important changes in their anatomical features during the period of the
ripening of the sexual cells. Thus in _Lar sabellarum_ the six radial
canals are simple in the first stage of development (A); but in the second
stage (B) each radial canal bifurcates before reaching the margin, and in
the adult stage shows a double bifurcation. The life-history has, however,
been worked out in very few of the Anthomedusae, and there can be little
doubt that as our knowledge grows several forms which are now known as
distinct species {267}will be found to be different stages of growth of the
same species.
[Illustration: FIG. 132.—Two stages in the development of the Medusa of
_Lar sabellarum_ (_Willsia stellata_). A, first stage with six canals
without branches; B, third stage with six canals each with two lateral
branches. The developing gonads may be seen on the manubrium in A. (After
Browne.)]
The movements of the Medusae are well described by Allman[301] in his
account of _Cladonema radiatum_:—"It is impossible to grow tired of
watching this beautiful medusa; sometimes while dashing through the water
with vigorous diastole and systole, it will all at once attach its grapples
to the side of the vessel, and become suddenly arrested in its career, and
then after a period of repose, during which its branched tentacles are
thrown back over its umbrella and extended into long filaments which float,
like some microscopic sea-weed in the water, it will once more free itself
from its moorings and start off with renewed energy." The Medusa of
_Clavatella_, "in its movements and mode of life, presents a marked
contrast to the medusiform zooid of other Hydrozoa. The latter is active
and mercurial, dancing gaily through the water by means of the vigorous
strokes of its crystalline swimming-bell. The former strides leisurely
along, or, using the adhesive discs as hands, climbs amongst the branches
of the weed. In the latter stage of its existence it becomes stationary,
fixing itself by means of its suckers; and {268}thus it remains, the
capitate arms standing out rigidly, like the rays of a starfish, until the
embryos are ready to escape."[302]
Among the Gymnoblastea there are many examples of a curious association of
the Hydroid with some other living animals. Thus _Hydractinia_ is very
often found on the shells carried by living Hermit crabs, _Dicoryne_ on the
shells of various Molluscs, _Tubularia_ has been found on a Cephalopod, and
_Ectopleura_ (a Corymorphid) on the carapace of a crab. There is but little
evidence, however, that in these cases the association is anything more
than accidental. The occurrence of the curious species, _Lar sabellarum_,
on the tubes of _Sabella_, of _Campaniclava cleodorae_ on the living shells
of the pelagic Mollusc _Cleodora cuspidata_, and of a _Gorgonia_ on the
tubes of _Tubularia parasitica_, appear to be cases in which there is some
mutual relationship between the two comrades. The genus _Stylactis_,
however, affords some of the most interesting examples of mutualism. Thus
_Stylactis vermicola_ is found only on the back of an _Aphrodite_ that
lives at the great depth of 2900 fathoms. _S. spongicola_ and _S.
abyssicola_ are found associated with certain deep-sea Horny Sponges. _S.
minoi_ is spread over the skin of the little rock perch _Minous inermis_,
which is found at depths of from 45 to 150 fathoms in the Indian seas.
In many cases it is difficult to understand what is the advantage of the
Hydroid to the animal that carries it, but in this last case Alcock[303]
suggests that the _Stylactis_ assists in giving the fish a deceitful
resemblance to the incrusted rocks of its environment, in order to allure,
or at any rate not to scare, its prey. Whether this is the real explanation
or not, the fact that in the Bay of Bengal and in the Laccadive and Malabar
seas the fish is never found without this Hydroid, nor the Hydroid without
this species of fish, suggests very strongly that there is a mutual
advantage in the association.
Cases of undoubted parasitism are very rare in this order. The remarkable
form _Hydrichthys mirus_,[304] supposed to be a Gymnoblastic Hydroid, but
of very uncertain position in the system, appears to be somewhat modified
in its structure by its parasitic habits on the fish _Seriola zonata_.
_Corydendrium {269}parasiticum_ is said to be a parasite living at the
expense of _Eudendrium racemosum_. _Mnestra_ is a little Medusa which
attaches itself by its manubrium to the Mollusc _Phyllirhoe_, and may
possibly feed upon the skin or secretions of its host.
Nearly all the species of the order are found in shallow sea water.
_Stylactis vermicola_ and the "Challenger" specimen of _Monocaulus
imperator_ occur at a depth of 2900 fathoms, and some species of the genera
_Eudendrium_ and _Myriothela_ descend in some localities to a depth of a
few hundred fathoms. _Cordylophora_ is the only genus known to occur in
fresh water. From its habit of attaching itself to wooden piers and
probably to the bottom of barges, and from its occurrence in navigable
rivers and canals, it has been suggested that _Cordylophora_ is but a
recent immigrant into our fresh-water system. It has been found in England
in the Victoria docks of London, in the Norfolk Broads, and in the
Bridgewater Canal. It has ascended the Seine in France, and may now be
found in the ponds of the Jardin des Plantes at Paris. It also occurs in
the Elbe and in some of the rivers of Denmark.
The classification of the Gymnoblastea is not yet on a satisfactory basis.
At present the hydrosome stage of some genera alone has been described, of
others the free-swimming Medusa only is known. Until the full life-history
of any one genus has been ascertained its position in the families
mentioned below may be regarded as only provisional. The principal families
are:—
FAM. BOUGAINVILLIIDAE.—The zooids of the hydrosome have a single circlet of
filiform tentacles at the base of the hypostome. In _Bougainvillia_
belonging to this family the gonophores are liberated in the form of
free-swimming Medusae formerly known by the generic name _Hippocrene_. In
the fully grown Medusa there are numerous tentacles arranged in clusters
opposite the terminations of the four radial canals. There are usually in
addition tentacular processes (labial tentacles) on the lips of the
manubrium. _Bougainvillia_ is a common British zoophyte of branching habit,
found in shallow water all round the coast. The medusome of _Bougainvillia
ramosa_ is said to be the common little medusa _Margelis ramosa_.[305] Like
most of the Hydroids it has a wide geographical distribution. Other genera
are _Perigonimus_, which has a Medusa with only two tentacles; and
{270}_Dicoryne_, which forms spreading colonies on Gasteropod shells and
has free gonophores provided with two simple tentacles, while the other
organs of the medusome are remarkably degenerate. In _Garveia_ and
_Eudendrium_ the gonophores are adelocodonic, in the former genus arising
from the body-wall of the axial zooids of the colony, and in the latter
from the hydrorhiza. _Stylactis_ is sometimes epizoic (p. 268). Among the
genera that are usually placed in this family, of which the medusome stage
only is known, are _Lizzia_ (a very common British Medusa) and _Rathkea_.
In _Margelopsis_ the hydrosome stage consists of a single free-swimming
zooid which produces Medusae by gemmation.
