204 THE DOCTRINE OF DESCENT. 
the Hydractinea seems superior, inasmuch as it is itself 
asexual form. The zone of spherical protuberances in 
the middle of the body are the ovaries or egg capsules 
corresponding to the sperm capsules of the male indi- 
vidual. Heterogenesis does not take place in our Hy- 
dractinia, but, as in the development 
of the ovum of the Cladonema into the 
Stauridium, there is a transformation 
of a ciliated lava into a sessile polype. 
But it is obvious that the part which 
in the Hydractinia is played by the 
male and female sexual organs is per- 
formed in the generative cycle of the 
Cladonema by the sexual animals. By 
foilowing the transition from the de- 
_ pendent organ into the independent 
animal, we find the solution and ex- 
planation of the process termed he- 
terogenesis. Between the genera reproduced like the 
Hydractinia, and those reproduced like the Cladonema, 
there are many others, of which the propagation shows 
the gradual transition of the rudimentary sexual organs 
into the sexual animal. We may so arrange the genera 
of the “ Medusa polypes” as to exhibit how the parts 
which in the Hydractinia are mere capsules, generating 
and enclosing the ova, become more and more perfect. 
They acquire a special branch of the alimentary canal 
and blood-vessels, and are provided with the marginal 
papille characteristic of the Meduse, and constituting 
their peculiar sensory organs. In short, what in one 
member of the systematic series may be termed an 
organ, is, in the next, the Medusa separating itself and 
FIG. 15. 
