ANIMALS WITH J OINTED LEGS— THE ARTHROPODS 187 



13. First swimmeret — biramous in male, but endopoclite and ex- 

 opodite fused together to form an organ for use in copulation ; reduced 

 and nonfunctional in female. 



14. Second swimmeret — biramous — used in copulation in male, but 

 small and nearly functionless in female. 



15. Third swimmeret — creates a current of water that may enter the 

 gill chamber, and in female serves for attachment of eggs. 



16. Fourth swimmeret — same as third. 



17. Fifth swimmeret — same as fourth. 



18. Uropod — used in swimming backward. 



The appendages of the crayfish may be used to illustrate a very im- 

 portant biological principal, homology. Body structures which arise in 

 similar ways in the embryo are said to be homologous. Thus, we can 

 say that the appendages of the crayfish are homologous to one another 

 even though they become greatly modified and assume different func- 

 tions in the adult. This not only applies to a series of similar structures 

 on the body of one animal, but to structures on different animals that 

 arise in the same way embryologically. For instance, the arm of a man, 

 the wing of a bird, the foreleg of a horse, and the pectoral fin of a fish 

 all arise as a little outpocketing of the ectoderm and mesoderm at the 

 same spot on each embryo. In the early embryo of each of these animals 

 the structure would appear as a little projection without any indication 

 as to its ultimate form and function. Therefore, we can say that these 

 four structures are homologous to one another. These all happen to 

 have different functions, but that need not necessarily be so. The arm 

 of a man and the arm of a monkey are used for similar functions and are 

 homologous also. 



Another biological principle, analogy, bears a close relation to homol- 

 ogy and the distinction between them should be made clear. Body struc- 

 tures are said to be analogous which have the same function, but a dif- 

 ferent embryonic background. As an example, the wing of a bird and 

 the wing of a house fly serve the same general purpose, flying, yet the 

 two arise in an entirely different way in the embryo. The bird wing, as 

 described previously, bears a definite relation to the front leg of other 

 animals, yet the wing of the fly arises as a little balloon-like puffing out 

 of the ectoderm on the back of the insect and bears no relation to its 

 legs. Since these two structures have no embryonic relationship, yet are 

 used for the same purpose, they are analogous to one another. 



A survey of the organ systems of the crayfish shows some of them 

 to be quite similar to the earthworm, while others show marked differ- 



