100 PROFESSOR ALLMAN ON THE GENETIC SUCCESSION 



indefinitely by budding (fig. 2) before the time arrives when an element of the 

 gonosome is to be budded off ; and a series of homomorphic zooids may thus 

 introduce themselves into the heteromorphic succession, as expressed in the 

 following formulae — 



V. h + h + /i + &c. + Ms +gph x h + h + h+ &c. + Ms +gph x &c. 



where the hydranth becomes indefinitely repeated in the formula of ternary 

 heteromorphism (III.) given above; and the same will apply to each of the other 

 two types of heteromorphism. 



Now, in all these cases, the succession from the primordial nutritive zooid to 

 the ultimate generative zooid, or gonophore, admits of being expressed in a 

 continuous line ; but one or more of the zooids of the trophosome may emit 

 buds which will diverge from the direct line of succession, and which may 

 then either form the starting-point for another similar fine of succession, or 

 may be destitute of all power of continuing the succession of the zooids. Thus, 

 (figs. 4 and 7) the primordial hydranth, or any of those derived from it, may 

 repeat itself by a bud which will diverge from the direct fine, produce other 

 zooids by gemmation, and thus start off a new series, as expressed in the 

 following formula : — 



*s 



VI. It 



( + It +h + It + &c. + bis + gph \ , ( ) o 



\ + k + h + It + &c. + Ms + gph ) ( '" J 



And this state of things may also repeat itself indefinitely, giving rise to an 

 indefinite number of collateral series diverging from one another, and from the 

 primary axis of succession. 



As already said, however, the diverging zooid may have no power of con- 

 tinuing the succession. Thus, the spiral hydranth of Hydractinia is not inter- 

 calated in the direct succession of zooids. It is a diverging zooid, like that 

 which starts off the collateral series in formula VI., but one which here never 

 gives rise to buds, and is therefore incapable of either continuing or originating 

 a new succession."" 



The following formula, where h' is the spiral hydranth, will express the 

 place and power of this zooid in Hydractinia : — 



/ 7 7 7 7 ( + Ms + gph x &c. 



VII. h \ + f + «P* x M + V 



The case expressed in the formulae given above is the simple one, where only 

 the last hydranth in the succession of buds composing a j3eriod is supposed to 



* The bifurcation occasionally observed in the spiral hydranth of Hydractinia is evidently 

 abnormal, and cannot be regarded as invalidating the above statement. 



