STAR-FISH AND SEA-URCHINS. 259 



is provided in its membraneous walls with a number 

 of annular or ring-shaped muscular fibres ; when 

 these fibres contract, the fluid contained in the tube 

 is forced back, Avliile, conversely, when these fibres 

 relax, the fluid runs into the tube. If the contrac- 

 tion of these fibres is strong, the tube shrinks up 

 entirely, i.e. is retracted within the body of the 

 animal ; but if the contraction of the fibres is not 

 so strong, the tube is only shortened. If, before its 

 shortening, its terminal expansion, or sucker, has 

 been applied to any flat surface, the effect of the 

 shortening is to cause the sucker to adhere to the 

 flat surface, in consequence of the pressure of 

 the surrounding sea-water being greater than that 

 of the fluid within the shortened tube. In this 

 way, by alternately contracting and relaxing the 

 muscular fibres in the walls of a tube-foot, a Star- 

 fish is able alternately to cause the terminal sucker 

 to fasten upon and to leave go of any flat surface 

 upon which the animal may be crawling. In other 

 words, when the tube-foot is about to form its 

 attachment to a flat surface, it is fully distended 

 wdth fluid ; but when the terminal sucker touches 

 the flat surface, this fluid is partly withdrawn, so 

 causing the sucker to adhere. 



When we dissect out one of these tube-feet, wo 

 find that at its base, within the body of the animal, 

 it bifurcates into two branches. One of these branches 

 passes immediately into a closed sac (Fig. 3C, /), 

 while the other passes into a large tube (Fig. 3G, k), 

 which runs all the way from base to tip of the ray, 

 receivinjx in its course similar branches from all the 



