192 GERM-CELL CYCLE IN ANIMALS 
as the tapeworm, to be able to form both male and 
female germ cells, since it may at any time become the 
only one of its species to occupy the alimentary canal 
of ahost. Hermaphroditism in such a case, however, 
is of no benefit if self-fertilization is not possible. 
Although there are thousands of hermaphroditic 
species of animals there are comparatively few whose 
eggs are known to be fertilized by spermatozoa from 
the same individual. We must therefore distinguish 
between morphological and physiological hermaphro- 
ditism and recognize the fact that the former condi- 
tion is much more prevalent than the latter. Among 
the species in which self-fertilization normally occurs 
are certain rhabdoccels, digenetic trematodes, ces- 
todes, ascidians, and mollusks. Van Baer, in 1835, 
claims to have observed self-copulation in the snail, 
Lymnea auricularia; that is, an individual with its 
penis inserted in its own female opening. That 
species of this genus fertilize their own eggs has 
frequently been stated by investigators. Frequently 
the spermatozoa of an hermaphrodite are capable 
of fertilizing the eggs of the same individual, 
but penetrate more readily the eggs of other individ- 
uals. Such is the case in the ascidian, Ciona in- 
testinalis (Castle, 1896; Morgan, 1905). 
Both sorts of germ cells are seldom produced at 
the same time by hermaphrodites. Those species 
in which spermatozoa mature first are called protan- 
dric; this is the usual condition. In a few cases 
eggs are formed first and later spermatozoa; in- 
dividuals in which this occurs are called protogynic. 
