' 
ON THE REPRODUCTIVE SYSTEM IN THE HYDROIDA. 393 
plainly recognizable as the body of the budding polypite, inyested with a 
strong chitinous covering, which is closely applied over its whole surface, and 
is continuous below with the periderm covering the rest of the branch. The 
interior of the young polypite is hollowed out into a wide cavity lined by a 
layer of loose cells—the most internal cells of the endoderm—which are filled 
with a granular pigment. 
The conical enlargement at the extremity of the branch continues to in- 
crease in size, and we soon see the soft parts within become contracted to- 
wards the proximal end of the cone, where they withdraw themselves from 
contact with the walls of the chitinous capsule, which had up to this time 
closely embraced them. At the wide or distal end of the cone they still re- 
main adherent to the capsule for some distance downwards, while at the 
proximal end itself there is also a distinct but narrow zone of contact and 
adhesion maintained between the internal soft parts and the external chiti- 
nous capsule. In the cavity which occupies the interior of the soft contents 
of the capsule very distinct rotating currents may be now seen, excited 
doubtless by the action of vibratile cilia, though a direct view of these cilia 
cannot be obtained through the thickness of the walls. 
Between the proximal and distal zones of contact, the internal structures 
become more and more withdrawn from the walls of the capsule, while the 
whole body continues to elongate ; and this may now be seen in the form of a 
cylindrical column occupying the axis of a conical cup of chitine, and ex- 
panded below into a narrow ring, which at this point connects it with the 
walls of the cup, while, above, it expands into a broad dise which fills up 
the distal extremity of the cup, like a lid or plug. The axis of the column 
is permeated by a tubular cavity in continuation below with the cavity of the 
branch, and expanding above into a wide chamber which occupies the interior 
of the plug-like enlargement of its distal end. It is now plain that, while 
the soft contents of the cup are the developing polypite, the cup itself is to 
become the hydrotheca. 
The excreting of the chitine and the shaping of the hydrotheca would 
seem to devolve on the terminal plug-lke disc alone, from the time that the 
lower parts of the nascent polypite had withdrawn themselves from contact 
with the walls of the external capsule; and as the polypite continues to 
elongate itself, the surrounding cup is extended at the same rate, by addition 
to its wider end from the sides of the disc, while the lower parts of the cup 
undergo little or no change. 
The upper surface of the disc has been all along covered with a thin layer 
of chitine, whose periphery is continuous with the chitinous walls of the cup, 
but which does not interfere with the growth of the young polypite ; for as 
the latter continues to extend itself, the layer of chitine on the upper surface 
of the disc is carried onwards before it, without becoming thereby detached 
from the side of the cup—a fact which we can scarcely explain otherwise 
than by supposing considerable extensibility in the recently deposited chitine 
of the cup. At last the hydrotheca has attained its complete size and shape, 
and now the young polypite becomes more or less retracted within it, the 
terminal plug-like disc withdrawing itself from the layer of chitine which it 
had excreted on its upper surface, and which is now left behind as a roof 
closing over the mouth of the cup. 
The whole circumference of the retracted disc now begins to develope a 
‘circle of minute tubercles, which gradually elongate themselves into short 
_thick tentacles, while the central part becomes elevated into a blunt conical 
proboscis (metastome), and the cylindrical tubular column which occupies the 
