Chap. 29 



ARTHROPODS CRUSTACEANS 



Generalized Biromous Appendage 



581 



). Antenna (loucfilng, fasting) 



18. Uropod (swimming) 



17. Swimmeret 



(carrying of 



eggs In 



female) 



[ Protopodite 



Endopodlle 



13. First abdominal 



appendage of male 



(copulating) 



13. First abdominal 



oppendoge of 



female 



(rudimentary) 



11. Fourth walking leg (walking) 



8. First walking leg (pinching) 

 I I Exopodite 



Fig. 29.10. Homology and the evolution of appendages. The appendages of the 

 left side of a crayfish. All these special structures are believed to have been derived 

 from a generalized two-branched appendage consisting of a basal piece, prodop- 

 odite; an inner branch, endopodite; and an outer one. exopodite as shown in the 

 figures. These basic structures are adapted for the different uses noted. They are 

 striking demonstrations of the changes that occur in evolution. (Courtesy, Hegner 

 and Stiles: College Zoology, ed. 6. New York, The Macmillan Co., 1951.) 



through the intestinal wall in the midgut, the part of the tract not lined with 

 chitin. Only a small amount of waste passes through the straight insignificant- 

 looking intestine. At certain times two limy bodies, the gastroliths, form in 

 pouches in the lining of the cardiac chamber of the stomach. These are asso- 

 ciated with molting to be discussed later. 



Blood and Circulation. The blood plasma is a watery fluid that contains 

 the bluish respiratory pigment hemocyanin composed of protein, copper, and 

 sulfur. It is similar to the pigment that makes clam broth bluish. Suspended in 

 it are numerous phagocytic cells. It clots very quickly and is probably a life 

 saver every time a claw of a crayfish is bitten off. It distributes food through 

 the body, carries respiratory gases to and from the gills, and waste products 

 to the kidneys. As in all arthropods, the circulation is the open type in which 

 blood vessels open into blood cavities, the sinuses or hemocoels. Blood flows 

 from the heart into the arteries and from them is carried by capillaries to the 

 various tissues, where it passes freely through minute open spaces and gradu- 



