DEVELOPMENT. 67 



appear still closer when we bear in mind that buds formed by entirely difTcrcnt bud-ljcaring types 

 may in their early stages be undistinguishable from one another. 



While the hydroid embryo, however, continues to develop as an independent organism, the 

 bud remains for a longer or shorter time dependent on the pai'ent stock ; but it will yet be seen 

 that the stage of differentiation on which the bud stands at the earliest period at wliich any 

 differentiation is perce])tible corresponds to that of the embryo at the moment when the ectoderm 

 and endoderm of the planula become differentiated as distinct structures, and the further 

 development of the planula and of the hydranth-bud present a close parallelism with one another. 



1. Development of the Bud. — Zooidal Development. 



Reproduction by budding or gemmation is the phenomenon which, of all others, most vividly 

 impresses us in our study of the Hydroida, and is that which confers upon this remarkaljle group 

 of organisms its peculiar and characteristic physiognomy. It struck with all its force the 

 earlier observers, and united with the flower-like form of the hydranth in suggesting the term 

 " zoophyte," by which the wonderful budding and blossoming plant-like animals which adorn our 

 rocks at low water, and are dredged up at various depths from the bottom of the sea, have long 

 been known to the naturalist. 



Gemmation in the Hydroida has for its object, 1, the extension of the trophosome; 2, the 

 origination and extension of the gonosome. 



The primordial trophosome (PI. I, fig. 11 ; PI. Ill, fig. S; PL XIII, fig. IG) is quite simple ; 

 but it soon begins to complicate itself by budding, and this complication is frequently carried to 

 a great extent, the primary buds giving rise to secondary buds, and these again to tertiary, while 

 buds of a fourth, fifth, or even higher order, may continue to be produced in succession ; and as 

 every bud may develop itself into a branch, the result will be the production of those complicated 

 dendritic groups (Pis. Ill, IV, IX, XIII, &c.) which attain to such perfection in numerous species 

 among the Tatjidarian, Campamdarian, and Sertidarian hydroids. 



The complex trophosomes which thus result from successive buddings may present sym- 

 metrical and asymmetrical forms. Symmetrical forms are, as a general rule, presented throughout 

 the Sertularians ; the hydranths, with their hydrothecai, being in these hydroids developed upon 

 points which are symmetrically disposed in relation to a common axis or a common plane ; while 

 the ramification of the trophosome is here also usually symmetrical — distichous in most species, 

 verticillate in others. The Campanularians, on the other hand, and especially the Tubidarians, 

 present in most cases an asymmetrical disposition of their branches. The genus Pennaria among 

 the Tuhularians aff'ords a remarkable exception in this respect, its gemmation being so singularly 

 symmetrical as to give to the entire trophosome a close resemblance to that of a Plumularia — so 

 close, indeed, as to have led the earlier systematists to place it in that genus. 



Under the general head of Gemmation, we may here consider the development of the 

 hydranth, the development of the blastostyle, the development of the sporosac, and the develop- 

 ment of the medusa. 



