a´, Pneumatocyst.
t, Palpons.
a, Axis of the colony.
m, Nectocalyx.
o, Orifice of nectocalyx.
n, Siphon.
g, Gonophore.
i, Tentacle.]
In the first place the cormus has been regarded as a single individual
and its appendages as _organs_. This is the so-called "polyorgan"
theory, especially connected with the name of Huxley; but it must be
borne in mind that Huxley regarded all the forms produced, in any
animal, between one egg-generation and the next, as constituting in
the lump one single individual. Huxley, therefore, considered a
hydroid colony, for example, as a single individual, and each separate
polyp or medusa budded from it as having the value of an organ and not
of an individual. Hence Huxley's view is not so different from those
held by other authors as it seems to be at first sight.
In more recent years Woltereck [59] has supported Huxley's view of
individuality, at the same time drawing a fine distinction between
"individual" and "person." The individual is the product of sexual
reproduction; a person is an individual of lower rank, which may be
produced asexually. A Siphonophore is regarded as a single individual
composed of numerous zoids, budded from the primary zoid (siphon)
produced from the planula. Any given zoid is a person-zoid if
equivalent to the primary zoid, an organ-zoid if equivalent only to a
part of it. Woltereck considers the siphonophores most nearly allied
to the Narcomedusae, producing like the buds from an aboral stolon,
the first bud being represented by the pneumatophore or protocodon, in
different cases.
Contrasting, in the second place, with the polyorgan theory are the
various "polyperson" theories which interpret the Siphonophore cormus
as a colony composed of more or fewer individuals in organic union
with one another. On this interpretation there is still room for
considerable divergence of opinion as regards detail. To begin with,
it is not necessary on the polyperson theory to regard each appendage
as a distinct individual; it is still possible to compare appendages
with parts of an individual which have become separated from one
another by a process of "dislocation of organs." Thus a bract may be
regarded, with Haeckel, as a modified umbrella of a medusa, a siphon
as its manubrium, and a tentacle as representing a medusan tentacle
shifted in attachment from the margin to the sub-umbrella; or a siphon
may be compared with a polyp, of which the single tentacle has become
shifted so as to be attached to the coenosarc and so on. Some authors
prefer, on the other hand, to regard every appendage as a separate
individual, or at least as a portion of an individual, of which other
portions have been lost or obliterated.
A further divergence of opinion arises from differences in the
interpretation of the persons composing the colony. It is possible to
regard the cormus (1) as a colony of medusa-persons, (2) as a colony
of polyp-persons, (3) as composed partly of one, partly of the other.
It is sufficient here to mention briefly the views put forward on this
point by C. Chun and R. Haeckel.
Chun (HYDROZOA [1]) maintains the older views of Leuckart and Claus,
according to which the cormus is to be compared to a floating hydroid
colony. It may be regarded as derived from floating polyps similar to
_Nemopsis_ or _Pelagohydra_, which by budding produce a colony of
polyps and also form medusa-buds. The polyp-individuals form the
nutritive siphosome or trophosome. The medusa-buds are either fertile
or sterile. If fertile they become free medusae or sessile gonophores.
If sterile they remain attached and locomotor in function, forming the
nectosome, the pneumatophore and swimming-bells.
Haeckel, on the other hand, is in accordance with Balfour in regarding
a Siphonophore as a medusome, that is to say, as a colony composed of
medusoid persons or organs entirely. Haeckel considers that the
Siphonophores have two distinct ancestral lines of evolution:
1. In the _Disconanthae_, i.e. in such forms as _Velella_, _Porpita_,
&c., the ancestor was an eight-rayed medusa (_Disconula_) which
acquired a pneumatophore as an ectodermal pit on the ex-umbrella, and
in which the organs (manubrium, tentacles, &c.) became secondarily
multiplied, just as they do in _Gastroblasta_ as the result of
incomplete fission. The nearest living allies of the ancestral
_Disconula_ are to be sought in the _Pectyllidae_.
[Illustration: After Haeckel, from Lankester's _Treatise on Zoology_.
FIG. 74.--_Stephalia corona_, a young colony.
p, Pneumatophore.
n, Nectocalyx.
l, Aurophore.
lo, Orifice of the aurophore.
s, Siphon.
t, Tentacle.]
