The Sexual Phases of Myzostoma. 
289 
mature spermatozoa, the deeply-shaded triangle heli the gradually 
diminishing male Organs after the appearance of mature reproductive 
cells. Similarly the region cofgm would represent the period preced- 
ing the appearance of the first mature ova, the triangle fhg the period 
of maturity. As there are very probahly many immature ova still 
present in the ovaries and body-cavity at death, I have extended 
this region to gm in the line hm. Thus four distinct phases may be 
recognized in the life of these Myzostomes: 
1) A phase of sexual neutrality, or indifference. 
2) A protandric phase extending from the appearance of 
the first ripe spermatozoa to the appearance of the first ripe 
ova (cf). 
3) An androgynous or functionally hermaphrodi te 
phase extending from the appearance of the first ripe ova 
to the disappearance of the last ripe spermatozoa (^). 
4) A hy sterogynous ph ase extending from the disappearance 
of the last spermatozoa to the disappearance of the last ripe 
ova — an event which is very probahly not attained at the 
time of the animaVs, death (g). 
A comparison of this Schema (Fig. 57) with Fig. 58 shows at a 
glance the peculiarities of forms like M. pulvinar. The same methods 
of shading indicate the morphologically equivalent regions in both 
diagrams. In M. pulvinar the male phase is very short and terminates 
at h before the retarded appearance of the first mature ova at f\ 
so that this species lacks a functionally hermaphrodite stage cor- 
responding to ^ in the Schema of M. glahrum^ and there intervenes 
a period during which the animal grows, but does not produce ova 
or spermatozoa. Hence, when a number of specimens are examined, 
this Sharp Separation of the male and female phases in time has 
the same superficial appearance as the Separation in space in gono- 
choristic forms with dwarf males h 
The genus Myzostoma is by no means exceptional in presenting 
this form of dichogamy, viz. the sequence of protandry, functional 
hermaphroditism and hysterogyny: an exactly similar asynchronism 
1 M. pulvinar is certainly peculiar in exhibiting two well-marked periods 
of sexual maturity during its life-time. In this respect it resembles the cases 
of dissogony which occur among the Ctenophora. In M. pulvinar the periods 
are functionally unisexual, in the Ctenophora functionally hermaphrodite. 
