ZOOPHYTA HYDROIDA. 
89 
which, in an old specimen, are to be found in a state of activity 
only near the summit, or on the new shoots. The Thuiaria 
thuja affords a remarkable example of this fact ; the branches 
which carry the polypes dropping off in regular succession as 
younger ones are successively formed, so that the polypidom re- 
tains, throughout its whole growth, the appearance of a bottle- 
brush, the naked stem and the branched top being kept in every 
stage in a due proportion to each other. Sertularia argentea, 
Plumularia falcata, &c. are subjected to the same law, — the pri- 
mary polypiferous shoots being deciduous, so that in them also 
the stalk becomes bare, while the upper parts are graced with 
a luxuriant ramification loaded with tiny architects. But in our 
eagerness to generalize, let us not forget that there are some 
species, as Sertularia pumila, abietina, &c., in which this pro- 
cess of successive denudation is not observable, —perhaps, how- 
ever, because of their form, which is not of a kind to be altered 
by it, and hence unnoticeable, or because the duration of the 
whole is too fugitive to permit the law to produce a visible ef- 
fect. 
There are facts which appear to prove that the life of the in- 
dividual polypes is even more transitory than their own cells ; 
that like a blossom they bud and blow and fall off or are absorb- 
ed, when another sprouts up from the medullary pulp to occupy 
the very cell of its predecessor, and in its turn to give way and 
be replaced by another. When speaking of flexible corallines 
Lamouroux says, “ Some there are that are entirely covered 
with polypi through the summer and autumn, but they perish 
with the cold of winter : no sooner, however, has the sun resum- 
ed his revivifying influence than new animals are developed, and 
fresh branches are produced upon the old ones.”* Of the Tu- 
bularia indivisa, Sir John G. Dalyell tells us that 66 the head is 
deciduous, falling in general soon after recovery from the sea. 
It is regenerated at intervals of from ten days to several weeks, 
but with the number of external organs successively diminishing, 
though the stem is always elongated. It seems to rise within 
this tubular stem from below, and to be dependent on the pre- 
sence of the internal tenacious matter with which the tube is oc- 
* Corail. Flex, p. xvi. 
