CHAPTER XXII 



CRUSTACEA, A CLASS OF ARTHROPODS 



Vocabulary 



Carapace, the external protective covering of the thorax in 



Crustacea. 



Mandibles, jaw-like organs. 

 Maxillae, little jaws, aid in holding food. 



Maxillipeds, jaw-feet, aid in catching, holding, and chewing food. 

 Literally, actually, truly. 



Our study of the worms showed us a group of animals in which 

 tissues and organs had become somewhat specialized, circulator}- 

 organs developed, and adaptations formed for an inactive under- 

 ground or parasitic kind of life. In the Crustacea we deal with 

 animals such as crayfish, lobster, and crab, which are adapted for 

 an active, aquatic (water) life, in which division of labor among 

 their various organs has been carried to a higher point. 



CRAYFISH 



External Structure. The crayfish, which we will study as a type, 

 has the body covered with a dark-colored, limey, external skeleton 

 '(exo-skeleton) divided into two regions, the cephalo-thorax (head 

 thorax) being covered by the united carapace, and the abdomen 

 made up of seven separate movable segments. This is the first 

 animal we have studied which has had any skeleton at all and it 

 may seem strange to find it on the outside of the body while ours 

 is inside. However the same functions are performed in both 

 cases, namely to protect the organs and act as levers for the muscles. 



Protection is most important in the Crustacea which really have 

 a suit of mail, such as the knights used to wear. Their joints are 



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