160 MANUAL OF ZOOLOGY SECT. 



In each of the ambulacral grooves there are two double 

 rows of soft tubular bodies ending in sucker-like extremities ; 

 these are the ta^-feet (Figs. 80, 81, 82, T. P.). In a living 

 specimen they will be seen to act as the locomotive organs 

 of the animal. They are capable of being greatly extended ; 

 when the Starfish is moving along, it will be observed to 

 do so by the tube-feet being extended outwards and for- 

 wards (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 

 forwards; 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, 82, 

 A, <*:.), over which is a median process, the tentacle (/.), 

 similar to the tube-feet but smaller and without the terminal 

 sucker. The tentacles have been ascertained by experi- 

 ment to be olfactory organs, the Starfish being guided 

 to its food much more by their means than by the sense of 

 sight. 



If one of the arms be cut across transversely (Fig. 81 and 

 Fig. 82, 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 

 connected 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 coslome or body-cavity, and below, between 

 the two arms of the V, is the ambulacral groove. The dorsal 



