THE FLATWORMS— PLATYHELMINTHES 143 



In planaria, during copulation, there seems to be a mutual exchange 

 of sperms with both animals serving a male and female at the same time. 

 Sperms are produced in the numerous testes and make their way through 

 the slender vasa efferentia (sing, vas efferens) into one of the two vasa 

 deferentia (sing, vas deferens) and thence down into the seminal vesicles 

 which act as a storage chamber for the sperms until the time comes for 

 their discharge. In copulation, the penis of each animal is thrust 

 through the genital pore of its partner and the seminal vesicles contract 

 to force the sperms into the seminal receptacle of the other. Then the 

 animals separate and a short time later eggs are produced by the ovaries 

 and make their way down the oviducts where they meet the sperms which 

 come out of the seminal receptacles and journey up the oviduct to unite 

 with them to form zygotes. As the zygotes continue down the oviducts 

 the yolk glands secrete many yolk cells to nourish the embryos. When 

 these reach the genital atrium a group of from four to twenty zygotes 

 and thousands of yolk cells will be surrounded by a cocoon which then 

 passes out of the genital pore and is usually attached to the under side 

 of a rock or a leaf or similar underwater structure. The embryonic 

 planaria develop within the cocoon, deriving nourishment from the yolk 

 cells for several weeks, and then break this confining wall and crawl 

 out to start life on their own. 



It may seem strange that, with complete reproductive organs of 

 both sexes in its body, planaria should copulate with another individual 

 and thus fertilize its eggs with sperms from another animal. However, 

 similar cases are found in many forms of life. Most flowering plants 

 have both male and female organs in each flower, yet elaborate 

 mechanisms have been developed which result in cross-pollination rather 

 than self-pollination. Insects and the wind may be used to spread the 

 pollen from one flower to another. We have already learned how 

 Paramecium undergoes a complicated reaction known as conjugation and 

 the only apparent biological value of such a union seems to be the 

 exchange of hereditary material contained in the nucleus. A process 

 which is so widespread in nature must have some biological value — 

 some value in survival which causes the process to become established in 

 so many cases. 



For one thing, the production of variety among the offspring which 

 results from the blending of genes from different parents is of major 

 importance. The process of natural selection cannot operate unless 

 there are inherited differences among living things. Also, there is often 

 an immediate advantage to the offspring. In general, the offspring 

 which come from two different parents are more vigorous than those 

 which result from self-fertilization. In higher forms of animals there 



