AND THEIR REAL NATURE. 107 



They exist, organically connected with each other, and are 

 normally free only in their first generation, and indeed 

 only in theii' earliest stage of development, and only for 

 a short time, since the free-smmming ciliated embryo 

 swims about in the water at most for some hours, in order 

 to find a suitable place for the foundation of a new polype 

 stem. In the Cory nee or claviform polypes, the organic 

 connexion between the individuals and generations is 

 rather more lax ; the perfect gemmiparous or ovigerous 

 individuals are usually quite free, often even at an early 

 age {Coryne fritiUaria, CorymorpJia,) so that they do 

 not attain then' full development until after their separa- 

 tion from the " ^z^r^m^" -generation. In the Medusce and 

 SaljjcB, the generations which are connected together into 

 one whole, become more hke each other ; the first gene- 

 ration of the Meduscs is still fixed but more active and 

 mobile in its parts ; the individuals of the perfect genera- 

 tion, leave the " nursing'' animal while still very small, and 

 undergo remarkable changes after they have become free 

 and are swimming freely about ; both generations of the 

 SaljjiB, finally, are free and free swimmers, only the indi- 

 viduals of one of them are organically connected with 

 each other ; they have, however, no common organs (in 

 the full-grown state,) and if my explanation of the alter- 

 nate generation of the compound Ascidians is correct, we 

 have in that instance precisely the development of the 

 Saljja at a somewhat lower stage ; the individuals of the 

 one generation are organically connected, without having 

 a common organ ; but both generations are fixed. 



In the class of Entozoa a similar progressive attempt at 

 becoming free and accomplishing a perfect growth appears 

 evident to me. 



In the Cestoidea the generation of perfect individuals, 

 constitutes externally a unity ; they are only successively 

 detached from each other as the term of their existence 

 approaches, and their whole existence is throughout con- 

 nected with the " nursing' animal. In some of the Tre- 

 maioda, the later generations remain within the earlier, 