FAM. PODOCORYNIDAE.—The zooids have the same general features as those of
the Bougainvilliidae, but the perisarc does not extend beyond the
hydrorhiza.
In _Podocoryne_ and _Hydractinia_ belonging to this family the hydrorhiza
forms an encrusting stolon which is usually found on Gasteropod shells
containing a living Hermit crab. In _Podocoryne_ the gonophores are
free-swimming Medusae with a short manubrium provided with labial
tentacles. _Hydractinia_ differs from _Podocoryne_ in having polymorphic
zooids and adelocodonic gonophores.
A fossil encrusting a _Nassa_ shell from the Pliocene deposit of Italy has
been placed in the genus _Hydractinia_, and four species of the same genus
have been described from the Miocene and Upper Greensand deposits of this
country.[306] These are the only fossils known at present that can be
regarded as Gymnoblastic Hydroids.
The Medusa _Thamnostylus_, which has only two marginal tentacles and four
very long and profusely ramified labial tentacles, is placed in this
family. Its hydrosome stage is not known.
FAM. CLAVATELLIDAE.—This family contains the genus _Clavatella_, in which
the zooids of the hydrosome have a single circlet of capitate tentacles.
The gonophore is a free Medusa provided with six bifurcated capitate
tentacles.
FAM. CLADONEMIDAE.—This family contains the genus _Cladonema_, in which the
zooids have two circlets of four tentacles, the labial tentacles being
capitate and the aboral filiform. The gonophore is a free Medusa with eight
tentacles, each provided with a number of curious capitate tentacular
processes (Fig. 131).
{271}FAM. TUBULARIIDAE.—This important and cosmopolitan family is
represented in the British seas by several common species. The zooids of
the hydrosome of _Tubularia_ have two circlets of numerous filiform
tentacles. The gonophores are adelocodonic, and are situated on long
peduncles attached to the zooid on the upper side of the aboral circlet of
tentacles. The larva escapes from the gonophore and acquires two tentacles,
with which it beats the water and, assisted by the cilia, keeps itself
afloat for some time. In this stage it is known as an "Actinula."[307]
[Illustration: FIG. 133.—_Ceratella fusca._ About nat. size. (After Baldwin
Spencer.[308])]
FAM. CERATELLIDAE.—The colony of _Ceratella_ may be five inches in height.
The stem and main branches are substantial, and consist of a network of
branching anastomosing tubes supported by a thick and fenestrated chitinous
perisarc. The {272}whole branch is enclosed in a common layer of ectoderm.
The zooids have scattered capitate tentacles. The Ceratellidae occur in
shallow water off the coast of New South Wales, extend up the coast of East
Africa as far as Zanzibar, and have also been described from Japan.
FAM. PENNARIIDAE.—In the hydrosome stage the zooids have numerous oral
capitate tentacles scattered on the hypostome, and a single circlet of
basilar filiform tentacles. The medusa of _Pennaria_, a common genus of
wide distribution, is known under the name _Globiceps_.
FAM. CORYNIDAE.—In the hydrosome stage the zooids of this family possess
numerous capitate tentacles arranged in several circlets or scattered.
In _Cladocoryne_ the tentacles are branched. _Syncoryne_ is a common and
widely distributed genus with numerous unbranched capitate tentacles
irregularly distributed over a considerable length of the body-wall of the
zooid. In many of the species the gonophores are liberated as Medusae,
known by the name _Sarsia_, provided with four filiform tentacles and a
very long manubrium. In some species (_S. prolifera_ and _S. siphonophora_)
the Medusae are reproduced asexually by gemmation from the long manubrium.
A common British Anthomedusa of this family is _Dipurena_, but its
hydrosome stage is not known. In the closely related genus _Coryne_ the
gonophores are adelocodonic, and exhibit very rudimentary medusoid
characters.
FAM. CLAVIDAE.—This is a large family containing many genera, some with
free-swimming Medusae, others with adelocodonic gonophores. In the former
group are included a number of oceanic Medusae of which the hydrosome stage
has not yet been discovered. The zooids of the hydrosome have numerous
scattered filiform tentacles. The free-swimming Medusae have hollow
tentacles.
_Clava_ contains a common British species with a creeping hydrorhiza
frequently attached to shells, and with adelocodonic gonophores.
_Cordylophora_ is the genus which has migrated into fresh water in certain
European localities (see p. 269). It forms well-developed branching
colonies attached to wooden gates and piers or to the brickwork banks of
canals. Several Anthomedusae, of which the hydrosome stage is not known,
appear to be related to the Medusae of this family, but are sometimes
separated as {273}the family TIARIDAE. Of these _Tiara_, a very brightly
coloured jelly-fish sometimes attaining a height of 40 mm., is found on the
British coasts, and _Amphinema_ is found in considerable numbers at
Plymouth in September. _Turritopsis_ is a Medusa with a hydrosome stage
like _Dendroclava_. For _Stomatoca_, see p. 415.
FAM. CORYMORPHIDAE.—This family contains the interesting British species
_Corymorpha nutans_. The hydrosome stage consists of a solitary zooid of
great size, 50-75 mm. in length, provided with two circlets of numerous
long filiform tentacles. The free-swimming Medusae are produced in great
numbers on the region between the two circlets of tentacles. These Medusae
were formerly known by the name _Steenstrupia_, and are noteworthy in
having only one long moniliform tentacle, opposite to one of the radial
canals.
The gigantic _Monocaulus imperator_ of Allman was obtained by the
"Challenger" at the great depth of 2900 fathoms off the coast of Japan. It
was nearly eight feet in length. More recently Miyajima[309] has described
a specimen from 250 fathoms in the same seas which was 700 mm. (27.5 in.)
in length. Miyajima's specimen resembles those described by Mark from 300
fathoms off the Pacific coast of North America as _Branchiocerianthus
urceolus_ in the remarkable feature of a distinct bilateral arrangement of
the circlets of tentacles. Owing to the imperfect state of preservation of
the only specimen of Allman's species it is difficult to determine whether
it is also bilaterally symmetrical and belongs to the same species as the
specimens described by Mark and Miyajima. These deep-sea giant species,
however, appear to differ from _Corymorpha_ in having adelocodonic
gonophores.
FAM. HYDROLARIDAE.—This family contains the remarkable genus _Lar_, which
was discovered by Gosse attached to the margin of the tubes of the marine
Polychaete worm _Sabella_. The zooids have only two tentacles, and exhibit
during life curious bowing and bending movements which have been compared
with the exercises of a gymnast. The Medusae (Fig. 132, A and B) have been
known for a long time by the name _Willsia_, but their life-history has
only recently been worked out by Browne.[310]
{274}FAM. MONOBRACHIIDAE.—_Monobrachium_, found in the White Sea by
Mereschkowsky, forms a creeping stolon on the shells of _Tellina_. The
zooids of the hydrosome have only one tentacle.
