2. _Podocorynidae_ (= medusa-families _Thamnostomidae_ and _Cytaeidae_
+ hydroid families _Podocorynidae_ and _Hydractiniidae_). Trophosome
encrusting with hydranths of _Bougainvillea_-type, polyps
differentiated into blastostyles, gastrozoids and dactylozoids;
gonosome free medusae or gonophores. The typical genus is the
well-known hydroid _Podocoryne_, budding the medusa known as
_Dysmorphosa_; _Thamnostylus_, _Cytaeis_, &c., are other medusae with
unknown hydroids. _Hydractinia_ (figs. 9, 10) is a familiar hydroid
genus, bearing gonophores.
3. _Cladonemidae_.--Trophosome, polyps with two whorls of tentacles,
the lower filiform, the upper capitate; gonosome, free medusae, with
tentacles solid and branched. The type-genus _Cladonema_ (fig. 20) is
a common British form.
4. _Clavatellidae._--Trophosome, polyps with a single whorl of
capitate tentacles; gonosome, free medusae, with tentacles branched,
solid. _Clavatella_ (fig. 21), with a peculiar ambulatory medusa is a
British form.
5. _Pennariidae_.--Trophosome, polyps with an upper circlet of
numerous capitate tentacles, and a lower circlet of filiform
tentacles. _Pennaria_, with a free medusa known as _Globiceps_, is a
common Mediterranean form. _Stauridium_ (fig. 2) is a British hydroid.
6. _Tubulariidae._--Trophosome, polyps with two whorls of tentacles,
both filiform. _Tubularia_ (fig. 4), a well-known British hydroid,
bears gonophores.
7. _Corymorphidae_ (including the medusa-family
_Hybocodonidae_).--Trophosome solitary polyps, with two whorls of
tentacles; gonosome, free medusae or gonophores. _Corymorpha_ (fig.
3), a well-known British genus, sets free a medusa known as
_Steenstrupia_ (fig. 22). Here belong the deep-sea genera _Monocaulus_
and _Branchiocerianthus_, including the largest hydroid polyps known,
both genera producing sessile gonophores.
[Illustration: After Haeckel, _System der Medusen_, by permission of
Gustav Fischer.
FIG. 53.--_Pteronema darwinii_. The apex of the stomach is prolonged
into a brood pouch containing embryos.]
8. _Dendroclavidae._--Trophosome, polyp with filiform tentacles in
three or four whorls. _Dendroclava_, a hydroid, produces the medusa
known as _Turritopsis_.
9. _Clavidae_ (including the medusa-family _Tiaridae_ (figs. 27 and
51). Trophosome, polyps with scattered filiform tentacles; gonosome,
medusae or gonophores, the medusae with hollow tentacles. _Clava_
(fig. 5), a common British hydroid, produces gonophores; so also does
_Cordylophora_, a form inhabiting fresh or brackish water. _Turris_
produces free medusae. _Amphinema_ is a medusan genus of unknown
hydroid.
10. _Bythotiaridae._--Trophosome unknown; gonosome, free medusae, with
deep, bell-shaped umbrella, with interradial gonads on the base of the
stomach, with branched radial canals, and correspondingly numerous
hollow tentacles. _Bythotiara_, _Sibogita_.
11. _Corynidae_ (= hydroid families _Corynidae_, _Syncorynidae_ and
_Cladocorynidae_ + medusan family _Sarsiidae_).--Trophosome polyps
with capitate tentacles, simple or branched, scattered or
verticillate; gonosome, free medusae or gonophores. _Coryne_, a common
British hydroid, produces gonophores; _Syncoryne_, indistinguishable
from it, produces medusae known as _Sarsia_ (fig. 51). _Cladocoryne_
is another hydroid genus; _Codonium_ and _Dipurena_ (fig. 50) are
medusan genera.
12. _Myriothelidae._--The genus _Myriothela_ is a solitary polyp with
scattered capitate tentacles, producing sporosacs.
13. _Hydrolaridae._--Trophosome (only known in one genus), polyps with
two tentacles forming a creeping colony; gonosome, free medusae with
four, six or more radial canals, giving off one or more lateral
branches which run to the margin of the umbrella, with the stomach
produced into four, six or more lobes, upon which the gonads are
developed; the mouth with four lips or with a folded margin; the
tentacles simple, arranged evenly round the margin of the umbrella.
