5. _Polyorchidae._--Trophosome unknown; gonosome, free medusae of deep
form, with radial canals branched in a feathery manner, and bearing
gonads on the main canal, but not on the branches, with numerous
hollow tentacles bearing ocelli, and without otocysts. _Polyorchis_,
_Spirocodon_.
6. _Campanularidae.-_-Trophosome as in _Eucopidae_; gonosome, sessile
gonophores. Many common or well-known genera belong here, such as
_Halecium_, _Campanularia_, _Gonothyraea_, &c.
7. _Lafoëidae._--Trophosome as in the preceding; gonosome, free
medusae or gonophores, the medusae with large open otocysts. The
hydroid genus _Lafoëa_ is remarkable for producing gonothecae on the
hydrorhiza, each containing a blastostyle which bears a single
gonophore; this portion of the colony was formerly regarded as an
independent parasitic hydroid, and was named _Coppinia_. Medusan
genera are _Mitrocoma_, _Halopsis_, _Tiaropsis_ (fig. 29, &c.).
(So far as the characters of the trophosome are concerned, the seven
preceding families are scarcely distinguishable, and they form a
section apart, contrasting sharply with the families next to be
mentioned, in none of which are free medusae liberated from the
colony, so that only the characters of the trophosome need be
considered.)
[Illustration: After E. T. Browne, _Proc. Zool. Soc. of London_, 1896.
FIG. 59.--Three stages in the development of _Phialidium temporarium_.
a, The youngest stage, is magnified about 22 diam.; b, older, is
magnified about 8 diam.; c, the adult medusa, is magnified.]
8. _Sertularidae._--Hydrothecae sessile, biserial, alternating or
opposite on the stem. _Sertularia_ and _Sertularella_ are two very
common genera of this family.
9. _Plumularidae._--Hydrothecae sessile, biserial on the main stem,
uniserial on the lateral branches or pinnules, which give the colony
its characteristic feathery form; with nematophores. A very abundant
and prolific family; well-known British genera are _Plumularia_,
_Antennularia_ and _Aglaophenia_.
10. _Hydroceratinidae_.--This family contains the single Australian
species _Clathrozoon wilsoni_ Spencer, in which a massive hydrorhiza
bears sessile hydrothecae, containing hydranths each with a single
tentacle, and numerous nematophores. See W. B. Spencer [53].
11. _Dendrograptidae_, containing fossil (Silurian) genera, such as
_Dendrograptus_ and _Thamnograptus_, of doubtful affinities.
[Illustration: FIG. 60.--Portion of the calcareous corallum of
_Millepora nodosa_, showing the cyclical arrangement of the pores
occupied by the "persons" or hydranths. About twice the natural size.
(From Moseley.)]
ORDER III. Hydrocorallinae.--Metagenetic colony-forming Hydromedusae, in
which the polyp-colony forms a massive, calcareous _corallum_ into which
the polyps can be retracted; polyp-individuals always of two kinds,
gastrozoids and dactylozoids; gonosome either free medusae or sessile
gonophores. The trophosome consists of a mass of coenosarcal tubes
anastomosing in all planes. The interspaces between the tubes are filled
up by a solid mass of lime, consisting chiefly of calcium carbonate,
which replaces the chitinous perisarc of ordinary hydroids and forms a
stony corallum or _coenosteum_ (fig. 60). The surface of the coenosteum
is covered by a layer of common ectoderm, containing large nematocysts,
and is perforated by pores of two kinds, gastropores and dactylopores,
giving exit to gastrozoids and dactylozoids respectively, which are
lodged in vertical pore-canals of wider calibre than the coenosarcal
canals of the general network. The coenosteum increases in size by new
growth at the surface; and in the deeper, older portions of massive
forms the tissues die off after a certain time, only the superficial
region retaining its vitality down to a certain depth. The living
tissues at the surface are cut off from the underlying dead portions by
horizontal partitions termed _tabulae_, which are formed successively as
the coenosteum increases in age and size. If the coenosteum of
_Millepora_ be broken across, each pore-canal (perhaps better termed a
polyp-canal) is seen to be interrupted by a series of transverse
partitions, representing successive periods of growth with separation
from the underlying dead portions.
