RUDIMENTARY ORGANS OF ANIMALS 133 



organism round about the shell has become 

 more marked in character. The shell has 

 become internal instead of external, and forms 

 the so-called cuttle-bone. This structure, being 

 only the vestige of what was originally an 

 external shell, must be regarded as a reduced 

 organ. 



5. Artliropoda. The group of Arthropods com- 

 prises the Myriopoda, the Crustacea, the Arachnida, 

 and the Insecta. 



Instances of rudimentary organs are very common 

 among the Crustacea, but our investigations with 

 regard to the appendages of the cray-fish were so 

 thorough that we will give examples from another 

 group, that of the Insecta, instead. 



Insects are characterized by the possession of 

 three pairs of legs and two pairs of wings. The 

 organs of flight exhibit a multitude of special 

 adaptations, and numerous instances of degenera- 

 tion are exhibited. In the Nenroptera (dragon 

 flies), Hymenoptera (saw flies and bees), and Lepi- 

 doptera (moths and butterflies), the four wings are 

 generally all alike and fully developed. In the 

 Diptera (flies), the posterior wings have atrophied 

 and are represented by two reduced organs, the 

 " balancers," these being absent in certain types. 

 The Strepsiptera form a group of which compara- 

 tively little is yet known; the larvae live in the nests 

 of bees ; the females have no wings ; in the males 

 the anterior wings are rudimentary and the pos- 



