106 ANIMAL ACTIVITIES. 
(2) A swimmeret from the third abdominal segment 
seen from behind x 6. 
(¢) Sixth swimmeret. 
Homologies. In the lobster, shrimp, and crayfish, 
the antennz, claws, legs, and swimmerets are seen to 
Fic. 92.—Longitudinal Section of Crayfish. a, 
anus; d.a., dorsal artery; 4.g., intestine or hind- 
gut; 4, heart; m.g., midgut; , liver; s, stom- 
ach; 4, kidney; ¢a, labrum or lip; s.a., sternal 
artery; #.c., nerve-chain; v.a., ventral artery. 
be similarly situated and to bear a strong resemblance 
in structure. If we should study the growth of these 
three animals, we should find that these appendages 
arise in a similar way in the process of development. 
Parts of animals similar in position, structure, and origin 
are said to be homologous. The wings of the butterfly 
are homologous with those of the grasshopper. The 
three pairs of jointed legs in the caterpillar are homol- 
ogous with the legs of the butterfly. 
When parts correspond simply in use and not in 
origin or structure, they are said to be analogous. 
Thus the wing of a bird and the wing of a butterfly are 
analogous but not homologous. 
