PHYSIOLOGICAL NOTES. 



The- locomotor movements of an ophiurid, upon 

 a casual observation, seem to consist of an uncoor- 

 dinated writhing and twisting not calculated to bring 

 the creature to rood or a place of safety except by- 

 chance; but a more careful study shows them to be 

 the result of an orderly and nicely coordinated mech- 

 anism. 



The rapid strides which characterize the move- 

 ments of a brittle-star are in strong contrast with 

 the slow creeping movements of a star-fish ot sea- 

 urchin, the difference being due to the employment 

 of different locomotor mechanisms in the two cases; 

 the star-fish and sea-urchin depending entirely upon 

 their tube feet and spines while in the ophiurids 

 the arms them selves are the efficient locomotor 

 organs, they being used much as we use our arms in 

 swimming . 



The arm of an ophiurid consists of a large num- 

 ber of segments each of whiGh contains a central 

 calcarious ossicle. The calcarious ossicles of ad- 

 jacent segments articulat; with each other li'.ce the 



e 



vertebra^ of the spinal columnand are joined together 



by two pairs of muscles in such a manner that motion 



