186 BIOLOGY FOR BEGINNERS 



of the head. Since the degree in which an animal can get in touch 

 with its environment marks the stage of its advancement, the 

 Crustacea far excel the worms in development. 



Locomotion. This function is provided for by the swimmerets 

 which carry the body slowly forward, by the tail flipper which drives 

 it swiftly backward, and by the four pairs of walking legs which 

 can travel in either direction and sideways as well. All are operated 

 by powerful muscles, assisted by the exo-skeleton. You can see 

 why the slang expression " to crayfish " means to back out of any 

 agreement. 



Protection. The Crustacea's adaptations for protection are the 

 exo-skeleton with its color and spines, the powerful jaws and claws 

 for attack, speed for escape, fairly keen senses, and a nervous 

 system to guide its actions. 



Respiration. Respiration in protozoa was accomplished by 



contact of the cell with dissolved oxygen in the water; in the worm 



by contact of the body wall with oxygen in the air; osmosis was 



the method in both cases. In the Crustacea we have organs called 



gills, specially developed for carrying on the exchange of oxygen 



and carbon dioxide. These gills are thin walled, to allow osmosis, 



feathery to expose much surface, provided with many blood vessels 



to receive oxygen and to liberate carbon dioxide, and also are 



arranged to insure a constant flow of fresh water over them. This 



last is brought about in part by the gill bailer, attached to the 



second maxilla and partly by the gills being attached to the 



appendages. These move in the water, with every motion of a 



leg or maxilliped. Finally, as the water passes under the carapace 



from behind toward the head, this flow is aided every time the 



animal swims backward. The gills are protected by the carapace, 



which extends over them and forms a chamber which will 



hold moisture for some time, thus keeping them alive when 



removed from the water. Notice the importance of the fact that 



oxygen is soluble in water; if it were not, the aquatic animals 



could not exist, since it is the oxygen dissolved from the air, 



and not the oxygen of the water (H^sO) itself which all water 



animals use. 



