1. The _eudoxome_ (Calycophorida), consisting of a bract, siphon,
tentacle and gonophore; when free it is known as _Eudoxia_.
2. The _ersaeome_ (Calycophorida), made up of the same appendages as
the preceding type but with the addition of a nectocalyx; when free
termed _Ersaea_.
3. The _rhodalome_ of some _Rhodalidae_, consisting of siphon,
tentacle and one or more gonophores.
4. The _athorome_ of _Physophora_, &c., consisting of siphon,
tentacle, one or more palpons with palpacles, and one or more
gonophores.
5. The _crystallome_ of _Anthemodes_, &c., similar to the athorome but
with the addition of a group of bracts.
[Illustration: FIG. 72.--A, _Diphyes campanulata_; B, a group of
appendages (cormidium) of the same _Diphyes_. (After C. Gegenbaur.)
a, Axis of the colony.
m, Nectocalyx.
c, Sub-umbral cavity of nectocalyx.
v, Radial canals of nectocalyx.
o, Orifice of nectocalyx.
t, Bract.
n, Siphon.
g, Gonophore.
i, Tentacle.]
_Embryology of the Siphonophora._--The fertilized ovum gives rise to a
parenchymula, with solid endoderm, which is set free as a
free-swimming planula larva, in the manner already described (see
HYDROZOA). The planula has its two extremities dissimilar
(Bipolaria-larva). The subsequent development is slightly different
according as the future cormus is headed by a pneumatophore
(Physophorida, Cystophorida) or by a nectocalyx (Calycophorida).
(i.) Physophorida, for example _Halistemma_ (C. Chun, HYDROZOA [1]).
The planula becomes elongated and broader towards one pole, at which a
pit or invagination of the ectoderm arises. Next the pit closes up to
form a vesicle with a pore, and so gives rise to the pneumatophore.
From the broader portion of the planula an outgrowth arises which
becomes the first tentacle of the cormus. The endoderm of the planula
now acquires a cavity, and at the narrower pole a mouth is formed,
giving rise to the primary siphon. Thus from the original planula
three appendages are, as it were, budded off, while the planula itself
mostly gives rise to coenosarc, just as in some hydroids the planula
is converted chiefly into hydrorhiza.
(ii.) Calycophorida, for example, _Muggiaea_. The planula develops, on
the whole, in a similar manner, but the ectodermal invagination
arises, not at the pole of the planula, but on the side of its broader
portion, and gives rise, not to a pneumatophore, but to a nectocalyx,
the primary swimming bell or _protocodon_ ("_Fallschirm_") which is
later thrown off and replaced by secondary swimming bells,
_metacodons_, budded from the coenosarc.
From a comparison of the two embryological types there can be no doubt
on two points; first, that the pneumatophore and the protocodon are
strictly homologous, and, therefore if the nectocalyx is comparable to
the umbrella of a medusa, as seems obvious, the pneumatophore must be
so too; secondly, that the coenosarcal axis arises from the
ex-umbrella of the medusa and cannot be compared to a manubrium, but
is strictly comparable to the "bud-spike" of a Narcomedusan.
_Theories of Siphonophore Morphology._--The many theories that have
been put forward as to the interpretation of the cormus and the
various parts are set forth and discussed in the treatise of Y. Delage
and E. Hérouard (HYDROZOA [4]) and more recently by R. Woltereck [59],
and only a brief analysis can be given here.
[Illustration: After C. Gegenbaur.