other Invertebrate- Animal Study 467 



The long antenna; are as flexible as braided whiplashes, large at the 

 base and ending in a threadlike tip. They are composed of many seg- 

 ments, the basal ones being quite large. Above the antennae on each side, 

 is a pair of shorter ones called antennules, which come from the same 

 basal segment; the lower one is the more slender and is usually directed 

 forward; the upper one is stouter, curves upward, and is kept always 

 moving, as if it were constantly on the alert for impressions. The anten- 

 nae are used for exploring far ahead or behind the creature, and are often 

 thrust down into the mud and gravel at the bottom of the aquarium, as if 

 probing for treasure. The antennules seem to give warning of things 

 closer at hand. Between the antennae and antennules is a pair of finger- 

 like organs, that are hinged at the outer ends and can be lifted back, if we 

 do it carefully. 



In looking down upon a crayfish, we can see six abdominal segments 

 and the flaring tail at the end, which is really another segment greatly 

 modified. The first segment, or that next to the cephalothorax, is narrow; 

 the others are about equal in size, each graceful in shape, with a widened 

 part at each side which extends down along the sides of the creature. 

 These segments are well hinged together so that the abdomen may be 

 completely curled beneath the cephalothorax. The plates along the sides 

 are edged with fringe. The tail consists of five parts, one semicircular in 

 the center, and two fan-shaped pieces at each side, and all are margined 

 with fringe. This tail is a remarkable organ. It can be closed or extended 

 sidewise like a fan ; it can be lifted up or curled beneath. 



Looking at the crayfish from below, we see on the abdomen some very 

 beautiful featherlike organs called swimmerets. Each swimmeret con- 

 sists of a basal segment with twin paddles joined to its tip, each paddle 

 being narrow and long and fringed with hairs. The mother crayfish has 

 four pairs of these, one pair on each of the second, third, fourth and fifth 

 segments; her mate has an additional larger pair on the first segment. 

 These swimmerets, when at rest, lie close to the abdomen and are directed 

 forward and slightly inward. When in motion, they paddle with a back- 

 ward, rhythmic motion, the first pair setting the stroke and the other pairs 

 following in succession. This motion sends the body forward, and the 

 swimmerets are chiefly used to aid the legs in forward locomotion. A 

 crayfish, on the bottom of a pond, seems to glide about with great ease; 

 but place it on land, and it is an awkward walker. The reason for this 

 difference lies, I believe, in the aid given by the swimmerets when the 

 creature is in water. Latter says: "In walking, the first three pairs of 

 legs pull and the fourth pair pushes. Their order of movement is as fol- 

 lows: The first on the right and the third on the left side move together, 

 next the third right and the first left, then the second right and fourth 

 left, and lastly the fourth right and second left." 



When the crayfish really wishes to swim, the tail is suddenly brought 

 into use; it is thrust out backward, lays hold of the water by spreading 

 out widely, and then doubles under with a spasmodic jerk which pulls the 

 creature swiftly backward. 



The crayfish's appearance is magically transformed when it begins to 

 swim; it is no longer a creature of sprawling awkward legs and great 

 clumsy nippers ; now, its many legs lie side by side supinely and the great 

 claws are limp and flow along in graceful lines after the body, all obedient 

 to the force which sends the creature flying through the water. I cannot 

 discover that the swimmerets help in this movement. 



