THE DECAPOD CRUSTACEA. 203 



the meaning of the evolution theory. All are constructed 

 on fundamentally the same plan, but display almost infinite 

 modification in detail. As already hinted in the preceding 

 chapters, it is clear that while the ancestral forms, like the 

 more primitive living forms, must have been free-swimming 

 animals inhabiting open water, the tendency of all has been 

 to acquire in many different ways the creeping habit, which 

 is an adaptation to life on the sea-bottom. Further, some 

 forms, like the swimming crabs, have secondarily re-acquired 

 the power of swimming, but accomplish this by the modified 

 legs, and not by the appendages of the tail as the primitive 

 forms do. 



The motion of the more primitive swimming Decapods 

 is very well worth study and is of much interest. It is 

 perhaps most easily observed in some of the smaller prawns, 

 which live well in confinement and require less space than 

 the larger forms. When undisturbed their swimming is 

 the perfection of graceful and apparently almost effortless 

 movement. The tail-fan is kept expanded, and serves as a 

 rudder to alter the direction of the movements as occasion 

 may require ; it must also be of much use as a float, by its 

 extent and lightness assisting to support the body in the 

 water. The antennal scales, which are often large, no doubt 

 also perform both functions. The propulsion of the body is 

 effected by the movements of the anterior swimmerets, 

 which by their constant motion can drive the body in any 

 direction. Startle your prawn and you will find that it 

 darts backwards or sideways by the sudden flexion of the 

 mobile tail. It is, however, characteristic of the Natantia 

 that their ordinary mode of movement is gentle swimming 

 by means of the anterior five pairs of swimmerets. The 

 creeping Decapods have lost this mode of motion, and 

 though they retain in many cases the power of jerking 

 themselves backward at a sudden alarm, their ordinary 

 method of locomotion is a leisurely creeping. The anterior 

 swimmerets may be retained, or may be largely aborted, 

 but they are never strong enough to propel the heavy body. 

 Beginning with this prime distinction of habit, it is easy 

 to deduce the structural characters of the two sets, and it is 

 of very much interest to note how the minute differences 

 between Crustacea, such as prawn, lobster, and crab, are 



