20*2 THE MECHANICAL FUNCTIONS. 



their expanded state, the Asterias is capable of 

 effecting a slow progressive motion ; so that these 

 processes may be regarded as corresponding to 

 feet, being levers for the advance of the body. 

 This, it may be remarked, is the first time that 

 we meet with organs of that description in our 

 progress through the animal kingdom. Each of 

 these feet is terminated by a concave disk, which 

 when applied to any flat surface acts as a sucker, 

 on the principles already adverted to.* Reau- 

 mur counted 304 of these feet in each of the five 

 rays of the star fish, making 1520 in all.f Each 

 foot consists of a tube, closed at the outer end, 

 and the stem of which, after passing through 

 the aperture in the integument, is dilated into a 

 bag or reservoir of fluid ; as is shown in Fig. 97. 

 By the contraction of this reservoir, the fluid it 

 contains is propelled into the outer portion of the 

 tube, which protrudes by being thus distended ; 

 the foot fixes itself by means of its terminal 

 fleshy disk to the point it touches, and then, by 

 retracting, draws the body along for a short dis- 

 tance. By the retreat of the fluid into its reser- 

 voir, the foot is again detached, and ready to be 

 moved forwards, and is thus made instrumental 

 in taking another step, by a repetition of the same 



* Page 137. 



f Memoires de 1'Academie des Sciences, 1710, p. 487. 



