128 THE NATURE AND VARIETIES OF 



4. The conclusion, therefore, to which a fair compari- 

 son of the reproductive process in the two cases now cited 

 the Trematoda and the Polypifera seems to lead is 

 this, that though in both we certainly have an alternation 

 of forms in the genetic cycle, still we cannot treat them as 

 parallel. It does not seem possible to do so without leav- 

 ing out of view two points of obvious contrast. 



1. That the polype stock has a much greater perma- 

 nence than the trematode redia or precursory zooid, and 

 may throw off many successive broods of medusoids. 



2. That the concluding links of the respective series have 

 really nothing in common but the single point of sexual 

 completeness the medusoids being sometimes of the most 

 rudimentary structure, more comparable to detached organs 

 than to perfect animals. For the parallel indeed, the single 

 character of sexual completeness must be taken to mark 

 the culminating point of the development of the species, 

 however defective the general organization ; and this is 

 precisely what some naturalists contend for. Thus Siebold 

 in his concluding note on the Comparative Anatomy of the 

 Acalepha remarks " However various the developing forms 

 may be, that one must be regarded as the real one, which 

 exists during the development of the testicles and ovaries." 

 So also Dr. A. Thomson "Some [Hydrozoa] we have 

 been accustomed to see principally in their largest and most 

 developed condition, as Medusae ; others are best known in 

 that polypoid condition, in which they remain for the 

 longest time ; but we must regard that condition in which 

 sexual reproduction takes place as the complete one, and 

 this we have seen is in both the Acaleph or Medusa form, 

 while the polype or polypoid state, however permanent it 

 may appear, is to be looked upon as a preparatory stage."* 



But the general analogies of living beings surely indicate 



* Cyclopsed. Anat. and Physiol., Art. Ovum, p. 22. 