The remarkable hydroid _Lar_ (fig. 11) grows upon the tubes of the
worm _Sabella_ and produces a medusa known as _Willia_. Another
medusan genus is _Proboscidactyla_.
14. _Monobrachiidae._--The genus _Monobrachium_ is a colony-forming
hydroid which grows upon the shells of bivalve molluscs, each polyp
having but a single tentacle. It buds medusae, which, however, are as
yet only known in an immature condition (C. Mereschkowsky [41]).
15. _Ceratellidae._--Trophosome polyps forming branching colonies of
which the stem and main branches are thick and composed of a network
of anastomosing coenosarcal tubes covered by a common ectoderm and
supported by a thick chitinous perisarc; hydranths similar to those of
_Coryne_; gonosome, sessile gonophores. _Ceratella_, an exotic genus
from the coast of East Africa, New South Wales and Japan. The genera
_Dehitella_ Gray and _Dendrocoryne_ Inaba should perhaps be referred
to this family; the last-named is regarded by S. Goto [16] as the type
of a distinct family, _Dendrocorynidae_.
Doubtful families, or forms difficult to classify, are: Pteronemidae,
Medusae of Cladonemid type, with hydroids for the most part unknown.
The British genus _Gemmaria_, however, is budded from a hydroid
referable to the family _Corynidae_. _Pteronema_ (fig. 53).
_Nemopsidae_, for the floating polyp _Nemopsis_, very similar to
_Tubularia_ in character; the medusa, on the other hand, is very
similar to _Hippocrene_ (_Margelidae_). See C. Chun (HYDROZOA [1]).
_Pelagohydridae_, for the floating polyp _Pelagohydra_, Dendy, from
New Zealand. The animal is a solitary polyp bearing a great number of
medusa-buds. The body, representing the hydranth of an ordinary
hydroid, has the aboral portion modified into a float, from which
hangs down a proboscis bearing the mouth. The float is covered with
long tentacles and bears the medusa-buds. The proboscis bears at its
extremity a circlet of smaller oral tentacles. Thus the affinities of
the hydranth are clearly, as Dendy points out, with a form such as
_Corymorpha_, which also is not fixed but only rooted in the mud. The
medusae, on the other hand, have the tentacles in four tufts of (in
the buds) five each, and thus resemble the medusae of the family
_Margelidae_. See A. Dendy [12].
[Illustration: _Fig. 54._--Diagram showing possible modifications of
the persons of a Calyptoblastic Hydromedusa. Letters a to h same as in
fig. 49. i, The horny cup or hydrotheca of the hydriform persons; l,
medusiform person springing from m, a modified, hydriform person
(blastostyle); n, the horny case or gonangium enclosing the
blastostyle and its buds. This and the hydrotheca i give origin to the
name _Calyptoblastea_. (After Allman.)]
_Perigonimus._--This common British hydroid belongs by its characters
to the family _Bougainvillidae_; it produces, however, a medusa of the
genus _Tiara_ (fig. 52), referable to the family _Clavidae_; a fact
sufficient to indicate the tentative character of even the most modern
classifications of this order.
SUB-ORDER II. HYDROIDEA CALYPTOBLASTEA (LEPTOMEDUSAE).--Trophosome with
polyps always differentiated into nutritive and reproductive individuals
(blastostyles) enclosed in hydrothecae and gonothecae respectively; with
sympodial type of budding. Gonosome with free medusae or gonophores; the
medusae typically with otocysts, sometimes with cordyli or ocelli (figs.
54, 55).
[Illustration: FIG. 55.--View of the Oral Surface of one of the
_Leptomedusae_ (_Irene pellucida_, Haeckel), to show the numerous
tentacles and the otocysts.
ge, Genital glands.
M, Manubrium.
ot, Otocysts.
rc, The four radiating canals.
Ve, The velum.]