FAM. MYRIOTHELIDAE.—This family contains the single genus _Myriothela_. The
zooid of the hydrosome stage is solitary and is provided, as in the
Corynidae, with numerous scattered capitate tentacles. The gonophores are
borne by blastostyles situated above the region of the tentacles. In
addition to these blastostyles producing gonophores there are, in _M.
phrygia_, supplementary blastostyles which capture the eggs as they escape
from the gonophores and hold them until the time when the larva is ready to
escape. They were called "claspers" by Allman. In some of the Arctic
species Frl. Bonnevie[311] has shown that they are absent. Each zooid of
_M. phrygia_ is hermaphrodite.
[Illustration: FIG. 134.—_Pelagohydra mirabilis._ _Fl_, The float; _M_,
position of the mouth; _Ten.Fl_, filamentous tentacles of the float. (After
Dendy.)]
FAM. PELAGOHYDRIDAE.—This family was constituted by Dendy[312] for the
reception of _Pelagohydra mirabilis_, a remarkable new species discovered
by him on the east coast of the South Island of New Zealand. The hydrosome
is solitary and free-swimming, the proximal portion of the body being
modified to form a float, the distal portion forming a flexible proboscis
terminated by the mouth and a group of scattered manubrial tentacles. The
tentacles are filiform and scattered over the surface of the float. Medusae
are developed on stolons between the tentacles of the float. They have
tentacles arranged in four radial groups of five each, at the margin of the
umbrella.
As pointed out by Hartlaub,[313] _Pelagohydra_ is not the only genus in
which the hydrosome floats. Three species of the genus _Margelopsis_ have
been found that have pelagic habits, and two {275}of them have been shown
to produce numerous free-swimming Medusae by gemmation; but at present
there is no reason to suppose that in these forms there is any extensive
modification of the aboral extremity of the zooid to form such a highly
specialised organ as the float of _Pelagohydra_.
The affinities of _Pelagohydra_ are not clear, as our knowledge of the
characters of the Medusa is imperfect; but according to Dendy it is most
closely related to the Corymorphidae. _Margelopsis_ belongs to the
Bougainvilliidae.
ORDER IV. CALYPTOBLASTEA—LEPTOMEDUSAE.
The hydrosome stage is characterised by the perisarc, which not only
envelops the stem and branches, as in many of the Gymnoblastea, but is
continued into a trumpet-shaped or tubular cup or collar called the
"hydrotheca," that usually affords an efficient protection for the zooids
when retracted. No solitary Calyptoblastea have been discovered. In the
simpler forms the colony consists of a creeping hydrorhiza, from which the
zooids arise singly (_Clytia johnstoni_), but these zooids may give rise to
a lateral bud which grows longer than the parent zooid.
[Illustration: FIG. 135.—Part of a hydrocladium of a dried specimen of
_Plumularia profunda_. _Gt_, Gonotheca; _Hc_, the stem of the hydrocladium
with joints (_j_); _Ht_, a single hydrotheca; _N_, nematophores. Greatly
enlarged. (After Nutting.)]
The larger colonies are usually formed by alternate right and left budding
from the last-formed zooid, so that in contrast to the Gymnoblast colony
the apical zooid of the stem is the youngest, and not the oldest, zooid of
the colony. In the branching colonies the axis is frequently composed of a
single tube of perisarc, which may be lined internally by the ectoderm and
endoderm tissues formed by the succession of zooids that have given rise to
the branches by gemmation. Such a stem is said to be monosiphonic.
{276}In some of the more complicated colonies, however, the stem is
composed of several tubes, which may or may not be surrounded by a common
sheath of ectoderm and perisarc, as they are in _Ceratella_ among the
Gymnoblastea. Such stems are said to be "polysiphonic" or "fascicled." The
polysiphonic stem may arise in more than one way, and in some cases it is
not quite clear in what manner it has arisen.[314]
In many colonies the zooids are only borne by the terminal monosiphonic
branches, which receive the special name "hydrocladia." The gonophores of
the Calyptoblastea are usually borne by rudimentary zooids, devoid of mouth
and tentacles (the "blastostyles"), protected by a specially dilated cup of
perisarc known as the "gonotheca" or "gonangium." The shape and size of the
gonothecae vary a good deal in the order. They may be simply oval in shape,
or globular (_Schizotricha dichotoma_), or greatly elongated, with the
distal ends produced into slender necks (_Plumularia setacea_). They are
spinulose in _P. echinulata_, and annulated in _P. halecioides_, _Clytia_,
etc.
In some genera there are special modifications of the branches and
hydrocladia, for the protection of the gonothecae. The name "Phylactocarp"
is used to designate structures that are obviously intended to serve this
purpose. The phylactocarp of the genera _Aglaophenia_ and _Thecocarpus_ is
the largest and most remarkable of this group of structures, and has
received the special name "corbula." The corbula consists of an axial stem
or rachis, and of a number of corbula-leaves arising alternately from the
rachis, bending upwards and then inwards to meet those of the other side
above, the whole forming a pod-shaped receptacle. The gonangia are borne at
the base of each of the corbula-leaves. There is some difference of opinion
as to the homologies of the parts of the corbula, but the rachis seems to
be that of a modified hydrocladium, as it usually bears at its base one or
more hydrothecae of the normal type. The corbula-leaves are usually
described as modified nematophores (_vide infra_), but according to
Nutting[315] there is no more reason to regard them as modified
nematophores than as modified hydrothecae, and he regards them as "simply
the modification of a structure originally intended to {277}protect an
indefinite person, an individual that may become either a sarcostyle[316]
or a hydranth."
The other forms of phylactocarps are modified branches as in _Lytocarpus_,
and those which are morphologically appendages to branches as in
_Cladocarpus_, _Aglaophenopsis_, and _Streptocaulus_.
The structures known as "nematophores" in the Calyptoblastea are the thecae
of modified zooids, comparable with the dactylozooids of _Millepora_. They
form a well-marked character of the very large family Plumulariidae, but
they are also found in species of the genera _Ophiodes_, _Lafoëina_,
_Oplorhiza_, _Perisiphonia_, _Diplocyathus_, _Halecium_, and _Clathrozoon_
among the other Calyptoblastea. The dactylozooids are usually capitate or
filiform zooids, without tentacles or a mouth, and with a solid or
occasionally a perforated core of endoderm. They bear either a battery of
nematocysts (_Plumularia_, etc.), or of peculiar adhesive cells
(_Aglaophenia_ and some species of _Plumularia_). The functions of the
dactylozooids are to capture the prey and to serve as a defence to the
colony. In the growth of the corbula of _Aglaophenia_ the dactylozooids
appear to serve another purpose, and that is, as a temporary attachment to
hold the leaves together while the edges themselves are being connected by
trabeculae of coenosarc.