[Illustration: FIG. 61.--Enlarged view of the surface of a living
_Millepora_, showing five dactylozooids surrounding a central
gastrozooid. (From Moseley.)]
Besides the wider vertical pore-canals and the narrower, irregular
coenosarcal canals, the coenosteum may contain, in its superficial
portion, chambers or _ampullae_, in which the reproductive zoids
(medusae or gonophores) are budded from the coenosarc.
The gastropores and dactylopores are arranged in various ways at the
surface, a common pattern being the formation of a cyclosystem (fig.
60), in which a central gastrozoid is surrounded by a ring of
dactylozoids (fig. 61). In such a system the dactylopores may be
confluent with the gastropore, so that the entire cyclosystem presents
itself as a single aperture subdivided by radiating partitions, thus
having a superficial resemblance to a madreporarian coral with its
radiating septa (figs. 62 and 63).
[Illustration: FIG. 62.--Diagrams illustrating the successive stages in
the development of the cyclosystems of the _Stylasteridae_. (After
Moseley.)
1, _Sporadopora dichotoma_.
2, 3, _Allopora nobilis_.
4, _Allopora profunda_.
5, _Allopora miniacea_.
6, _Astylus subviridis_.
7, _Distichopora coccinea_.
s, Style.
dp, Dactylopore.
gp, Gastropore.
b, In fig. 6, inner horseshoe-shaped mouth of gastropore.]
The gastrozoids usually bear short capitate tentacles, four, six or
twelve in number; but in _Astylus_ (fig. 63) they have no tentacles. The
dactylozoids have no mouth; in _Milleporidae_ they have short capitate
tentacles, but lack tentacles in _Stylasteridae_.
The gonosome consists of free medusae in _Milleporidae_, which are
budded from the apex of a dactylozoid in _Millepora murrayi_, but in
other species from the coenosarcal canals. The medusae are produced by
direct budding, without an entocodon in the bud. They are liberated in a
mature condition, and probably live but a short time, merely sufficient
to spread the species. The manubrium bearing the gonads is mouthless,
and the umbrella is without tentacles, sense-organs, velum or radial
canals. In the _Stylasteridae_ sessile gonophores are formed, always by
budding from the coenosarc. In _Distichopora_ the gonophores have radial
canals, but in other genera they are sporosacs with no trace of medusoid
structure.
[Illustration: FIG. 63.--Portion of the corallum of _Astylus subviridis_
(one of the Stylasteridae), showing cyclosystems placed at intervals on
the branches, each with a central gastropore and zone of slit-like
dactylopores. (After Moseley.)]
_Classification._---Two families are known:--
1. _Milleporidae._--Coenosteum massive, irregular in form; pores
scattered irregularly or in cyclosystems, without styles, with
transverse tabulae; free medusae. A single genus, _Millepora_ (figs.
60, 61).
2. _Stylasteridae._--Coenosteum arborescent, sometimes fanlike, with
pores only on one face, or on the lateral margins of the branches;
gastropores with tabulae only in two genera, but with (except in
_Astylus_) a _style_, i.e. a conical, thorn-like projection from the
base of the pore, sometimes found also in dactylopores; sessile
gonophores. _Sporadopora_ has the pores scattered irregularly.
_Distichopora_ has the pores arranged in rows. _Stylaster_ has
cyclosystems. In _Allopora_ the cyclostems resemble the calyces of
Anthozoan corals. In _Cryptohelia_ the cyclosystem is covered by a cap
or operculum. In _Astylus_ (fig. 63) styles are absent.
_Affinities of the Hydrocorallinae._--There can be no doubt that the
forms comprised in this order bear a close relationship to the
Hydroidea, especially the sub-order Gymnoblastea, with which they
should perhaps be classed in a natural classification. A
hydrocoralline may be regarded as a form of hydroid colony in which
the coenosarc forms a felt-work ramifying in all planes, and in which
the chitinous perisarc is replaced by a massive calcareous skeleton.
So far as the trophosome is concerned, the step from an encrusting
hydroid such as _Hydractinia_ to the hydrocoralline _Millepora_ is not
great.
