216 



ZOOLOGY 



behind ? What is this, animal or mineral matter ? Examine the shell of a 

 dead crayfish and try to find out how the segments of the abdomen are 

 joined together. Notice the different positions taken by the abdomen in 

 the living animal. How might this be accounted for mechanically? 



The small appendages attached to the under surface of the segments are 

 called the swimmerets or pleopods. How many are attached to each seg- 

 ment? Observe the move- 

 ment when in the water.' Why 

 are they called swimmerets? 

 Notice that each plcopod is 

 made up of three pieces, a 

 base and two branches. Now 

 look at the broad appendage 

 that, together with the last 

 segment of the abdomen, 

 fcjrms a broad finlike appa- 

 ratus, the caudal -fin. You 

 will find this appendage like- 

 wise composed of three pieces ; 

 it is homologous to the pleo- 

 pods. This appendage is 

 known as the uropod. Cray- 

 fish normally swim very rap- 

 idly in a backward direction 

 by means of a sudden jerk- 

 ing of the caudal fin. Other 

 methods of locomotion may 

 be observed by allowing the 

 animal to walk in a shallow 

 pan with a little water in it. 

 Note the position and number 

 of jointed legs that are used 

 in locomotion. Is the large 

 pair of pincher legs used for 

 this purpose? If so, to what 

 extent ? Crayfish have a defi- 

 nite method in the movement 

 of the walking legs, the first 

 and third moving in alterna- 

 tion with the second and 

 fourth legs on the same side of the animal. Test a crayfish to see if this rule 

 holds true. Dp you find any other methods of locomotion than the ones 

 mentioned? 



Watch the animal in its movements to see if, in avoiding objects, it first 

 allows certain parts of the body to touch the object. The longer of the 

 two pairs of feelers (the antenna) function as organs of touch. Hairs 

 which are sensitive to touch are also found in various other parts of the 

 bod3^ The bases of the antennse are broad, and a small flat piece projects 

 outward from this basal portion. The antennae are believed to have also 

 the function of smell. Crayfish are thus able to learn of the presence of 

 food at a considerable distance away. The short appendages immediately 

 in front of the antennse are called the antennulce (little antennse). Notice 

 that the four stalks are in reality two branches from one base on each 

 side. Part of this appendage is believed to contain the organ of hearing. 

 Test in any manner that you can the sight of the crayfish. Test with 



Female lobster, showing eggs attached to the swim- 

 merets. From photograph loaned by the American 

 Museum of Natural History. 



