38o PHYSIOLOGY OF THE INVERTEBRATA. 



studied by Drs. Romanes and Ewart ;* and tliey liave shown 

 that the ambulacral system is instrumental in the locomotion 

 of all these animals, except the Opliivridca. 



■ The Astcridca. — {a) The common starfish (Urastcr rubens) 

 crawls upon a flat horizontal surface at the rate of two inches 

 per minute. "The animal usually crawls in a determinate 

 direction, and, while crawling, the ambulacral feet at the end 

 of each ray are protruded forwards as feelers ; this is parti- 

 cularly the case with the terminal feet on the ray, or rays, 

 facing the direction of advance. When in the course of their 

 advance, these tentacular feet happen to come into contact with 

 a solid body, the animal may either continue its direction of 

 advance unchanged, or may deflect that direction towards the 

 solid body." Urastcr rubens has the power of ascending per- 

 pendicular surfaces, and also of attaching itself to solid bodies. 

 The ambulacral feet are so strong in holding on to a perpen- 

 dicular surface, that the feet of one or two rays are sufficient 

 to support the animal when its body is distended in a hori- 

 zontal position (Fig. 74). If Uraster is turned over on its 

 dorsal surface upon the flat floor of a tank, it does not occupy 

 more than half a minute in righting itself. This is done by 

 a number of the ambulacral feet of three rays getting a firm 

 hold of the floor of the tank ; this being done, the animal 

 turns a complete somersault — the disc and inactive rays being 

 thrown over the active ones with considerable rapidity. 



(/>) The sun-stars (Solastcr) move about in a similar manner 

 to Urastcr ; but the method of righting themselves is slightly 

 different from that just described. 



(c) Astro2)cctcii airrantiacus. — Eomanes and Ewart state 

 that the ordinary locomotor movements of this species are 

 highly peculiar. The general form of the animal resembles 



Urastcr, although its disc is proportionally larger, and the 

 whole animal smaller. Its ambulacral feet are pointed tubes, 

 about a quarter of an inch long, and unprovided with any 



* Philosophical Transactions, 1 881, p. S29. 



