Chapter V: V V (4)

1. _Siphons_ or nutritive appendages, from which the order takes its
name; never absent and usually present in great numbers (fig. 68, e).
Each is a tube dilated at or towards the base and containing a mouth
at its extremity, leading into a stomach placed in the dilatation
already mentioned. The siphons have been compared to the manubrium of
a medusa-individual, or to polyps, and hence are sometimes termed
gastrozoids.

2. _Palpons_ (fig. 68, g), present in some genera, especially in
Physonectae; similar to the siphons but without a mouth, and purely
tactile in function, hence sometimes termed dactylozoids. If a distal
pore or aperture is present, it is excretory in function; such
varieties have been termed "cystons" by Haeckel.

3. _Tentacles_ ("_Fangfäden_"), always present, and implanted one at
the base of each siphon (fig. 68, f). The tentacles of siphonophores
may reach a great length and have a complex structure. They may bear
accessory filaments or _tentilla_ (f'), covered thickly with batteries
of nematocysts, to which these organisms owe their great powers of
offence and defence.

4. _Palpacles_ ("_Tastfäden_"), occurring together with palpons, one
implanted at the base of each palpon (fig. 68, h). Each palpacle is a
tactile filament, very extensile, without accessory filaments or
nematocysts.

5. _Bracts_ ("_hydrophyllia_"), occur in _Calycophorida_ and some
_Physophorida_ as scale-like appendages protecting other parts (fig.
68, l). The mesogloea is greatly developed in them and they are often
of very tough consistency. By Haeckel they are considered homologous
with the umbrella of a medusa.

FIG. 70.--Diagram of the structure of _Velella_, showing the central
and peripheral thirds of a half-section of the colony, the middle
third being omitted. The ectoderm is indicated by close hatching, the
endoderm by light hatching, the mesogloea by thick black lines, the
horny skeleton of the pneumatophore and sail by dotting.

BL, Blastostyle.
C, Centradenia.
D, Palpon.
EC, Edge of colony prolonged beyond the pneumatophore.
G, Cavity of the large central siphon.
M, Medusoid gonophores.
PN, Primary central chamber, and
PN', concentric chamber of the pneumatophore, showing an opening to
the exterior and a "trachea."
S, Sail.]

6. _Gonostyles_, appendages which produce by budding medusae or
gonophores, like the blastostyles of a hydroid colony. In their most
primitive form they are seen in _Velella_ as "gonosiphons," which
possess mouths like the ordinary sterile siphons and bud free medusae.
In other forms they have no mouths. They may be branched, so-called
"gonodendra," and amongst them may occur special forms of palpons,
"gonopalpons." The gonostyles have been compared to the blastostyles
of a hydroid colony, or to the manubrium of a medusa which produces
free or sessile medusa-buds.

7. _Gonophores_, produced either on the gonostyles already mentioned
or budded, as in hydrocorallines, from the coenosarc, i.e. the stem
(fig. 68, i.). They show every transition between free medusae and
sporosacs, as already described, for hydroid colonies. Thus in
_Velella_ free medusae are produced, which have been described as an
independent genus of medusae, _Chrysomitra_. In other types the
medusae may be set free in a mature condition as the so-called
"genital swimming bells," comparable to the _Globiceps of Pennaria_.
The most usual condition, however, is that in which sessile medusoid
gonophores or sporosacs are produced.

The various types of appendages described in the foregoing may be
arranged in groups termed _cormidia_. In forms with a compact
coenosarc such as _Velella_, _Physalia_, &c., the separate cormidia
cannot be sharply distinguished, and such a condition is described
technically as one with "scattered" cormidia. In forms in which, on
the other hand, the coenosarc forms an elongated, tubular axis or
stem, the appendages are arranged as regularly recurrent cormidia
along it, and the cormidia are then said to be "ordinate." In such
cases the oldest cormidia, that is to say, those furthest from the
nectosome, may become detached (like the segments or proglottides of a
tape-worm) and swim off, each such detached cormidium then becoming a
small free cormus which, in many cases, has been given an independent
generic name. A cormidium may contain a single nutritive siphon
("monogastric") or several siphons ("polygastric"):

FIG. 71.--Upper surface of _Velella_, showing pneumatophore and sail.]

The following are some of the forms of cormidia that occur:--

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.

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.

FIG. 73.--_Physophora hydrostatica._

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_.

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: