THE CRAWFISH. 123 



One pair of antennules. 



One pair of antennae. 



One pair of mandibles. 



Two pairs of maxillae. 



Three pairs of maxillipeds. 



Five pairs of legs. 



Six pairs of swimmerets. 



Draw all these accurately and in order, twice natural 

 size. If the parts of the smaller appendages are not 

 easily made out, place such between two glass slips, and 

 examine with low power of microscope. 



Make a diagrammatic drawing of a longitudinal section 

 of the body, showing the relative size and position of the 

 principal internal organs. 



Make a drawing of a crawfish in its natural position, 

 as seen from above. 



Serial Homology. Observe the striking similarity in 

 structure which underlies the great diversity in size and 

 use in all the appendages. Similar likeness in the 

 arrangement of the parts of the segments which bear the 

 appendages is also evident; and these things, together 

 with a more marked correspondence during the early 

 stages of development, indicate that each segment is 

 homologous with every other. The segments being 

 serially arranged, this is called serial Jiomology. 



Development. If crawfishes be collected in early spring, 

 the females will be found with large, berry-like clusters 

 of small, round, dark-colored eggs, glued fast to the 

 swimmerets. As these are lashed backward and forward 

 through the water, the eggs are continually washed, and 

 kept free from mud and other impurities. When the 

 eggs hatch, they split in halves, and the emerging young, 

 already armed with well-developed forceps, cling to the 



