384 PHYSIOLOGY OF THE INVERTEBRATA. 



method. The muscular system of the rays is very pei 

 developed, enabling these long and snake-like appenda 

 perform with energy and quickness a great variety of s 

 like writhings. As the movement of all the arms is co 

 nated, the animal is able by these writhings to shuffle 

 along flat horizontal surfaces at a considerable speed, 

 when it desires to move still more rapidly, it adopts ai 

 plan. If the animal is advancing in the direction c 

 arrow (Fig. 75), one of its rays, i, is pointed straight! 

 direction ; the two adjacent rays, 2 and 3, are throw: 

 wards as far as possible, and then, by a strong contr 

 downwards upon the floor of the tank, these two rays 

 elevate the disc, and, while keeping the disc so ele 

 throw themselves violently backwards into the foi 

 crescents, as represented in 2' and 3'. The result 

 movement is to propel the animal forwards — ^ray i 

 pushed into the position i', while rays 4 and 5 are dr 

 along into the position 4' and 5'. As soon as the rays 2 

 have assumed the position 2' and 3', they are again, w 

 an instant's delay, protruded straight, to be again ins 

 thrown into the form of the curves 2' and 3'. Thi 

 animal advances by a series of leaps and bounds, whicl 

 between i\ and 2 inches in length, and which folio 

 another with so much rapidity, that a lively Ophiur 

 easily travel at the rate of 6 feet per minute. While 

 travelling, the ray, i, is usually kept straight pointe 

 partly uplifted — doubtless in order to act as a feelei 

 sometimes the animal varies its method of progression, 

 to use two pairs of arms for the propelling movement 

 in this case the remaining arm is, of course, dragged b 

 and so rendered useless as a feeler. Ophiura is able 

 any pair, or pairs, of its arms as propellers, indifferent! 

 in all cases it so uses them by resting their outer, or 

 thirds upon the tank floor, and at each leap raising 

 remaining two-thirds, together with the anterior part ' 

 disc, off the floor; at the end of each leap, howeve 



