160 MANUAL OF ZOOLOGY sect. 



of the ambulacral grooves two double rows of soft tubular 

 bodies ending in sucker-like extremities ; these are the tube- 

 feet (Figs. 86, 88, T. F), of which, in Asterias, there are 

 four rows in each arm. In a living specimen they will 

 be seen to act as the locomotive organs of the animal. They 

 are capable of being greatly extended, and when the star- 

 fish is moving along, it will be observed to do so by the 

 tube-feet being extended outwards and forwards {i.e., in the 

 direction in which the animal is moving), their extremities 

 becoming fixed by the suckers, and then the whole tube- 

 foot contracting so as to draw the body forward ; the hold 

 of the sucker then becomes relaxed, the tube-foot is stretched 

 forwards again, and so on. The action of all the tube-feet, 

 extending and contracting in this way, results in the steady 

 progress of the starfish over the surface. With the aid of 

 the tube-feet the starfish is also able to right itself if it is 

 turned over on its back. 



At the extremity of each of the ambulacral grooves is to 

 be distinguished a small bright red spot, the eye (Fig. 88, A, 

 oc), and over it a median process, the tentacle (t), similar 

 to the tube-feet but smaller and without the terminal sucker. 

 The tentacles have been ascertained by experiment to be 

 olfactory organs, the starfish being guided to its food much 

 more by this means than by the sense of sight. If one of 

 the arms be cut across transversely (Fig. 87 and Fig. 88, B) 

 and the cut surface examined, the dorsal part of the thick, 

 hard wall of the arm will present the appearance of an arch 

 (with its convexity upwards), and the ventral part the form 

 of an inverted V, the ends of the limbs of which are con- 

 nected with the ventral ends of the dorsal arch by a very 

 short, flat, horizontal portion. Enclosed by these parts is a 

 space, a part of the ccelome or body-cavity, and below, be- 

 tween the two arms of the V, is the ambulacral groove. 



