taken at random out of the tanks and placed mouth downwards on 

 a table, about half of them will begin to travel with considerable freedom, 

 the rest remaining sluggish. It is hard to say what causes the difference in 

 each case. Small and medium-sized urchins start off more readily than large 

 ones, but the same urchin may go freely at one time, and at another hardly 

 shift its place. In the latter event the lantern is either not in use at all, 

 such movement as occurs being caused by the spines, or more rarely 

 the teeth are only knocking or biting up and down against the surface. In 

 either case irritation, as by scratching one side with a needle, is sometimes, 