The calyptoblastic polyp of the nutritive type is very uniform in
character, its tendency to variation being limited, as it were, by the
enclosing hydrotheca. The hydranth almost always has a single circlet of
tentacles, like the _Bougainvillea_-type, in the preceding sub-order; an
exception is the curious genus _Clathrozoon_, in which the hydranth has
a single tentacle. The characteristic hydrotheca is formed by the bud at
an early stage (fig. 56); when complete it is an open cup, in which the
hydranth develops and can be protruded from the opening for the capture
of food, or is withdrawn into it for protection. Solitary polyps are
unknown in this sub-order; the colony may be creeping or arborescent in
form; if the latter, the budding of the polyps, as already stated, is of
the sympodial type, and either biserial, forming stems capable of
further branching, or uniserial, forming pinnules not capable of further
branching. In the biserial type the polyps on the two sides of the stem
have primitively an alternating, zigzag arrangement; but, by a process
of differential growth, quickened in the 1st, 3rd, 5th, &c., members of
the stem, and retarded in the 2nd, 4th, 6th, &c., members, the polyps
may assume secondarily positions opposite to one another on the two
sides of the stem. Other variations in the mode of growth or budding
bring about further differences in the building up of the colony, which
are not in all cases properly understood and cannot be described in
detail here. The stem may contain a single coenosarcal tube
("monosiphonic") or several united in a common perisarc
("polysiphonic"). An important variation is seen, in the form of the
hydrotheca itself, which may come off from the main stem by a stalk, as
in _Obelia_, or may be sessile, without a stalk, as in _Sertularia_.
[Illustration: After Allman, _Gymnoblastic Hydroids_, by permission of
the council of the Ray Society.
FIG. 56.--Diagrams to show the mode of formation of the Hydrotheca and
Gonotheca in Calyptoblastic Hydroids. A-D are stages common to both;
from D arises the hydrotheca (E) or the gonotheca (F); th, theca; st,
stomach; t, tentacles; m, mouth; mb, medusa-buds.]
In many Calyptoblastea there occur also reduced defensive polyps or
dactylozoids, which in this sub-order have received the special name of
_sarcostyles_. Such are the "snake-like zoids" of _Ophiodes_ and other
genera, and as such are generally interpreted the "machopolyps" of the
_Plumularidea_. These organs are supported by cuplike structures of the
perisarc, termed nematophores, regarded as modified hydrothecae
supporting the specialized polyp-individuals. They are specially
characteristic of the family _Plumularidae_.
The medusa-buds, as already stated, are always produced from
blastostyles, reduced non-nutritive polyps without mouth or tentacles.
An apparent, but not real, exception is _Halecium halecinum_, in which
the blastostyle is produced from the side of a nutritive polyp, and both
are enclosed in a common theca without a partition between them (Allman
[1] p. 50, fig. 24). The gonotheca is formed in its early stage in the
same way as the hydrotheca, but the remains of the hydranth persists as
an operculum closing the capsule, to be withdrawn when the medusae or
genital products are set free (fig. 56).
The blastostyles, gonophores and gonothecae furnish a series of
variations which can best be considered as so many stages of
evolution.
Stage 1, seen in _Obelia_. Numerous medusae are budded successively
within the gonotheca and set free; they swim off and mature in the
open sea (Allman [1], p. 48, figs. 18, 19).
Stage 2, seen in _Gonothyraea_. Medusae, so-called "meconidia," are
budded but not liberated; each in turn, when it reaches sexual
maturity, is protruded from the gonotheca by elongation of the stalk,
and sets free the embryos, after which it withers and is replaced by
another (Allman [1], p. 57, fig. 28).
Stage 3, seen in _Sertularia_.--The gonophores are reduced in varying
degree, it may be to sporosacs; they are budded successively from the
blastostyle, and each in turn, when ripe, protrudes the spadix through
the gonotheca (fig. 57, A, B). The spadix forms a gelatinous cyst, the
so-called acrocyst (ac), external to the gonotheca (gth), enclosing
and protecting the embryos. Then the spadix withers, leaving the
embryos in the acrocyst, which may be further protected by a so-called
marsupium, a structure formed by tentacle-like processes growing out
from the blastostyle to enclose the acrocyst, each such process being
covered by perisarc like a glove-finger secreted by it (fig. 57, C).
(Allman [1], pp. 50, 51, figs. 21-24; Weismann [58], p. 170, pl. ix.,
figs. 7, 8.)
