ORGANIC EVOLUTION 



169 



free in the body cavity. Later they enter the funnel-shaped 

 end of a short oviduct, that penetrates the septum at the 

 rear of the thirteenth segment, and opens to the 

 exterior on the ventral surface of the fourteenth segment, as 

 before noted. 



A preliminary change in the clitellum precedes the dis- 

 charge of the eggs. The glands, to which the clitellum owes 

 its thickness, secrete a milky fluid within it that loosens it 

 from the body. It breaks its moorings, and is gradually 

 worked forward and finally slipped off over the front end. 

 Its front and rear openings are elastically held close to the 

 sides of the worm, retaining all its fluid content, into which 

 while passing segment fourteen, the eggs are discharged, and 

 while passing segment ten, the sperms, also, stored there 

 previously, derived from another worm. Passing off at the 

 front, the ends close elastically, making a cylindric capsule 



with pointed ends. Within this fertili- 

 zation takes place — cross-fertiHzation, 

 of necessity, and the eggs pass the early 

 stages of their development in the milky 

 fluid in which they float ; it is for the 

 young worms both cradle and food. 

 These capsules are left lying under 

 leaves on damp soil, into w^hich, the 

 little worms on emergence may enter. 



Thus the worm, like the hydra and 

 many of the other lower animals is 

 bisexual (hermaphroditic). Cross-fer- 

 tilization in the hydra, secured by the 

 earlier maturing of the sperm cells than 

 of the eggs, is in the worm secured by 

 the complicated method just described. For the sperm cells 

 are essentially aquatic; in the water they may reach the 

 egg unaided, but in order to fulfill their function in terres- 

 trial animals they require to be transported. 



Fig. 106. Ovary and sex 

 cells of the earth 

 worm, s, sperm; e, 

 egg. 



