ANATOMY OF ASTEEIAS. 173 



through the ambulacral apertures, forming the principal agents whereby 

 locomotion is effected, next require our notice. In the annexed figure 

 (fig. 87) they are seen fully extended, projecting for some distance 



Fig. 87. 



Asteriaa. 



beyond the margins of the ambulacral grooves that occupy the middle 

 of each ray, every one of them being furnished at its extremity with a 

 sucking disk, adapted to take firm hold upon any smooth surface. The 

 mechanism whereby these suckers, or feet, as they are usually called, 

 are extended from the body and again retracted, is very simple. That 

 portion of each foot which is external to the shell is a muscular tube, 

 closed at one extremity, namely that whereunto the sucker is appended ; 

 whilst by the opposite it communicates, through the corresponding 

 ambulacral hole, with a globular contractile vesicle situated within the 

 body of the animal. Both the tubular foot, and the vesicle appended 

 to it, are endowed with a power of independent action ; so that, if the 

 vesicle contracts, the fluid within it is forced into the external tubular 

 portion of the organ, which thus becomes distended and rendered erect ; 

 but if, on the other hand, the muscular tube shrinks in turn, the con- 



