ON THE REPRODUCTIVE SYSTEM IN THE HYDROIDA,. 
373 
by a tubular cavity, which communicates with that of the blastostyle. As the 
arms continue to elongate, we find them next (h) with their free extremities 
bending towards one another, until finally, 
by the meeting of their extremities, they 
completely enclose an oval space (c), which 
is entirely shut in by the lateral coalescence 
of the wide chitinous tubes with which the 
radiating processes are each invested. 
In the mean time the eight longitudinal 
ribs of the gonangium continue themselves 
upon the radiating arms, and ultimately 
extend beyond their extremities as free 
pointed processes. Two of them, however, 
situated opposite to one another, greatly 
surpass the others in size, and mainly con- 
tribute to the peculiar and characteristic 
form of the gonangium*. 
If we compare the structure now de- 
scribed with an ordinary polypite, we shall 
have no difficulty in recognizing an exact 
parallelism ; for the eight tubular processes 
which are developed from the summit of 
the blastostyle may be regarded as homo- 
logous with the tentacles of a polypite. 
They have, however, undergone a special 
modification, by which they become subser- 
vient to an entirely different function from 
that of the tentacles of the polypite; for, 
no mouth being developed on the blastostyle, 
they are no longer prehensile organs admi- 
nistering to the alimentation of the colony, 
but, like the blastostyle itself, have assigned 
to them functions appertaining to repro- 
duction rather than nutrition, and are 
destined to circumscribe a cavity for the 
retention and development of the ova. 
The ova would seem to continue in the 
marsupial cavity until they have acquired 
the condition of ciliated embryos. 
The modification of marsupial receptacle 
which occurs in Sertularia tamarisca is also 
very interesting. The female gonangia 
(fig. 11) are here of an oval form for about 
the proximal half of their length, and then 
become trihedral, with the sides diverging 
upwards, while the whole is terminated by 
a three-sided pyramid. The sides of the 
Fig. 10.—Female 
gonangium 
with marsupial chamber of 
Sertularia rosacea. 
a, lateral walls and, 6, roof of the 
gonangium proper; ec, chitinous 
walls of the marsupial chamber ; d, 
three of the six smaller and, e, the 
two larger spine-like processes which 
crown the marsupial chamber; jf, 
blastostyle; g, disc-like summit of 
the blastostyle; 4, the eight radiating 
tubular processes from the summit of 
the blastostyle; 72, ova in the mar- 
supial chamber, the germinal vesicle 
having already disappeared ; , go- 
nophore still attached to the blasto- 
style, and containing two ova with 
the germinal vesicle distinct ; /, dila- 
tation of the blastostyle, probably an 
incipient gonophore. 
pyramid are cut into two or three short teeth along their edges, and each 
of their basal angles is prolonged into a short spine. 
* In Sertularia fallax also a marsupial chamber is developed in the gonangium, and, 
judging from the figures and short description given by Dr. Strethill Wright (Proc. R. 
Phys. Soc. Edin. 28th April, 1858), it would seem that the structure is very nearly 
identical with that here described in S. rosacea. 
