DEVELOPMENT. 105 



but one which never gives rise to buds, and is therefore incapable of citlicr continuing or 

 originating a new succession.' 



The following formula, where //' is the spiral hydranth, will express the place and jjower of 

 this zooid in Hi/dractinia : 



+ */■« + 'JP^>- X . . . . &c. 



va *{ + «• + '"" x^lit^;' 



The case expressed in Formula VI is the simple one where only the last hydranth in the 

 succession of buds composing a period is supposed to give origin to a bud of the gonosome. But 

 any other hydranth in the succession may just as well bud otf a member of the gonosome, which 

 may thus form a collateral gonosomal axis. This, which is by far the most usual case, is what is 

 actually represented in the diagrams (see figs. 41, 42, 43). The axis, however, thus formed will 

 be necessarily definite, and will contrast in this respect with the indefinitely extended axis of 

 the trophosome, while it will differ from the diverging bud, // in VII, by the fact of its having 

 the power of repeating the colony by sexual reproduction, while // has no power of reproduction 

 either sexual or non-sexual. 



This condition may be' expressed by the following formula, in which not only the last 

 hydranth of the period gives off a bud of the gonosome, but the primordial hydranth itself emits 

 a collateral gonosomal axis : 



) -j- i/5 -i_ ,jph ) I ) 



Besides the particular cases now given certain other modifications of the plan of gemmation 

 will at once occur to any one who has made the Hydhoida a subject of study. Those here 

 adduced, however, will serve to convey a sufficiently adequate idea of the more important features 

 in hydroid gemmation. It is thus, by the combination of heteromorphic and homomorphic 

 multiplication, and of direct and diverging series indefinitely repeated, that the animal attains to 

 the condition of those wonderful complex colonies which impress themselves so strongly on the 

 mind of the observer. 



So also the gonosome may present, not only a heteromorphic, but a homomorphic multipli- 

 cation of zooids. In no case, however, so far as I am aware, does any zooid of the gonosome 

 repeat itself by homomorphic gemmation, except in some comparatively rare instances of budding 

 in the Medusa ; for though the homomorphic repetition of zooids may be in the gonosome, as in 

 the trophosome, carried to a great extent, it is almost always the resiJt of budding from a zooid 

 of a different form. Thus, the blastostyle never emits buds destined to repeat its own form, and 

 this form, however frequently repeated in the gonosome, is always budded oft' from the hydranthal 

 element in the trophosome, its own buds, however numerous, being always heteromorphic with 

 itself. 



It is a universal law in the succession of zooids that no retogression ever takes place in the 

 series. In other words, no bud ever becomes developed into a zooid which is of a different 



1 The bifurcation occasioually observed in the spiral hydranth of Hydractinia is evidently abnormal, 

 and cannot be regarded as contradicting the above statement. 



14 



