244 THE INTERNAL ENVIRONMENT OF THE BODY Part III 



cells, are surrounded by the body fluid. These flame cells are so named because 

 a tuft of flagella in the funnel-shaped hollow of the cell flickers like a flame. 

 Actually the flagella constantly wave fluid into the tubes, ultimately to pass out 

 of the body through numerous fine pores. Planarians that live in fresh water 

 have well-developed flame cells; whereas in those living in brackish water, the 

 entire excretory system is reduced. As with marine amebas, the osmotic pres- 

 sure of salt in the surrounding water and that of the protoplasm are balanced. 



The kidneys (nephridia) of earthworms repeat the essentials of kidney form 

 and function, tubules closely associated with blood and body fluid, each one 

 a guardian of the content of the blood. There are two kidneys in nearly every 

 segment of the earthworm (Fig. 14.2). Their inner ends are immersed in the 

 watery coelomic fluid; their outer ends open on the body surface; the tubules 

 themselves are entwined with blood capillaries. The inner end of each ne- 

 phridium is a funnel formed by ciliated cells arranged in beautiful symmetry 

 like the ribs of a palm-leaf fan, coming together at the mouth of the tubule 

 which receives fluid from the body cavity. The funnels draw in fluid and thus 

 keep down any excess of incoming water. 



The kidneys of crayfishes and lobsters are hardly recognizable as such either 

 in shape or position, but they actually are tubular and are guardians of the 

 content of the blood (Fig. 14.2). In lobsters they are the paired green glands, 

 one on each side of the head near the eye. Each one is a two-lobed, saclike 

 tube whose inner end opens into a body cavity (hemocoel). The outer open- 

 ing is a hole easily seen on the basal segment of the antenna. Excretory sys- 

 tems usually include pairs of kidneys located well forward in the body like 

 those of lobsters and crayfishes. This does not occur in adult vertebrates but 

 as an embryo every vertebrate animal goes through a stage when it has "head 

 kidneys" (Fig. 14.3). 



Kidneys of Vertebrates 



Likeness of Structure and Function. The vertebrate kidney is an assemblage 

 of excretory tubules, always in a dorsal location and composed of many units, 

 each one basically similar to a kidney of an earthworm. In the kidneys of the 

 most primitive fishes, there are only a few of these units in one kidney; one 

 human kidney, however, contains at least one million of such tubules. During 

 their evolution the various types of kidneys of vertebrates that have appeared 

 are: those connected with coelomic fluid, that is, coelomic blood, free in the 

 main cavities, (pronephros); with coelomic fluid and circulating blood, 

 (mesonephros); and solely with circulating blood (metanephros), the kidneys 

 of adult reptiles, birds, and mammals. 



Historical Succession of Kidneys — Pronephros, Mesonephros, Metanephros. 

 The first or pronephric kidneys are near the anterior end of the animal and 

 consist of a few tubules. The inner ends of these are ciliated funnels immersed 



