ON THE REPRODUCTIVE SYSTEM IN THE HYDROIDA. 3889 
Agass., the sterile polypites of the male colony are described as differing from 
those of the female colony by their more elongated proboscis *. 
V. DEVELOPMENT. 
Reproduction in the Hyprorpa is sexual and non-sexual, the former taking 
place by means of ova and spermatozoa, the latter by buds and occasionally 
by spontaneous division. 
A. Non-sexual Reproduction. 
a. Gemmation. 
Reproduction by gemmation is the phenomenon which, of all others, most 
vividly impresses us in our study of the Hyproma, and is that which confers 
upon this remarkable group of organisms their peculiar and characteristic 
physiognomy. It struck with all its force the earlier observers, and united 
with the flower-like form of the polypite in suggesting the term ‘“ zoophyte” 
by which the wonderful budding and blossoming plant-lke 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 Hyprorma has for its object, 1, the extension of the tro- 
phosome; 2, the formation and extension of the gonosome. 
The primordial trophosome is quite simple ; but it soon begins to complicate 
itself by budding, and this complication is frequently carried to a great ex- 
tent, the primary buds giving rise to secondary buds, and these again to ter- 
tiary, while buds of a fourth, fifth, or even higher order may continue to be 
produced in succession ; and as every bud may develope itself into a branch, 
the result will be the production of those complicated dendritic groups which 
attain to such perfection in numerous species among the T’ubularian, Cam- 
panularian, and Sertularian hydroids. 
The complex trophosome which thus results from successive buddings may 
present symmetrical or asymmetrical forms. Symmetrical forms are, as a 
general rule, presented throughout the Sertularians ; the polypites, with their 
hydrothece, being in these hydroids developed upon points which are symme- 
trically 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 Tubularians, present in most cases an asymmetrical 
disposition of their polypites, and, as a necessary consequence, an asymmetri- 
eal ramification. The genus Pennaria among the Tubularida affords a re- 
markable exception in this respect, its gemmation being so singularly sym- 
metrical 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 this genus. 
Under the general head of Gemmation, we may here consider the develop- 
ment of the polypite, the development of the gonoblastidium, and the develop- 
ment of the gonophore. 
1. Development of the Polypite. 
It is exceedingly rare to find the trophosome retaining through life the 
simple condition which it presents during its primordial state. Cases, 
however, of permanently simple trophosomes occur. We meet with them, 
* Agassiz, Nat. Hist. United States, vol. iv. p. 228. 