Hickson considers that the families _Milleporidae_ and _Stylasteridae_
should stand quite apart from one another and should not be united in
one order. The nearest approach to the _Stylasteridae_ is perhaps to
be found in _Ceratella_, with its arborescent trophosome formed of
anastomosing coenosarcal tubes supported by a thick perisarc and
covered by a common ectoderm. _Ceratella_ stands in much the same
relation to the _Stylasteridae_ that _Hydractinia_ does to the
_Milleporidae_, in both cases the chitinous perisarc being replaced by
the solid coenosteum to which the hydrocorallines owe the second half
of their name.
ORDER IV. Graptolitoidea (Rhabdophora, Allman).--This order has been
constituted for a peculiar group of palaeozoic fossils, which have been
interpreted as the remains of the skeletons of Hydrozoa of an extinct
type.
A typical graptolite consists of an axis bearing a series of tooth-like
projections, like a saw. Each such projection is regarded as
representing a cup or hydrotheca, similar to those borne by a
calyptoblastic hydroid, such as _Sertularia_. The supposed hydrothecae
may be present on one side of the axis only (monoprionid) or on both
sides (diprionid); the first case may be conjectured to be the result of
uniserial (helicoid) budding, the second to be produced by biserial
(scorpioid) budding. In one division (_Retiolitidae_) the axis is
reticulate. In addition to the stems bearing cups, there are found
vesicles associated with them, which have been interpreted as gonothecae
or as floats, that is to say, air-bladders, acting as hydrostatic organs
for a floating polyp-colony.
Since no graptolites are known living, or, indeed, since palaeozoic
times, the interpretation of their structure and affinities must of
necessity be extremely conjectural, and it is by no means certain that
they are Hydrozoa at all. It can only be said that their organization,
so far as the state of their preservation permits it to be ascertained,
offers closer analogies with the Hydrozoa, especially the
Calyptoblastea, than with any other existing group of the animal
kingdom.
See the treatise of Delage and Hérouard (HYDROZOA, [4]), and the
article GRAPTOLITES.
ORDER V. Trachylinea.--Hydromedusae without alternation of generations,
i.e. without a hydroid phase; the medusa develops directly from the
actinula larva, which may, however, multiply by budding. Medusae with
sense-organs represented by otocysts derived from modified tentacles
(tentaculocysts), containing otoliths of endodermal origin, and
innervated from the ex-umbral nerve-ring.
This order, containing the typical oceanic medusae, is divided into two
sub-orders.
SUB-ORDER 1. TRACHOMEDUSAE.--Tentacles given off from the margin of the
umbrella, which is entire, i.e. not lobed or indented; tentaculocysts
usually enclosed in vesicles; gonads on the radial canals. The medusae
of this order are characterized by the tough, rigid consistence of the
umbrella, due partly to the dense nature of the mesogloea, partly to the
presence of a marginal rim of chondral tissue, consisting of thickened
ectoderm containing great numbers of nematocysts, and forming, as it
were, a cushion-tyre supporting the edge of the umbrella. Prolongations
from the rim of chondral tissue may form clasps or _peronia_ supporting
the tentacles. The tentacles are primarily four in number, perradial,
alternating with four interradial tentaculocysts, but both tentacles and
sense-organs may be multiplied and the primary perradii may be six
instead of four (fig. 26). The tentacles are always solid, containing an
axis of endoderm-cells resembling notochordal tissue or
plant-parenchyma, and are but moderately flexible. The sense-organs are
tentaculocysts which are usually enclosed in vesicles and may be sunk
far below the surface. The gonads are on the radial canals or on the
stomach (_Ptychogastridae_), and each gonad may be divided into two by a
longitudinal sub-umbral muscle-tract. The radial canals are four, six,
eight or more, and in some genera blindly-ending centripetal canals are
present (fig. 26). The stomach may be drawn out into the manubrium,
forming a proboscis ("Magenstiel") of considerable length.
The development of the Trachomedusae, so far as it is known, shows an
actinula-stage which is either free (larval) or passed over in the egg
(foetal) as in _Geryonia_; in no case does there appear to be a free
planula-stage. The actinula, when free, may multiply by larval budding,
but in all cases both the original actinula and all its descendants
become converted into medusae, so that there is no alternation of
generations. In _Gonionemus_ the actinula becomes attached and
polyp-like and reproduces by budding.
[Illustration: After Haeckel, _System der Medusen_, by permission of
Gustav Fischer.