EXCRETION 83 



70. Life History. — In the starfish group the sexes are 

 distinct. The sexes in the annelids are distinct in some 

 forms and in others the same individuals have both 

 ovaries and spermaries. However, the sperms that unite 

 with eggs always come from another worm. During the 

 season when the ovaries and spermaries are forming eggs 

 and sperms, certain segments, usually six in number, be- 

 ginning with the twenty-eighth segment, and known as 

 the clitellum (kli-tel'liim), pour out a gelatinous secretion 

 which hardens into a collar-like sac around the worm. 



This sac is worked forward and as it passes the openings 

 of the reproductive organs, eggs and the sperms from 

 another worm are pushed into it. The sac continues to 

 move forward and finally leaves the worm as a closed 

 capsule. This capsule contains eggs, sperms, and fluid 

 food. After the fusion of the eggs and sperms, the re- 

 suiting embryonic worms begin to feed upon the fluid 

 food in the capsule; later they feed upon each other 

 until but one may remain eventually to bore or eat 

 its way to the earth outside. From now on the food of 

 the young worm is the soil. 



The earthworm is an example of an animal which has 

 both ovaries and spermaries. 



71. Respiration. — Oxygen passes through the skin di- 

 rectly into the blood, which then carries the oxygen to the 

 various cells of the body. The outer surface must be 

 kept moist to permit the skin to act as a lung. 



72. Excretion. — In each segment is found a pair of 

 organs known as nephridia (ne-fiid'i-a), which look like 

 little threads. These remove the liquid waste and carry 

 it to the outside of the body. It is believed that carbon 

 dioxide passes off through the skin, much as oxygen 

 passes in. This taking in and giving off of these gases 

 is accomplished by osmosis. 



