4 6 



ANIMAL BIOLOGY 



"Wtr 



FIG. 76. Two PAIRS 

 OF NEPHRIDIA. 



(Fig. 76). Each nephridium has an inner open end within 

 the body cavity, and^its outer end opens by a pore on the 

 surface between the setae (Fig. 78). 

 The nephridia absorb waste water 

 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 dioxid 

 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 ova or egg glands 

 consist of two pairs of small pores found 

 on the ventral surface of the fourteenth 

 and fifteenth segments in most species 

 (see Fig. 77). There are also two pairs 

 of small receptacles for temporarily 

 holding the foreign sperm cells. One 

 pair of the openings from these recepta- 

 cles is found (with difficulty) in the 

 wrinkle behind the ninth segment (Fig. 

 7 7), and the other pair behind the tenth 

 segment. The sperm glands are in front of the ovaries (Fig. 77), 

 but the sperm ducts are longer than the oviducts, and open behind 

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



XV 



FIG. 77. Sperm (sp~) and 

 egg glands (es) of earth- 

 worm. 



