ON THE REPRODUCTIVE SYSTEM IN THE HYDROIDA. 391 
it remains fixed as a permanent part of the hydrosoma in the Tubularida, it 
is in Hydra destined to become detached and enjoy henceforth an independent 
existence. 
Decapitation and Re-formation of successive Polypite-heads. Polarity of the 
Hydroid.—Our account of the development of the polypite-bud in the 7ubu- 
larida would be incomplete without some reference to a very remarkable 
phenomenon presented by certain species of Tubularia, namely, the periodical 
shedding and renewal of the polypite-heads. This phenomenon was several 
years ago observed by Dalyell*, and described with all his usual accuracy by 
this excellent observer. I cannot find, however, that any author has followed 
the process with that exactness which is necessary to enable us to form a 
correct idea of its nature. My own observations have been principally made 
on Tubularia indivisa, where I have bestowed upon the process in question a 
very careful examination. 
When the polypite of this species, with its clusters of gonophores, has 
acquired full maturity, the time is come when it is to be cast off and its place 
taken by a successor, A breach of continuity now occurs in the endoderm of 
the stem at a short distance behind the polypite; while the ectoderm haying 
already become detached from the endoderm in the space between this breach 
and the base of the polypite, the endoderm of the upper end of the stem slips 
out of the ectoderm, carrying the polypite with it, and leaving behind it the 
empty ectoderm as a thin, collapsed, membranous sheath, surrounded by the 
periderm, which here exists as a very delicate loose pellicle. 
The polypite thus detached falls to the ground, where it retains for some 
time its vitality, the gonophores which still hang from it discharging such of 
their contents as had not escaped before the decapitation. 
In the mean time the wound which had been formed in the ccenosare by 
the detachment of the polypite heals over, and the truncated end of the 
ceenosarc becomes closed. 
Two slight constrictions, one a little behind the other, are next seen to 
take place in the ccenosare at a short distance from the decapitated extremity, 
while the peculiar tubular lacunze which exist in the coenosare of Tubularia, 
and which had hitherto extended as separate canals through the whole cceno- 
sarc of the stem, now coalesce in front of the anterior constriction, where 
they form a single cavity by the breaking down of the partitions of endoderm 
which had up to this time separated them from one another, 
A girdle of minute tubercles may next be seen budding forth from the 
ceenosarc¢, at the site of the posterior constriction. These soon become extended 
into tentacles, which embrace the anterior part of the coenosare, round which 
they appear twisted in a very elongated spiral. 
In the next stage a similar zone of tubercles, becoming, like the others, 
elongated into tentacles, shows itself close behind the anterior constriction ; 
and there are thus established the two sets of tentacles, the posterior and 
anterior ones of the new polypite. 
By the elongation of the.ccenosare from behind, the new polypite is gra- 
dually lifted up out of the tube of the periderm, when the tentacles, having 
room to expand, immediately fall into their normal position, while the rudi- 
mental clusters of gonophores may be seen as minute lobulated elevations 
between the anterior and posterior series of tentacles, and the free extremity 
of the polypite has by this time become perforated by a mouth. 
The polypite now increases in size, raised higher and higher on the elon- 
gating ccenosarc, which clothes itself with a periderm as it lengthens, and the 
* Rare and Remarkable Animals, 1847, vol. i. p. 4. 