In a very large number of Calyptoblastea the gonophore is a reduced Medusa
which never escapes from the gonotheca, but in the family Eucopidae the
gonophores escape as free-swimming Medusae, exhibiting certain very
definite characters. The gonads are situated not on the manubrium, as in
the Anthomedusae, but on the sub-umbrellar aspect of the radial canals. The
marginal sense-organs may be ocelli or vesiculate statocysts. The bell is
usually more flattened, and the velum smaller than it is in the
Anthomedusae, and the manubrium short and quadrangular. Such Medusae are
called Leptomedusae.
Leptomedusae of many specific forms are found abundantly at the surface of
the sea in nearly all parts of the world, but with the exception of some
genera of the Eucopidae and a few others, their connexion with a definite
Calyptoblastic hydrosome has not been definitely ascertained. It may be an
assumption that time will prove to be unwarranted that all the Leptomedusae
pass through a Calyptoblastic hydrosome stage.
{278}FAM. AEQUOREIDAE.—In this family the hydrosome stage is not known
except in the genus _Polycanna_, in which it resembles a Campanulariid. The
sense-organs of the Medusae are statocysts. The radial canals are very
numerous, and the genital glands are in the form of ropes of cells
extending along the whole of their oral surfaces. _Aequorea_ is a fairly
common genus, with a flattened umbrella and a very rudimentary manubrium,
which may attain a size of 40 mm. in diameter.
FAM. THAUMANTIIDAE.—The Medusae of this family are distinguished from the
Aequoreidae by having marginal ocelli in place of statocysts. The hydrosome
of _Thaumantias_ alone is known, and this is very similar to an _Obelia_.
FAM. CANNOTIDAE.—The hydrosome is quite unknown. The Medusae are ocellate,
but the radial canals, instead of being undivided, as in the Thaumantiidae,
are four in number, and very much ramified before reaching the ring canal.
The tentacles are very numerous. In the genus _Polyorchis_, from the
Pacific coast of North America, the four radial canals give rise to
numerous lateral short blind branches, and have therefore a remarkable
pinnate appearance.
FAM. SERTULARIIDAE.—In this family the hydrothecae are sessile, and
arranged bilaterally on the stem and branches. The general form of the
colony is pinnate, the branches being usually on opposite sides of the main
stem. The gonophores are adelocodonic. _Sertularia_ forms more or less
arborescent colonies, springing from a creeping stolon attached to stones
and shells. There are many species, several of which are very common upon
the British coast. Many specimens are torn from their attachments by storms
or by the trawls of fishermen and cast up on the sand or beach with other
zoophytes. The popular name for one of the commonest species (_S.
abietina_) is the "sea-fir." The genus has a wide geographical and
bathymetrical range. Another common British species frequently thrown up by
the tide in great quantities is _Hydrallmania falcata_. It has slender
spirally-twisted stems and branches, and the hydrothecae are arranged
unilaterally.
The genus _Grammaria_, sometimes placed in a separate family, is
distinguished from _Sertularia_ by several characters. The stem and
branches are composed of a number of tubes which are considerably
compressed. The genus is confined to the southern seas.
{279}FAM. PLUMULARIIDAE.—The hydrothecae are sessile, and arranged in a
single row on the stem and branches. Nematophores are always present.
Gonophores adelocodonic. This family is the largest and most widely
distributed of all the families of the Hydrozoa. Nutting calculates that it
contains more than one-fourth of all the Hydroids of the world. Over 300
species have been described, and more than half of these are found in the
West Indian and Australian regions. Representatives of the family occur in
abundance in depths down to 300 fathoms, and not unfrequently to 500
fathoms. Only a few species have occasionally been found in depths of over
1000 fathoms.
The presence of nematophores may be taken as the most characteristic
feature of the family, but similar structures are also found in some
species belonging to other families (p. 277).
The family is divided into two groups of genera, the ELEUTHEROPLEA and the
STATOPLEA. In the former the nematophores are mounted on a slender pedicel,
which admits of more or less movement, and in the latter the nematophores
are sessile. The genera _Plumularia_ and _Antennularia_ belong to the
Eleutheroplea. The former is a very large genus, with several common
British species, distinguished by the terminal branches being pinnately
disposed, and the latter, represented by _A. antennina_ and _A. ramosa_ on
the British coast, is distinguished by the terminal branches being arranged
in verticils.
The two most important genera of the Statoplea are _Aglaophenia_ and
_Cladocarpus_. The former is represented by a few species in European
waters, the latter is only found in American waters.
FAM. HYDROCERATINIDAE.—The colony consists of a mass of entwined
hydrorhiza, with a skeleton in the form of anastomosing chitinous tubes.
Hydrothecae scattered, tubular, and sessile. Nematophores present.
Gonophores probably adelocodonic.
This family was constituted for a remarkable hydroid, _Clathrozoon
wilsoni_, described by W. B. Spencer from Victoria.[317] The zooids are
sessile, and spring from more than one of the numerous anastomosing tubes
of the stem and branches. The whole of the surface is studded with an
enormous number of small and very simple dactylozooids, protected by
tubular nematophores. Only {280}a few specimens have hitherto been
obtained, the largest being 10 inches in height by 4 inches in width. In
general appearance it has some resemblance to a dark coloured fan-shaped
_Gorgonia_.
FAM. CAMPANULARIIDAE.—The hydrothecae in this family are pedunculate, and
the gonophores adelocodonic.
In the cosmopolitan genus _Campanularia_ the stem is monosiphonic, and the
hydrothecae bell-shaped. Several species of this genus are very common in
the rock pools of our coast between tide marks. _Halecium_ is characterised
by the rudimentary character of its hydrothecae, which are incapable of
receiving the zooids even in their maximum condition of retraction. The
genus _Lafoea_ is remarkable for the development of a large number of
tightly packed gonothecae on the hydrorhiza, each of which contains a
blastostyle, bearing a single gonophore and, in the female, a single ovum.
This group of gonothecae was regarded as a distinct genus of Hydroids, and
was named _Coppinia_.[318] _Lafoea dumosa_ with gonothecae of the type
described as _Coppinia arcta_ occurs on the British coast.
_Perisiphonia_ is an interesting genus from deep water off the Azores,
Australia, and New Zealand, with a stem composed of many distinct tubes.
The genus _Zygophylax_, from 500 fathoms off the Cape Verde, is of
considerable interest in having a nematophore on each side of the
hydrotheca. According to Quelch it should be placed in a distinct family.