2. In the _Siphonanthae_, i.e. in all other Siphonophores, the
ancestral form was a _Siphonula_, a bilaterally symmetrical
Anthomedusa with a single long tentacle (cf. _Corymorpha_), which
became displaced from the margin to the sub-umbrella. The _Siphonula_
produced buds on the manubrium, as many Anthomedusae are known to do,
and these by reduction or dislocation of parts gave rise to the
various appendages of the colony. Thus the umbrella of the _Siphonula_
became the protocodon, and its manubrium, the axis or stolon, which,
by a process of dislocation of organs, escaped, as it were, from the
sub-umbrella through a cleft and became secondarily attached to the
ex-umbrella. It must be pointed out that, however probable Haeckel's
theory may be in other respects, there is not the slightest evidence
for any such cleft in the umbrella having been present at any time,
and that the embryological evidence, as already pointed out, is all
against any homology between the stem and a manubrium, since the
primary siphon does not become the stem, which arises from the
ex-umbral side of the protocodon and is strictly comparable to a
stolon.
_Classification._--The Siphonophora may be divided, following Delage and
Hérouard, into four sub-orders:
I. CHONDROPHORIDA (_Disconectae_ Haeckel, _Tracheophysae_ Chun). With an
apical chambered pneumatophore, from which tracheal tubes may take
origin (fig. 70); no nectocalyces or bracts; appendages all on the lower
side of the pneumatophore arising from a compact coenosarc, and
consisting of a central principal siphon, surrounded by gonosiphons, and
these again by tentacles.
Three families: (1) _Discalidae_, for _Discalia_ and allied genera,
deep-sea forms not well known; (2) _Porpitidae_ for the familiar genus
_Porpita_ (fig. 69) and its allies; and (3) _Velellidae_, represented
by the well-known genus _Velella_ (figs. 70, 71), common in the
Mediterranean and other seas.
II. CALYCOPHORIDA (_Calyconectae_, Haeckel). Without pneumatophore, with
one, two, rarely more nectocalyces.
Three families: (1) _Monophyidae_, with a single nectocalyx; examples
_Muggiaea_, sometimes found in British seas, _Sphaeronectes_, &c.; (2)
_Diphyidae_, with two nectocalyces; examples _Diphyes_ (fig. 72),
_Praya_, _Abyla_, &c.; and (3) _Polyphyidae_, with numerous
nectocalyces; example _Hippopodius_, _Stephanophyes_ and other genera.
[Illustration: From G. H. Fowler, modified after G. Cuvier and E.
Haeckel, Lankester's _Treatise on Zoology_.
FIG. 75.--A. _Physalia_, general view, diagrammatic; B, cormidium of
Physalia; D, palpon; T, palpacle; G, siphon; GP, gonopalpon; M[male],
male gonophore; M[female], female gonophore, ultimately set free.]
III. PHYSOPHORIDA (_Physonectae_ + _Auronectae_, Haeckel). With an
apical pneumatophore, not divided into chambers, followed by a series of
nectocalyces or bracts.
A great number of families and genera are referred to this group,
amongst which may be mentioned specially--(1) _Agalmidae_, containing
the genera _Stephanomia_, _Agalma_, _Anthemodes_, _Halistemma_, &c.;
(2) _Apolemidae_, with the genus _Apolemia_ and its allies; (3)
_Forskaliidae_, with _Forskalia_ and allied forms; (4)
_Physophoridae_, for _Physophora_ (fig. 73) and other genera, (5)
_Anthophysidae_, for _Anthophysa_, _Athorybia_, &c.; and lastly the
two families (6) _Rhodalidae_ and (7) _Stephalidae_ (fig. 74),
constituting the group Auronectae of Haeckel. The Auronectae are
peculiar deep-sea forms, little known except from Haeckel's
descriptions, in which the large pneumatophore has a peculiar duct,
termed the aurophore, placed on its lower side in the midst of a
circle of swimming-bells.
IV. CYSTOPHORIDA (_Cystonectae_, Haeckel). With a very large
pneumatophore not divided into chambers, but without nectocalyces or
bracts. Two sections can be distinguished, the Rhizophysina, with long
tubular coenosarc-bearing ordinate cormidia, and Physalina, with compact
coenosarc-bearing scattered cormidia.
A type of the Rhizophysina is the genus _Rhizophysa_. The Physalina
comprise the families _Physalidae_ and _Epibulidae_, of which the
types are _Physalia_ (figs. 74, 75) and _Epibulia_, respectively.
_Physalia_, known commonly as the Portuguese man-of-war, is remarkable
for its great size, its brilliant colours, and its terrible stinging
powers.
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