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BEGINNERS' ZOOLOGY 



Fig. 76. —Two pairs 



OF NEPHRIDIA in a 



worm (diagram). 



ridium has an inner open end within the body cavity, and 



its outer end opens by a pore on the surface between the 



setae. The nephridia absorb waste 

 from the liquid in the celom, or body 

 cavity surrounding the food tube, 

 and convey it to the outside. 



Respiration. — The skin of the 

 earthworm is moist, and the blood 

 capillaries approach so near to the 

 surface of the body that the oxygen 

 is constantly passing in from the air, 

 and carbon dioxide passing out; hence 



it is constantly breathing through all parts of its skin. 



It needs no lungs nor special respiratory organs of any 



kind. 



Reproduction. — When one individual animal produces both 

 sperm cells and egg cells, it is said to be hermaphrodite. This 

 is true of the earthworm. The egg cell 

 is always fertilized, however, not by the 

 sperm cells of the same worm, but by 

 sperm cells formed by another worm. 

 The openings of these ovaries consist of 



two pairs of small pores found in most 

 species on the ventral surface of the 

 fourteenth segment (see Fig. 77). There 



are also two pairs of small irccptacles 

 for temporarily holding the foreign sperm 

 cells. One pair of the openings from 

 these receptacles is found (with diffi- 

 culty) in the wrinkle behind the ninth 

 segment (Fig. 77), and the other pair 

 behind the tenth segment. The sperm- 

 atids are in front of the ovaries (Fig. 77), but the sperm ducts 

 are longer than the oviducts, and open behind them on the fifteenth 

 segment (Figs. 77, 78). The worms exchange sperm cells, but not 



Fig. 77. — Sperm (.<■/) and 

 egg glands (es) of worm. 



