THE ARTHROPODS 369 



position of the break in the skeleton through which the 

 body is withdrawn varies. If possible, examine "molts" or 

 cast-off skeletons of a crab, king-crab, and an insect (e.g., 

 grasshopper or cicada). 



Molting affects crayfishes and all other arthropods so 

 severely that many die from exhaustion, or possibly because 

 blood is lost from broken appendages. However, the disad- 

 vantages of molting appear to be more than counterbalanced 

 by the great protective value of the external skeleton. 



313. Near Relatives of the Crayfish. The remarkably 

 close similarity between crayfish and lobster can only be 



Fio. 118. A common shrimp (Palaemonetes). Eggs shown attached to the 

 swimmerets of the abdomen. Its structure is very similar to that of cray- 

 fish arid lobster. (From Davenport.) 



explained on the assumption that they are closely related. 

 If we see two strange people who look alike, we assume that 

 they belong to the same family; and likewise we believe 

 that similarity among animals and plants indicates relation- 

 ship. 



The crayfishes, lobsters, shrimps, prawns, crabs, each 

 with numerous species, are alike in the general plan of 

 the body, the same number of parts, and the same arrange- 

 ment of appendages. Since all of these have five pairs of 

 large legs, they are grouped together in the order Decapoda 

 (decapods), meaning ten legs. 



Now, the crabs are different from the lobsters in that their 

 abdomens (popularly called " tails ") have failed to grow as 

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