Sex as an Adaptation 417 



ences in the sexes besides those connected with the organs 

 of reproduction. Such differences are found, as we have 

 seen, in insects, in some spiders, crustaceans, and in many 

 birds and mammals. In a few cases the difference between 

 the sexes is very great, especially when the female is par- 

 asitic and the male free, as in some of the crustaceans. In 

 some other cases the male is parasitic on the female. Thus 

 in Bonellia the male is microscopic in size, being in length 

 only one-hundredth part of the female. In Hydatina senta 

 the male is only about a third as large as the female. It has 

 no digestive tract, and lives only a few days. In another 

 rotifer the males are mere sacs enclosing the male reproduc- 

 tive organs. 



2. Hermaphroditic Forms. — There are many species of 

 animals and plants in which each individual contains both 

 the male and the female organs of reproduction, and there 

 are whole groups in which only these hermaphroditic forms 

 occur. Thus in the ctenophors the eggs develop along 

 one side of each radial canal and spermatozoa along the 

 other. The group of flatworms is almost exclusively her- 

 maphroditic. The earthworms and the leeches have only 

 these bisexual forms, and in the mollusks, while a few groups 

 have separate sexes, yet certain groups of gasteropods and 

 of bivalve forms are entirely hermaphroditic. 



In the common garden snail, although there are two sets 

 of sexual ducts closely united, yet from the same reproduc- 

 tive sac both eggs and sperm are produced. The barnacles 

 and the ascidians are for the most part hermaphroditic forms. 

 Many other examples might be cited, but these will suffice 

 to show that it is by no means unusual in the animal kingdom 

 for the same individual to produce both male and female germ- 

 cells. However, one of the most striking facts in this connec- 

 tion is that self-fertilization seldom takes place, so that the 

 result is the same in certain respects as though separate sexes ex- 

 isted. This point will come up later for further consideration. 



