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.
[Illustration: From G. H. Fowler, after A. Agassiz, Lankester's
_Treatise on Zoology_.
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"):
[Illustration: From G. H. Fowler, after G. Cuvier, Lankester's
_Treatise on Zoology_.