I*JQ HOLMES. [\'OL. II. 



as before. Commonly during swimming the pleopods beat 

 more rapidly, but this is not always the case. When the 

 swimming ceases they drop back into their usual rhythm, 

 whether faster or slower than before. In their motion the 

 three pairs of pleopods act as a unit, keeping perfect time 

 like well-trained oarsmen. If the abdomen be removed from 

 the rest of the body, the pleopods, after a few spasmodic move- 

 ments clue to the shock of the operation, continue to beat 

 rhythmically for several minutes ; the three pairs all move with 

 the same rhythm, though somewhat more slowly than before the 

 operation. Moreover, if a single segment with' its pair of 

 appendages be isolated, the rhythmic motion of the appendages 

 still goes on for some minutes, but gradually becomes slower 

 and more irregular, until nothing but small twitches indicate 

 the existence of irritability. 



When the animal is in a vigorous condition the beat of the 

 pleopods is rapid, but when the creature becomes weakened the 

 beat becomes slower, yet as long as life lasts the pleopods con- 

 tinue their movements. The beat of the pleopods may still 

 persist after the rest of the animal refuses to respond to any 

 sort of stimulation. 



The swimming of Amphithoe is mainly effected by the pleo- 

 pods. The first impulse, however, is gained by the sudden 

 extension of the abdomen, which gives the body a rapid forward 

 movement. The abdomen is then held in an extended position 

 and the pleopods, which then hang nearly at right angles to the 

 body, serve to continue the forward motion. When swimming 

 against the force of gravity the motion of the pleopods alone 

 is not sufficient to keep the body going, and when the original 

 impetus becomes exhausted the abdomen is bent forward and 

 again suddenly extended, giving the animal a fresh start. The 

 flexure of the abdomen before every stroke tends to draw the 

 body backward. This, combined with the weight of the animal, 

 causes ground to be lost between every stroke. Swimming 

 towards the surface is therefore accomplished by a series of 

 springs, between each of which the animal falls back more or 

 less. While swimming horizontally the beating of the pleo- 

 pods is all that is required to keep up the motion ; specimens 



