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 respectsas though separate sexes ex- 
isted. This point will come up later for further consideration. 
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