_Ophiodes_ has long and very active defensive zooids, protected by
nematophores. It is found in the Laminarian zone on the English coast.
FAM. EUCOPIDAE.—The hydrosome stage of this family is very similar to that
of the Campanulariidae, but the gonophores are free-swimming Medusae of the
Leptomedusan type.
One of the best-known genera is _Obelia_, of which several species are
among the commonest Hydroids of the British coast.
_Clytia johnstoni_ is also a very common Hydroid, growing on red algae or
leaves of the weed _Zostera_. It consists of a number of upright, simple,
or slightly branched stems springing from a creeping hydrorhiza. When
liberated the Medusae are globular in form, with four radial canals and
four marginal tentacles, but {281}this Medusa, like many others of the
order, undergoes considerable changes in form before it reaches the
sexually mature stage.
_Phialidium temporarium_ is one of the commonest Medusae of our coast, and
sometimes occurs in shoals. It seems probable that it is the Medusa of
_Clytia johnstoni_.[319] By some authors the jelly-fish known as
_Epenthesis_ is also believed to be the Medusa of a _Clytia_.
FAM. DENDROGRAPTIDAE.—This family includes a number of fossils which have
certain distinct affinities with the Calyptoblastea. In _Dictyonema_,
common in the Ordovician rocks of Norway, but also found in the Palaeozoic
rocks of North America and elsewhere, the fossil forms fan-shaped colonies
of delicate filaments, united by many transverse commissures, and in
well-preserved specimens the terminal branches bear well-marked uniserial
hydrothecae. In some species thecae of a different character, which have
been interpreted to be gonothecae and nematophores respectively, are found.
Other genera are _Dendrograptus_, _Thamnograptus_, and several others from
Silurian strata.
ORDER V. GRAPTOLITOIDEA.
A large number of fossils, usually called Graptolites, occurring in
Palaeozoic strata, are generally regarded as the skeletal remains of an
ancient group of Hydrozoa.
In the simpler forms the fossil consists of a delicate straight rod bearing
on one side a series of small cups. It is suggested that the cups contained
hydroid zooids, and should therefore be regarded as the equivalent of the
hydrothecae, and that the axis represents the axis of the colony or of a
branch of the Calyptoblastea. In some of the forms with two rows of cups on
the axis (_Diplograptus_), however, it has been shown that the cups are
absent from a considerable portion of one end of the axis, and that the
axes of several radially arranged individuals are fused together and united
to a central circular plate. Moreover, there is found in many specimens a
series of vesicles, a little larger in size than the cups, attached to the
plate and arranged in a circle at the base of the axes. These vesicles are
called the gonothecae.
The discovery of the central plate and of the so-called {282}gonothecae
suggests that the usual comparison of a Graptolite with a Sertularian
Hydroid is erroneous, and that the colony or individual, when alive, was a
more or less radially symmetrical floating form, like a Medusa, of which
only the distal appendages (possibly tentacles) are commonly preserved as
fossils.
The evidence that the Graptolites were Hydrozoa is in reality very slight,
but the proof of their relationship to any other phylum of the animal
kingdom does not exist.[320] It is therefore convenient to consider them in
this place, and to regard them, provisionally, as related to the
Calyptoblastea.
The order is divided into three families.
FAM. 1. MONOPRIONIDAE.—Cups arranged uniserially on one side of the axis.
The principal genera are _Monograptus_, with the axis straight, curved, or
helicoid, from many horizons in the Silurian strata; _Rastrites_, with a
spirally coiled axis, Silurian; _Didymograptus_, Ordovician; and
_Coenograptus_, Ordovician.
FAM. 2. DIPRIONIDAE.—Cups arranged in two or four vertical rows on the
axis.
_Diplograptus_, Ordovician and Silurian; _Climacograptus_, Ordovician and
Silurian; and _Phyllograptus_, in which the axis and cups are arranged in
such a manner that they resemble an ovate leaf.
FAM. 3. RETIOLITIDAE.—Cups arranged biserially on a reticulate axis.
_Retiolites_, Ordovician and Silurian; _Stomatograptus_, _Retiograptus_,
and _Glossograptus_, Ordovician.
FOSSIL CORALS POSSIBLY ALLIED TO HYDROZOA.
Among the many fossil corals that are usually classified with the Hydrozoa
the genus _Porosphaera_ is of interest as it is often supposed to be
related to _Millepora_. It consists of globular masses about 10-20 mm. in
diameter occurring in the Upper Cretaceous strata. In the centre there is
usually a foreign body around which the coral was formed by concentric
encrusting growth. Running radially from pores on the surface to the
centre, there are numerous tubules which have a certain general resemblance
to the pore-tubes of _Millepora_. The monomorphic {283}character of these
tubes, their very minute size, the absence of ampullae, and the general
texture of the corallum, are characters which separate this fossil very
distinctly from any recent Hydroid corals. _Porosphaera_, therefore, was
probably not a Hydrozoon, and certainly not related to the recent
_Millepora_.
Closely related to _Porosphaera_ apparently are other globular,
ellipsoidal, or fusiform corals from various strata, such as _Loftusia_
from the Eocene of Persia, _Parkeria_ from the Cambridge Greensand, and
_Heterastridium_ from the Alpine Trias. In the last named there is
apparently a dimorphism of the radial tubes.
Allied to these genera, again, but occurring in the form of thick,
concentric, calcareous lamellae, are the genera _Ellipsactinia_ and
_Sphaeractinia_ from the Upper Jurassic.
Another important series of fossil corals is that of the family
STROMATOPORIDAE. These fossils are found in great beds of immense extent in
many of the Palaeozoic rocks, and must have played an important part in the
geological processes of that period. They consist of a series of calcareous
lamellae, separated by considerable intervals, encrusting foreign bodies of
various kinds. Sometimes they are flat and plate-like, sometimes globular
or nodular in form. The lamellae are in some cases perforated by tabulate,
vertical, or radial pores, but in many others these pores are absent. The
zoological position of the Stromatoporidae is very uncertain, but there is
not at present any very conclusive evidence that they are Hydrozoa.
_Stromatopora_ is common in Devonian and also occurs in Silurian strata.
_Cannopora_ from the Devonian has well-marked tabulate pores, and is often
found associated commensally with another coral (_Aulopora_ or
_Syringopora_).
ORDER VI. STYLASTERINA.
The genera included in this order resemble _Millepora_ in producing a
massive calcareous skeleton, and in showing a consistent dimorphism of the
zooids, but in many respects they exhibit great divergence from the
characters of the Milleporina.
