PHYSIOLOGY OF THE INVERTEBRATA. 385 



whole animal (except, perhaps, the elevated feeler-ray) lies 

 flat upon the floor." (Romanes and Ewart.) 



Ophiura when placed on its back has the power of righting 

 itself; but it is unable to ascend perpendicular surfaces 

 owing to the rudimentary condition of its ambulacral ap- 

 paratus. 



(e) The UcMnidea. — Unlike the rapid movements of the 

 starfishes, the Echini have a slow rate of locomotion. Along 

 a horizontal surface it is six inches per minute, while up a 

 perpendicular surface it is only a quarter of an inch per 

 minute. The ambulacral feet or pedicels have a greater 

 power of anchorage than the same appendages of the star- 

 fishes. In Echinus the pedi'cels are also used as feelers. 

 When a perpendicular surface is reached, the animal may 

 either ascend it or crawl along for an indefinite distance, 

 feeling it all the way with its pedicels. When an Echinus 

 is inverted upon its aboral pole, it has the power df righting 

 itself, although it is a much more difficult task than is the 

 case with the starfishes. This is due to the formation of its 

 body — for it is a rigid, non-muscular, and globular mass, 

 whose only motive power available for conducting the evolu- 

 tion is that which is supplied by relatively feeble pedicels. 



The spines and lantern* are also used in locomotion. 

 When the animal is taken out of the water and placed upon 

 a table, Romanes and Ewart observed that it began to walk 

 in some definite direction — i.e., in a straight line, and in doing 

 so the only organs used for the purposes of locomotion are 

 the spines and the lantern, the ambulacral feet under these 

 circumstances not being protruded at all. The rate of 

 locomotion is very slow — viz., about one inch per minute. The 

 so-called " Aristotle's lantern " is capable of being protruded 

 and retracted ; and these movements are perfectly rhythmical, 

 at the rate of three or four revolutions per minute. The 

 pedicellariae of Echinus assist in locomotion. It is by means 



» "Aristotle's lantern," or dental apparatus, in Echinus is worked by 

 thirty muscles. 



2 B 



