384 PHYSIOLOGY OF THE INVERTEBRATA. 



method. The muscular system of the rays is very perfectly 

 developed, enabling these long and snake-like appendages to 

 perform with energy and quickness a great variety of snake- 

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

 nated, the animal is able by these writhings to shuffle itself 

 along flat horizontal surfaces at a considerable speed. But 

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

 plan. If the animal is advancing in the direction of the 

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

 direction ; the two adjacent rays, 2 and 3, are thrown for- 

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

 downwards upon the floor of the tank, these two rays partly 

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

 throw themselves violently backwards into the form of 

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

 movement is to propel the animal forwards — ray i being 

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

 along into the position 4' and 5'. As soon as the rays 2 and 3 

 have assumed the position 2' and 3', they are again, without 

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

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

 animal advances by a series of leaps and bounds, which vary 

 between i| and 2 inches in length, and which follow one 

 another with so much rapidity, that a lively Ophiura can 

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

 travelling, the ray, i, is usually kept straight pointed and 

 partly uplifted — doubtless in order to act as a feeler ; but 

 sometimes the animal varies its method of progression, so as 

 to use two pairs of arms for the propelling movements, and 

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

 and so rendered useless as a feeler. Ophiura is able to use 

 any pair, or pairs, of its arms as propellers indifferently, and 

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

 thirds upon the tank floor, and at each leap raising their 

 remaining two-thirds, together with the anterior part of the 

 disc, ofl: the floor ; at the end of each leap, however, the 