The colony is arborescent in growth, the branches arising frequently only
in one plane, forming a flabellum. The calcareous skeleton is perforated to
a considerable depth by the gastrozooids, dactylozooids, and nutritive
canals, and the {284}gastropores and dactylopores are not provided with
tabulae except in the genera _Pliobothrus_ and _Sporadopora_. The character
which gives the order its name is a conical, sometimes torch-like
projection at the base of the gastropore, called the "style," which carries
a fold of the ectoderm and endoderm layers of the body-wall, and may serve
to increase the absorptive surface of the digestive cavity. In some genera
a style is also present in the dactylopore, in which case it serves as an
additional surface for the attachment of the retractor muscles. The pores
are scattered on all aspects of the coral in the genera _Sporadopora_,
_Errina_, and _Pliobothrus_; in _Spinipora_ and _Steganopora_ the scattered
dactylopores are situated at the extremities of tubular spines which
project from the general surface of the coral, the gastropores being
situated irregularly between the spines. In _Phalangopora_ the pores are
arranged in regular longitudinal lines, and in _Distichopora_ they are
mainly in rows on the edges of the flattened branches, a single row of
gastropores being flanked by a single row of dactylopores on each side. In
the remaining genera the pores are arranged in definite cycles, which are
frequently separated from one another by considerable intervals, and have,
particularly in the dried skeleton, a certain resemblance to the calices of
some of the Zoantharian corals.
In _Cryptohelia_ the cycles are covered by a lid-like projection from the
neighbouring coenenchym (Fig. 136, _l_ 1, _l_ 2). The gastrozooids are
short, and are usually provided with a variable number of small capitate
tentacles. The dactylozooids are filiform and devoid of tentacles, the
endoderm of their axes being solid and scalariform.
The gonophores of the Stylasterina are situated in large oval or spherical
cavities called the ampullae, and their presence can generally be detected
by the dome-shaped projections they form on the surface of the coral. The
female gonophore consists of a saucer-shaped pad of folded endoderm called
the "trophodisc," which serves the purpose of nourishing the single large
yolk-laden egg it bears; and a thin enveloping membrane composed of at
least two layers of cells. The egg is fertilised while it is still within
the ampulla, and does not escape to the exterior until it has reached the
stage of a solid ciliated larva. All the Stylasterina are therefore
viviparous. The male gonophore has a very much smaller trophodisc, which is
sometimes (_Allopora_) prolonged into a columnar process or spadix,
penetrating the {285}greater part of the gonad. The spermatozoa escape
through a peculiar spout-like duct which perforates the superficial wall of
the ampulla. In some genera (_Distichopora_) there are several male
gonophores in each ampulla.
The gonophores of the Stylasterina have been regarded as much altered
medusiform gonophores, and this view may possibly prove to be correct. At
present, however, the evidence of their derivation from Medusae is not
conclusive, and it is possible that they may have had a totally independent
origin.
_Distichopora_ and some species of _Stylaster_ are found in shallow water
in the tropics, but most of the genera are confined to deep or very deep
water, and have a wide geographical distribution. No species have been
found hitherto within the British area.
[Illustration: FIG. 136.—A portion of a branch of _Cryptohelia ramosa_,
showing the lids _l_ 1 and _l_ 2 covering the cyclosystems, the swellings
produced by the ampullae in the lids _amp_^1, _amp_^2, and the
dactylozooids, _dac._ × 22. (After Hickson and England.)]
A few specimens of a species of _Stylaster_ have been found in Tertiary
deposits and in some raised beaches of more recent origin, but the order is
not represented in the older strata.
FAM. STYLASTERIDAE.—All the genera at present known are included in this
family.
_Sporadopora_ is the only genus that presents a superficial general
resemblance to _Millepora_. It forms massive, branching white coralla, with
the pores scattered irregularly on the surface, and, like many varieties of
_Millepora_, not arranged in cyclosystems. It may, however, be
distinguished at once by the presence of a long, brush-like style in each
of the gastropores. The ampullae are large, but are usually so deep-seated
in the coenenchym that their presence cannot be detected from the surface.
It was found off the Rio de la Plata in 600 fathoms of water by the
"Challenger."
{286}In _Errina_ the pores are sometimes irregularly scattered, but in _E.
glabra_ they are arranged in rows on the sides of the branches, while in
_E. ramosa_ the gastropores occur at the angles of the branches only. The
dactylopores are situated on nariform projections of the corallum. The
ampullae are prominent. There are several gonophores in each ampulla of the
male, but only one in each ampulla of the female. This genus is very widely
distributed in water from 100 to 500 fathoms in depth.
_Phalangopora_ differs from _Errina_ in the absence of a style in the
gastropore; Mauritius.—_Pliobothrus_ has also no style in the gastropore,
and is found in 100-600 fathoms of water off the American Atlantic shores.
_Distichopora_ is an important genus, which is found in nearly all the
shallow seas of the tropical and semi-tropical parts of the world, and may
even flourish in rock pools between tide marks. It is nearly always
brightly coloured—purple, violet, pale brown, or rose red. The colony
usually forms a small flabellum, with anastomosing branches, and the pores
are arranged in three rows, a middle row of gastropores and two lateral
rows of dactylopores on the sides of the branches. There is a long style in
each gastropore. The ampullae are numerous and prominent, situated on the
anterior and posterior faces of the branches. Each ampulla contains a
single gonophore in the female colony and two or three gonophores in the
male colony.
_Spinipora_ is a rare genus from off the Rio de la Plata in 600 fathoms.
The branches are covered with blunt spines. These spines have a short
gutter-like groove at the apex, which leads into a dactylopore. The
gastropores are provided with a style and are situated between the spines.
_Steganopora_[321] from the Djilolo Passage, in about 600 fathoms, is very
similar to _Spinipora_ as regards external features, but differs from it in
the absence of styles in the gastropores, and in the wide communications
between the gastropores and dactylopores.
_Stylaster_ is the largest and most widely distributed genus of the family,
and exhibits a considerable range of structure in the many species it
contains. It is found in all the warmer seas of the world, living between
tide marks at a few fathoms, and extending to depths of 600 fathoms. Many
specimens, but especially those from very shallow water, are of a beautiful
rose {287}or pink colour. The corallum is arborescent and usually
flabelliform. The pores are distributed in regular cyclosystems, sometimes
on one face of the corallum only, sometimes on the sides of the branches,
and sometimes evenly distributed. There are styles in both gastropores and
dactylopores.
_Allopora_ is difficult to separate from _Stylaster_, but the species are
usually more robust in habit, and the ampullae are not so prominent as they
are on the more delicate branches of _Stylaster_. It occurs at depths of
100 fathoms in the Norwegian fjords. A very large red species (_A.
nobilis_) occurs in False Bay, Cape of Good Hope, in 30 fathoms of water.
In this locality the coral occurs in great submarine beds or forests, and
the trawl that is passed over them is torn to pieces by the hard, thick
branches, some of which are an inch or more in diameter.
