370 - Multicellular Animals, Especially Man 



INF. VENA CAVA 



TUNIC 

 CORTEX 

 PYRAMID 

 PELVIC CHAMBER 

 MEDULLA 

 RENAL VEIN 

 RENAL ARTERY 



Fig. 20-1. Diagram of the human urinary system, posterior view. 



becomes less and less adequate as the mass 

 of an animal increases. In flatworms, such as 

 Planaria, the body encompasses a consider- 

 able mass of mesodermal tissues, intervening 

 between the ectoderm and endoderm. Such 

 an increase in the mass of the body requires 

 the development of specialized excretory 

 organs, and planarians possess a large num- 

 ber of flame cells (Fig. 20-2), scattered 

 throughout the mesoderm. Each flame cell is 

 a hollow structure with a tuft of active cilia 

 in the central cavity; and the beating of these 

 accounts for the "flickering" that can be seen 

 in a flame cell when it is viewed with a micro- 

 scope. The many flame cells on each side of 

 the planarian are connected by a branching 

 system of fine tubes to main excretory ducts. 

 The excretory ducts run anteriorly on either 

 side of the mid-line of the body, emptying 



through a series of excretory pores, on the 

 dorsal surface of the animal. 



Precisely how the flame cells function is 

 not well understood. Apparently these cells 

 extract water and other excretory wastes 

 from the body fluid — which fills the tissue 



TUFT OF CILIA 



EXCRETORY 

 CHANNEL 



■NUCLEUS 



Fig. 20-2. A flame cell. Such primitive excretory units 

 are characteristic of flatworms. 



