408 REPORT—1863. 
is now plainly recognizable as the proboscis or metastome of the future polypite. 
The tentacles now rapidly multiply by the intercalation of others between those 
already formed. The second set may at first be easily distinguished by their 
shortness ; but the bases of all seem to be on the same level, and the whole 
appear to constitute a single uninterrupted series. The tentacles, though 
short and thick, will have thus soon attained the full number which we meet 
with in the adult. They consist in this stage of an endodermal and an 
ectodermal layer, the ectoderm apparently formed of a single layer of pris- 
matic cells, while the endoderm seems to fill the entire axis with a mass of 
minute, spherical, loosely aggregated cells. Just behind the tentacles the 
body of the young polypite is seen to be excavated by a large cavity, in which 
is a multitude of loose spherical cells, filled with a red granular pigment, 
and undoubtedly thrown off from the inner surface of the walls. 
The whole of the young hydroid is still completely enveloped by the deli- 
cate chitinous periderm, which forms a sheath extending over even the distal 
free extremity, and within which the various changes just described, in- 
cluding even the formation of the tentacles, have been going on. We now 
find, however, that this sheath (which has for some time lain loosely over the 
distal parts of the hydroid, and which it seemed to invest as in a sac) becomes 
ruptured in front of the tentacles, so that the water gains direct access to 
the surface of the young polypite, and the tentacles have full freedom to ex- 
tend themselves. It would seem, too, that the distal extremity of the pro- 
boscis had now, for the first time, ‘become perforated by a mouth; for, up to 
this stage, no undoubted evidence of an oral aperture could be detected. 
The young Eudendrium has thus acquired the form of a true polypite borne on 
the extremity of a short simple cylindrical stem, which still springs from the 
centre of the radiating disc. The stem elongates itself, and the body, tentacles, 
and proboscis rapidly acquire all the characters of the adult; the hydroid, 
however, is still simple, and it remains for it to develope from its base a 
creeping stolon which will take the place of the primordial disc, to complicate 
itself by the budding of new polypites and the development of branches, and, 
finally, by the formation of sexual zooids to combine a gonosome with the pri- 
mary trophosome, in order that the little hydroid whose progressive changes we 
have been thus following may attain the condition of the adult Hudendrium. 
The developmental phenomena above descyibed are in all their essential 
points, so far as we know, universal among the marine Hyprorpa, with one 
exception—that, namely, which is presented by the genus Tubwlaria. 
In this genus a minutely granular plasma, entirely similar to that which 
in other Hydroida becomes differentiated into ordinary ova, may be seen en- 
veloping the spadix of the young gonophore. Instead, however, of becoming 
transformed in the usual way into ova, portions become detached from the 
mass and lie loose in the cavity of the endotheca, where they undergo a deve- 
lopment into free embryos in the manner to be presently described, while the 
residual plasma continues to detach from its mass fresh fragments, ‘which are 
in their turn transformed into embryos. 
In the portions thus successively detached from the central plasma it is 
impossible, as has already been said, to detect any trace of germinal vesicle 
or germinal spot, and yet we should certainly not be justified in regarding 
them as mere gemme, or in attributing to them any other significance than 
that of true ova*. The plasma in which they originate holds in the gono- 
* Agassiz calls the central plasma in Tubularia the “ germ-basis,” and refuses to regard 
as ova the masses which are thrown off from it and become developed into polypoid youn: 
(Gp, ett. vol. iv. pp. 255 & 269.) 