_Astylus_ is a genus found in the southern Philippine sea in 500 fathoms of
water. It is distinguished from _Stylaster_ by the absence of a style in
the gastropore.
_Cryptohelia_ is an interesting genus found both in the Atlantic and
Pacific Oceans at depths of from 270 to about 600 fathoms. The cyclosystems
are covered by a projecting lid or operculum (Fig. 136, _l_ 1, _l_ 2).
There are no styles in either the gastropores or the dactylopores. The
ampullae are prominent, and are sometimes situated in the lids. There are
several gonophores in each ampulla of the female colony, and a great many
in the ampulla of the male colony.
{288}CHAPTER XI
HYDROZOA (_CONTINUED_): TRACHOMEDUSAE—NARCOMEDUSAE—SIPHONOPHORA
ORDER VII. TRACHOMEDUSAE.
The orders Trachomedusae and Narcomedusae are probably closely related to
one another and to some of the families of Medusae at present included in
the order Calyptoblastea, and it seems probable that when the
life-histories of a few more genera are made known the three orders will be
united into one. Very little is known of the hydrosome stage of the
Trachomedusae, but Brooks[322] has shown that in _Liriope_, and
Murbach[323] that in _Gonionema_, the fertilised ovum gives rise to a
_Hydra_-like form, and in the latter this exhibits a process of
reproduction by gemmation before it gives rise to Medusae. Any general
statement, therefore, to the effect that the development of the
Trachomedusae is direct would be incorrect. The fact that the hydrosomes
already known are epizoic or free-swimming does not afford a character of
importance for distinction from the Leptomedusae, for it is quite possible
that in this order of Medusae the hydrosomes of many genera may be similar
in form and habits to those of _Liriope_ and _Gonionema_.
The free border of the umbrella of the Trachomedusae is entire; that is to
say, it is not lobed or fringed as it is in the Narcomedusae. The
sense-organs are statocysts, each consisting of a vesicle formed by a more
or less complete fold of the surrounding wall of the margin of the
umbrella, containing a reduced clapper-like tentacle loaded at its
extremity with a statolith.
{289}[Illustration: FIG. 137.—_Liriope rosacea_, one of the Geryoniidae,
from the west side of North and Central America. Size, 15-20 mm. Colour,
rose. _cp_, Centripetal canal; _gon_, gonad; _M_, mouth at the end of a
long manubrium; _ot_, statocyst; _t_, tentacle; _to_, tongue. (After
Maas.)]
This statocyst is innervated by the outer nerve ring. There appears to be a
very marked difference between these marginal sense-organs in some of the
best-known examples of Trachomedusae and the corresponding organs of the
Leptomedusae. The absence of a stalk supporting the statolith and the
innervation of the otocyst by the inner instead of by the outer nerve ring
in the Leptomedusae form characters that may be of supplementary value, but
cannot be regarded as absolutely distinguishing the two orders. The
statorhab of the Trachomedusae is probably the more primitive of the two
types, and represents a marginal tentacle of the umbrella reduced in size,
loaded with a statolith and enclosed by the mesogloea. Intermediate stages
between this type and an ordinary tentacle have already been discovered and
described. In the type that is usually found in the Leptomedusae the
modified tentacle is still further reduced, and all that can be recognised
of it is the statolith attached to the wall of the statocyst, but
intermediate stages between the two types are seen in the family
Olindiidae, in which the stalk supporting the statolith passes gradually
into the tissue surrounding the statolith on the one hand and the vesicle
wall on the other. The radial canals are four or eight in number or more
numerous. They communicate at the margin of the umbrella with a ring canal
from which a number of short blind tubes run in the umbrella-wall towards
the centre of the Medusa (Fig. 137, _cp_). These "centripetal canals" are
subject to {290}considerable variation, but are useful characters in
distinguishing the Trachomedusae from the Leptomedusae. The tentacles are
situated on the margin of the umbrella, and are four or eight in number or,
in some cases, more numerous. The gonads are situated as in Leptomedusae on
the sub-umbrella aspect of the radial canals.
In _Gonionema murbachii_ the fertilised eggs give rise to a free-swimming
ciliated larva of an oval shape with one pole longer and narrower than the
other. The mouth appears subsequently at the narrower pole. The larva
settles down upon the broader pole, the mouth appears at the free
extremity, and in a few days two, and later two more, tentacles are formed
(Fig. 138).
At this stage the larva may be said to be _Hydra_-like in character, and as
shown in Fig. 138 it feeds and lives an independent existence. From its
body-wall buds arise which separate from the parent and give rise to
similar _Hydra_-like individuals. An asexual generation thus gives rise to
new individuals by gemmation as in the hydrosome of the Calyptoblastea. The
origin of the Medusae from this _Hydra_-like stage has not been
satisfactorily determined, but it seems probable that by a process of
metamorphosis the hydriform persons are directly changed into the
Medusae.[324]
[Illustration: FIG. 138.—Hydra-like stage in the development of _Gonionema
murbachii_. One of the tentacles is carrying a worm (_W_) to the mouth. The
tentacles are shown very much contracted, but they are capable of extending
to a length of 2 mm. Height of zooid about 1 mm. (After Perkins.)]
In the development of _Liriope_ the free-swimming larva develops into a
hydriform person with four tentacles and an enormously elongated hypostome
or manubrium; and, according to Brooks, it undergoes a metamorphosis which
directly converts it into a Medusa.
There can be very little doubt that in a large number of Trachomedusae the
development is direct, the fertilised ovum giving rise to a medusome
without the intervention of a hydrosome stage. In some cases, however
(_Geryonia_, etc.), the tentacles {291}appear in development before there
is any trace of a sub-umbrella cavity, and this has been interpreted to be
a transitory but definite Hydroid stage. It may be supposed that the
elimination of the hydrosome stage in these Coelenterates may be associated
with their adaptation to a life in the ocean far from the coast.
During the growth of the Medusa from the younger to the adult stages
several changes probably occur of a not unimportant character, and it may
prove that several genera now placed in the same or even different families
are stages in the development, of the same species. In the development of
_Liriantha appendiculata_,[325] for example, four interradial tentacles
appear in the first stage which disappear and are replaced by four radial
tentacles in the second stage.
As with many other groups of free-swimming marine animals the Trachomedusae
have a very wide geographical distribution, and some genera may prove to be
almost cosmopolitan, but the majority of the species appear to be
characteristic of the warmer regions of the high seas. Sometimes they are
found at the surface, but more usually they swim at a depth of a few
fathoms to a hundred or more from the surface. The Pectyllidae appear to be
confined to the bottom of the sea at great depths.