Stage 4, seen in _Plumularidae_.--The generative elements are produced
in structures termed corbulae, formed by reduction and modification of
branches of the colony. Each corbula contains a central row of
blastostyles enclosed and protected by lateral rows of branches
representing stunted buds (Allman [1], p. 66, fig. 30).
[Illustration: After Allman, _Gymnoblastic Hydroids_, by permission of
the council of the Ray Society.
Fig. 57.--Diagrams to show the mode of formation of an Acrocyst and a
Marsupium. In A two medusa-buds are seen within the gonotheca (gth), the
upper more advanced than the lower one. In B the spadix of the upper bud
has protruded itself through the top of the gonotheca and the acrocyst
(ac) is secreted round it. In C the marsupium (m) is formed as
finger-like process from the summit of the blastostyle, enclosing the
acrocyst; b, medusa-buds on the blastostyle.]
The _Leptomedusa_ in form is generally shallow, more or less
saucer-like, with velum less developed than in Anthomedusae (fig. 55).
The characteristic sense-organs are ectodermal otocysts, absent,
however, in some genera, in which case cordyli may replace them. When
otocysts are present, they are at least eight in number, situated
adradially, but are often very numerous. The cordyli are scattered on
the ring-canal. Ocelli, if present, are borne on the tentacle-bulbs. The
tentacles are usually hollow, rarely solid (_Obelia_). In number they
are rarely less than four, but in _Dissonema_ there are only two.
Primitively there are four perradial tentacles, to which may be added
four interradial, or they may become very numerous and are then
scattered evenly round the margin, never arranged in tufts or clusters.
In addition to tentacles, there may be marginal cirri (_Laodice_) with a
solid endodermal axis, spirally coiled, very contractile, and bearing a
terminal battery of nematocysts. The gonads are developed typically
beneath the radial canals or below the stomach or its pouches, often
stretching as long bands on to the base of the manubrium. In
_Octorchidae_ (fig. 58) each such band is interrupted, forming one mass
at the base of the manubrium and another below the radial canal in each
radius, in all eight separate gonad-masses, as the name implies. In some
Leptomedusae excretory "marginal tubercles" are developed on the
ring-canal.
_Classification._---As in the Gymnoblastea, the difficulty of uniting
the hydroid and medusan systems into one scheme of classification is
very great in the present state of our knowledge. In a great many
Leptomedusae the hydroid stage is as yet unknown, and it is by no
means certain even that they possess one. It is quite possible that
some of these medusae will be found to be truly hypogenetic, that is
to say, with a life-cycle secondarily simplified by suppression of
metagenesis. At present, ten recent and one extinct family of
Calyptoblastea (Leptomedusae) may be recognized provisionally:
1. _Eucopidae_ (figs. 55, 59).--Trophosome with stalked hydrothecae;
gonosome, free medusae with otocysts and four, rarely six or eight,
unbranched radial canals. Two of the commonest British hydroids belong
to this family, _Obelia_ and _Clytia_. _Obelia_ forms numerous
polyserial stems of the characteristic zigzag pattern growing up from
a creeping basal stolon, and buds the medusa of the same name. In
_Clytia_ the polyps arise singly from the stolon, and the medusa is
known as _Phialidium_ (fig. 59).
2. _Aequoridae._--Trophosome only known in one genus (_Polycanna_),
and similar to the preceding; gonosome, free medusae with otocysts and
with at least eight radial canals, often a hundred or more, simple or
branched. _Aequorea_ is a common medusa.
3. _Thaumantidae._--Trophosome only known in one genus
(_Thaumantias_), similar to that of the _Eucopidae_; gonosome, free
medusae with otocysts inconspicuous or absent, with usually four,
sometimes eight, rarely more than eight, radial canals, simple and
unbranched, along which the gonads are developed, with numerous
tentacles bearing ocelli and with marginal sense-clubs. _Laodice_ and
_Thaumantias_ are representative genera.
4. _Berenicidae._--Trophosome unknown; gonosome, free medusae, with
four or six radial canals, bearing the gonads, with numerous
tentacles, between which occur sense-clubs, without otocysts.
_Berenice_, _Staurodiscus_, &c.
[Illustration: After Haeckel, _System der Medusen_, by permission of
Gustav Fischer.