STAR- FISH AND SEA-URCIIINS. 309 



quicAly as possible. Now, if the righting action of 

 the Icct were entirely and only of a serial character, 

 the rigliiing would require to be performed by rear- 

 ing the animal upwards; the etfect of foot after 

 foot in the same rows being applied in succession to 

 the side of the tank, would require to be that of 

 rotating: the lobular shell ao-ainst the side of the 

 tank towards the surface of the Avater, and therefore 

 against the action of gravity. This is sometimes 

 clone, Avhich proves that the energy required to per- 

 form the feat is not more than a healthy P^chinus 

 can expend. But much more frequently the 

 Echinus adopts another device, and the only one by 

 which it is possible for him to attain his pui'pose 

 without the labour of rotating upwards: he rotates 

 laterally and downwards in the form of a spiral. 

 Thus, let us call the five feet-rows, 1, 2, 3, 4, and 5 

 (Figs. 59, 60, 61), and suppose that 1 and 2 are in 

 use near their ab-oral ends in holding the animal 

 inverted against the perpendicular side of a tank. 

 The downward spiral rotation would then be 

 eflected by gradually releasing the outer feet in row 

 1, and simultaneously attaching the outer feet in 

 row 2 (i.e. those nearest to row 3, and furthest from 

 row 1), as far as possible to the outer side of that 

 row. The eflcc' of this is to make the globe roll far 

 enough to that side to enable the inner feet of row^ 

 o {i.e. those nearest to row 2), when fully protruded, 

 to touch the side of the tank. They establish their 

 adhesions, and the residue of feet in row 1, now 

 leaving go their hold, these new adhesions serve to 

 roll the globe still further round in the same 



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