OF ZOOIDS IN THE HYDKOIDA. 101 



give origin to a bud of the gonosome. But any other hydranth in the succes- 

 sion may just as well bud off a member of the gonosome, which may thus 

 form a collateral gonosomal axis. This, indeed, is by far the most usual case, 

 and is what is actually represented in the diagrams (see figs. 2, 4, 7). The 

 axis, however, thus produced 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, h' in formula VII., by the fact of its having 

 the power of repeating the colony by sexual reproduction, while h' 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 emits a collateral gonosomal axis : — 



viii. h | + k + + *<£»* + &c - + Us + ,Jph } x h | } x &c. 



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 Hydroida 

 a subject of study. Those here adduced, however, will serve to convey an 

 adequate idea of the essential features in hydroid gemmation. 



It is thus, by the combination of heteromorphic and homomorphic multipli- 

 cation, 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 homo- 

 morphic multiplication 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 tropho- 

 some, carried to a great extent, it is almost always the result 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 gono- 

 some, is always budded off from the hyclranthal element in the trophosome, 

 its own buds, however numerous, being always heteromorphic with itself. 



In the formulae now given, one fact is obvious, namely, that the groups 

 included between every two acts of embryonal development are exactly similar 

 to one another in the nature and succession of their heteromorphic elements ; in 

 other words, that the life series of the hydroid may be represented by definite 

 groups of zooids exactly repeated after each generative act.""' We are indebted 

 to Huxley for having assigned to our conception of the biological individual its 



* The mere number of zooids in two or more of these groups may of course vary, depending an 

 this does on the accident of abundant or deficient nutrition and the like. 



VOL. XXVI. PART I. 2 D 



