EXCRETION 



145 



the fluid of the body cavity from which they are removed 

 directly by the kidney. In the group of flat worms (Platy- 

 helminthes), in which neither a blood-circulatory system nor a 

 coelomic cavity is present, a very primitive type of excretory 

 organ is found consisting of numerous capillaries whose blind 

 ends are formed of single cells, known as flame cells (Fig. 91). 

 The flame cell is provided with irregular projections extending 

 into the surrounding tissues and bears a bundle of cilia extending 

 into the cavity of the capillary. The flickering movement of 

 the cilia is like that of a flame. Leaving the flame cells, the 

 capillaries unite to form large tubes, 

 which eventually drain into one of two 

 longitudinal ducts that open separately 

 or by a single pore on the dorsal side of 

 the posterior end of the body. Excre- 

 tory products dissolved in water are 

 absorbed from the surrounding tissues 

 through the flame cells and the walls of 

 the tubules, and are propelled through 

 the tubes to the outside by the vibra- 

 tions of the cilia. Such organs con- 

 stitute a -protonephridial system or a 

 water-vascular system. 



Malpighian Tubules. — The hemocoel 

 of insects is a poorly developed body -;%£*-£-*-. 

 cavity filled with blood. The Malpigh- 

 ian tubules of the grasshopper are a clump of fine hairlike tubules 

 attached to the alimentary canal at the anterior end of the intes- 

 tine, from which they extend into the hemocoel where they are 

 bathed in blood (Fig. 53). They are blind at their free ends 

 and open at their attached ends into the intestine. These tubes 

 absorb nitrogenous and other excretory products from the blood 

 and deposit them in the intestine, whence they pass out of the 

 body with the feces. These products consist of urea, uric acid, 

 urates of calcium, sodium and ammonia, as well as oxalates, 

 carbonates and phosphates of calcium and potassium. 



Nephridia. — The nephridial tubules of the earthworm are 

 involved in connection with development of a coelomic cavity 

 and a blood-vascular system, both of which are present in the 

 earthworm. The earthworm is a segmented animal, which 



Fig. 91. — Flame cell of a 

 protonephridium of a flat- 

 worm, ci, cilia within the 

 funnel-shaped cavity of flame- 



