450 TUBULAR SUCKERS OF ECHINIDA. 



membranous tubes, each of them furnished with a sucker at its 

 extremity, are capable of being projected. These tubes consist 

 of two layers of muscular fibre, the outer one of which is circular, 

 whilst the inner one is longitudinal. Each tube is connected 

 with a little vesicle containing fluid, which is found within the 

 shell, just behind the minute orifice through which the tube 

 is projected. The several vesicles, which form five double rows 

 (//, Fig. 605), are connected by a set of vessels adapted to 

 distend them with fluid ; but every one seems to have a perfect 

 command over its own tubular foot. When the vesicle propels 

 fluid into it, the tube is projected to a considerable length ; and 

 the bands of muscular fibre which it possesses enable it to be 

 turned in any direction, and to apply its sucker in the most 

 advantageous manner. "When the distension is withdrawn, by 

 the relaxation of the walls of the vesicle, the longitudinal fibres 

 contract, and the tube is shortened with considerable force ; the 

 water which it contained flowing back through the aperture of 

 the shell into the vesicle behind. In many species this apparatus 

 appears to afford the principal means both of locomotion and of 

 the prehension of food. When several tubes are projected in any 

 particular direction, and their suckers are attached to a fixed 

 point beyond, it is obvious that, if the tubes then contract, the 

 whole body will be drawn towards that point. Appearing, as 

 it will then do, to turn upon the spines, it is not remarkable that 

 many observers should have regarded the spines as the real 

 instruments of locomotion ; since the transparency of the tubes 

 will prevent their being observed, unless they are carefully 

 looked for. 



1007. The action of these tubular suckers may be well seen 

 in the common Asterias or Star-fish ; from the under side of 

 whose arms they project in great numbers. If one of these 

 animals be placed in a glass vessel of sea- water, it may be seen 

 to climb up its sides by the use of this apparatus. One set of 

 suckers is fixed, and the animal is drawn towards them by the 

 contraction of their tubes ; it then takes a fresh attachment by 

 a set which were previously free ; and thus the numerous feet 

 are all employed, fixing and detaching themselves alternately, 



