204 



THE DOCTRINE OF DESCENT. 



the Hydractinea seems superior, inasmuch as it is itself 

 a sexual 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 

 following 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 s]*ecial branch of the alimentary canal 

 and blood-vessels, and are provided with the marginal 

 papillae characteristic of the Medusae, 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 



