181 THE MECHANICAL FUNCTIONS. 



sponding to feet, being levers for the advance of 

 the body. Each of these feet is terminated by a 

 concave disk, which, when applied to any flat sur- 

 face, acts as a sucker, on the principles already 

 adverted to.* Reaumur counted 304 of these feet 

 in each of the five rays of the star-fish, making 

 1520 in all.t Each foot consists of a tube, closed 

 at the outer end, and the stem of which, after pass- 

 ing 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 por- 

 tion 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 distance. By the retreat of the fluid into its 

 reservoir, 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 

 process.^ From the shortness of these feet, not- 

 withstanding their great number, the advance which 



* Page 126. 



+ Memoires de I'Acadeniie des Sciences, 1710, p. 487. 



X The mechanism by which the feet are protruded and retracted is 

 ilhistrated by the diagram, Fig. 97, which exhibits the bladders 

 connected with them, in different states of distention and contrac- 

 tion. Fig. 96 shows the upper side of the ambulacra, and of the 

 bladders connected with the feet. These bladders are supplied with 

 fluid by a set of canals quite distinct, according to Tiedemann, from 

 the vessels which circulate the nutritious juices, and which will be 

 hereafter described. 



In addition to these larger tubes, constituting the feet, there exists 

 also a smaller set, which pierce the skin in different places, and are 

 channels for the absorption of the water used in respiration. These 

 also will be noticed more particularly hereafter. 



