CRUSTACEANS 



crayfishes the body segments show most clearly on the abdo- 

 men (Fig. 122). 



f«l9ion 





iuoJIut)^ le^s 



Fig. 122. — Diagram of the crayfish, a higher crus- 

 tacean. 



The jointed appendages are the most diversified and yet the 

 most characteristic structures of crustaceans, big or little. 

 Legs, swimmerets, and mouth parts are all built on a ground 

 plan so uniform that they can be compared part with .part, 

 yet they have the most varied functions — feeling, eating, walk- 

 ing, swimming, and reproduction. Their shapes tell how 

 their owners live, what the crayfish eats, and how the fairy 

 shrimp swims. They are good examples of the zoological 

 saying that the outside of an animal tells where it has been. 



Nearly all crustaceans breathe by means of gills; in cray- 

 fishes these are attached on each side of the thorax, protected 

 by the carapace, the shell covering the head and thorax, where 

 water continually flows over and between them (Fig. 122) ; in 

 fairy shrimps they are on the ventral side of the body (Fig. 

 123). 



The male and female organs are in different individuals 

 and the young develop from fertilized eggs, but in many 

 forms, like the common little Cyclops (p. 166), there are periods 

 and seasons when the young are produced parthenogenetically, 

 from eggs which have never been fertilized by male cells. 



Two main groups of crustaceans. — There are two main 



159 



