No. 4.] AMT1I1 rtlOE LOXG1MAXA SMITH. 171 



may be seen swimming about for a considerable time without 

 employing the abdomen. 



Amphithoe has a decided disinclination for continuous swim- 

 ming. Ordinarily it makes only short excursions from one 

 place of concealment to another and generally stops upon 

 meeting with the first solid object that comes in its way, 

 although when situated where there is nothing to which it can 

 lay hold it may swim for some time in a uniform manner. It 

 may swim in various ways, on its side, or with either the dorsal 

 or the ventral surface uppermost, and apparently gets along 

 with about equal facility in any of these positions. 



The beat of the pleopods tends to propel the body not in a 

 straight line forward but in a circular course. The pleopods 

 being on the ventral side tend to cause the body to veer around 

 towards the dorsal side. When the body is held somewhat 

 concave on the ventral side, as it often is, this tendency may 

 be balanced or overcome by the tendency to move in circles in 

 the opposite direction. Such a condition is analogous to a 

 person rowing on one side of a boat with the rudder turned 

 toward the side of the oar. By having the body extended to 

 the right degree a straight course may be maintained. The 

 direction of movement is often changed by the animal turning 

 now on one side and now on the other. Circular movements 

 in one direction are thus compensated for by circular move- 

 ments in another as the animal turns over and a certain general 

 direction of motion is maintained. When swimming on the 

 back a nearly straight course is kept by rolling the body 

 slightly to the one or the other side. Rolling is probably 

 effected by the movements of the hinder pairs of thoracic legs. 

 When the animal is swimming these legs project outward and 

 backward. A downward stroke of these appendages on one 

 side would push the same side of the body upward and roll it 

 over toward the opposite side. In a larger species of amphipod, 

 whose movements are not so exceedingly rapid as those of 

 Amphithoe, I was able to see that the rolling of the body was 

 effected in just this way. It is highly improbable that in 

 Amphithoe a different method would be employed to produce 

 the same result. However this may be, it is certain that 



