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PROCEEDINGS OF THE 



[April, 1857. 



nate generation and direct metamorphosis, this must appear as an 

 incongruous association. 



But alternate generation is indeed only an individualized form 

 of metamorphosis, wherein the budding power is specialized 

 among the different parts of the embryonic mass, so that some of the 

 individuals produced by it are permanently adapted to a lower 

 rble than others, and thus (Siphonophorse) some become free sexual 

 Medusae, while others having neither digestive cavity nor sexual 

 organ, are doomed to remain as mere motor-machines, or swim- 

 bells, still others, as the canal-bearing bracts, are specialized to per- 

 form, perhaps, respiratory functions only, while other individuals 

 assume the form of long tentacula, having nothing much developed 

 about them but their thread-ceil bunches, and many others remain 

 still as simple digestive trunks provided with mouths. Now, in 

 this there is no alternation of generations, for no one of these 

 classes of individuals form a separate generation; there is but a 

 single generation the result of a single generative act, and that 

 generation includes all the classes of individuals, and would not 

 be complete if any one of them were omitted. The only differ- 

 ence, then, between this and the direct metamorphosis of Cunina, 

 is that there gemmation produces no permanent classes of indi- 

 viduals, but each individual entering into that single generation, 

 unless abnormally aborted, is equal in rank to every other. There 

 is therefore no distinction in kind, no essential and fundamental 

 difference between these two forms of metamorphosis united with 

 gemmation; the difference is one of degree only, and manner, in 

 carrying out a single fundamental plan. 



But we may bring these two modes of metamorphosis still closer 

 together by the link supplied in the valuable observation of Ge- 

 genbaur, that his genus Trachynema, so nearly allied to the Euco- 

 pidee is developed from a ciliated larva like Aeginopsis, which, 

 as he observes, separates the Trachynemidse from the Eucopidre. 

 On the other hand, they approach quite nearly to Eucope, in a 

 structural point of view. It is to be remembered, also, that noth- 

 ing is known as yet of the development of Circe, Aequorea, Gery- 

 onia, Liriope, as well as Sminthea and Eurybiopsis, while there 

 is not much probability were it from fixed hydroids it would have 

 escaped observation up to this time, in some one of these genera, 

 and their affinity to Trachynemidae on the one hand, and AeginidaB 

 on the other, render it probable that they have a direct form of 

 metamorphosis. Also, the development of Siomobrachium into 



