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true sexual reproduction in these animals the existence of such 
a type is a necessary condition. 
In Hydra fusca the organs in question consist, in their 
early stage, of minute depressed tubercles, attached by the 
whole of their broad base to the body of the Hydra. In their 
completely developed condition they present the appearance of 
more or less spherical capsules, attached by a contracted base 
to the Hydra, and slightly prolonged at the opposite point into 
a small mamilla, which is alternately perforated by an aperture 
for the egress of the contents. 
Into the interior of these bodies the gastric cavity of the 
Hydra is continued as a blind sac, which occupies the axis of 
the capsule and gives off from its sides numerous ramified 
tubes, which extend themselves in the interval between the 
central sac and the walls of the capsule. In these tubes, whose 
cavity, however, does not seem to communicate with that of the 
central sac, the spermatozoa appear to be developed, and sub- 
sequently escape into the cavity of the capsule to be finally 
expelled through the aperture in its summit. 
We have here, as in Cordylophora and the marine Hy- 
droida, the walls of the capsule representing the dise of a Me- 
dusa, the central sac homologous with the stomach, and the 
spermatophorous tubes with the radiating gastro-vascular 
canals. 
The spermatozoa possess a conical body with a long caudal 
filament attached to the larger end. They are produced in the 
interior of vesicles of evolution, a single spermatozoon being 
formed in each vesicle, and the vesicles themselves are produced 
in the interior of mother-cells. These mother-cells may ge- 
nerally be seen to contain two or three spermatozoa, which ap- 
pear to be often free on the mother-cell, but which are also 
doubtless frequently contained each in its own vesicle of evolu- 
tion, though the extreme delicacy of the latter renders it in- 
visible through the walls of the surrounding cell. Very fre- 
quently the bodies of the spermatozoa may be seen still in- 