The principal families of the Trachomedusae are:—
FAM. OLINDIIDAE.—This family appears to be structurally and in development
most closely related to the Leptomedusae, and is indeed regarded by
Goto[326] as closely related to the Eucopidae in that order. They have two
sets of tentacles, velar and exumbrellar; the statocysts are numerous, two
on each side of the exumbrellar tentacles. Radial canals four or six.
Manubrium well developed and quadrate, with distinct lips. There is an
adhesive disc on each exumbrellar tentacle.
Genera: _Olindias_, _Olindioides_, _Gonionema_ (Fig. 139), and _Halicalyx_.
As in other families of Medusae the distribution of the genera is very
wide. _Olindias mülleri_ occurs in the Mediterranean, _Olindioides formosa_
off the coast of Japan, _Gonionema murbachii_ is found in abundance in the
eel pond at Wood's Holl, United States of America, and _Halicalyx_ off
Florida.
Two genera may be referred to in this place, although their {292}systematic
position in relation to each other and to other Medusae has not been
satisfactorily determined.
[Illustration: FIG. 139.—_Gonionema murbachii._ Adult Medusa, shown
inverted, and clinging to the bottom. Nat. size. (After Perkins.)]
_Limnocodium sowerbyi_ is a small Medusa that was first discovered in the
_Victoria regia_ tanks in the Botanic Gardens, Regent's Park, London, in
the year 1880. It has lately made its appearance in the _Victoria regia_
tank in the Parc de la Bête d'Or at Lyons.[327] As it was, at the time of
its discovery, the only fresh-water jelly-fish known, it excited
considerable interest, and this interest was not diminished when the
peculiarities of its structure were described by Lankester and others. It
has a rather flattened umbrella, with entire margin and numerous marginal
tentacles, the manubrium is long, quadrate, and has four distinct lips.
There are four radial canals, and the male gonads (all the specimens
discovered were of the male sex) are sac-like bodies on the sub-umbrellar
aspect of the middle points of the four radial canals. In these characters
the genus shows general affinities with the Olindiidae. The difficult
question of the origin of the statoliths from the primary germ layers of
the embryo and some other points in the minute anatomy of the Medusa have
{293}suggested the view that _Limnocodium_ is not properly placed in any of
the other orders. Goto,[328] however, in a recent paper, confirms the view
of the affinities of _Limnocodium_ with the Olindiidae.
The life-history of _Limnocodium_ is not known, but a curious Hydroid form
attached to _Pontederia_ roots was found in the same tank as the Medusae,
and this in all probability represents the hydrosome stage of its
development. The Medusae are formed apparently by a process of transverse
fission of the Hydroid stock[329] similar in some respects to that observed
in the production of certain Acraspedote Medusae. This is quite unlike the
asexual mode of formation of Medusae in any other Craspedote form. The
structure of this hydrosome is, moreover, very different to that of any
other Hydroid, and consequently the relations of the genus with the
Trachomedusae cannot be regarded as very close.
_Limnocodium_ has only been found in the somewhat artificial conditions of
the tanks in botanical gardens, and its native locality is not known, but
its association with the _Victoria regia_ water-lily seems to indicate that
its home is in tropical South America.
_Limnocnida tanganyicae_ is another remarkable fresh-water Medusa, about
seven-eights of an inch in diameter, found in the lakes Tanganyika and
Victoria Nyanza of Central Africa.[330] It differs from _Limnocodium_ in
having a short collar-like manubrium with a large round mouth two-thirds
the diameter of the umbrella, and in several other not unimportant
particulars. It produces in May and June a large number of Medusa-buds by
gemmation on the manubrium, and in August and September the sexual organs
are formed in the same situation.
[Illustration: FIG. 140.—_Limnocnida tanganyicae._ × 2. (After Günther.)]
The fixed hydrosome stage, if such a stage occurs in the life-history, has
not been discovered; but Mr. Moore[331] believes that {294}the development
is direct from ciliated planulae to the Medusae. The occurrence of
_Limnocnida_ in Lake Tanganyika is supposed by the same authority to afford
a strong support to the view that this lake represents the remnants of a
sea which in Jurassic times spread over part of the African continent. This
theory has, however, been adversely criticised from several sides.[332]
The character of the manubrium and the position of the sexual cells suggest
that _Limnocnida_ has affinities with the Narcomedusae or Anthomedusae, but
the marginal sense-organs and the number and position of the tentacles,
showing considerable similarity with those of _Limnocodium_, justify the
more convenient plan of placing the two genera in the same family.
FAM. PETASIDAE.—The genus _Petasus_ is a small Medusa with four radial
canals, four gonads, four tentacles, and four free marginal statorhabs. A
few other genera associated with _Petasus_ show simple characters as
regards the canals and the marginal organs, but as very little is known of
any of the genera the family may be regarded as provisional only. _Petasus_
is found in the Mediterranean and off the Canaries.
FAM. TRACHYNEMIDAE.—In this family there are eight radial canals, and the
statorhabs are sunk into a marginal vesicle. _Trachynema_, characterised by
its very long manubrium, is a not uncommon Medusa of the Mediterranean and
the eastern Atlantic Ocean. Many of the species are small, but _T.
funerarium_ has sometimes a disc two inches in diameter. _Homoconema_ and
_Pentachogon_ have numerous very short tentacles.
FAM. PECTYLLIDAE.—This family contains a few deep-sea species with
characters similar to those of the preceding family, but the tentacles are
provided with terminal suckers. _Pectyllis_ is found in the Atlantic Ocean
at depths of over 1000 fathoms.
FAM. AGLAURIDAE.—The radial canals are eight in number and the statorhabs
are usually free. In the manubrium there is a rod-like projection of the
mesogloea from the aboral wall of the gastric cavity, covered by a thin
epithelium of endoderm, which occupies a considerable portion of the lumen
of the manubrium. This organ may be called the tongue. _Aglaura_ has an
octagonal umbrella, and a manubrium which does not project beyond the
velum. It occurs in the Atlantic Ocean and Mediterranean Sea.
{295}FAM. GERYONIIDAE.—In this family there are four or six radial canals,
the statorhabs are sunk in the mesogloea, and a tongue is present in the
manubrium. _Liriope_ (Fig. 137) is sometimes as much as three inches in
diameter. It has a very long manubrium, and the tongue sometimes projects
beyond the mouth. There are four very long radial tentacles. It is found in
the Atlantic Ocean, the Mediterranean Sea, and the Pacific and Indian
Oceans. _Geryonia_ has a wider geographical distribution than _Liriope_,
and is sometimes four inches in diameter. It differs from _Liriope_ in
having six, or a multiple of six, radial canals. _Carmarina_ of the
Mediterranean and other seas becomes larger even than _Geryonia_, from
which it differs in the arrangement of the centripetal canals.
_Liriantha appendiculata_ sometimes occurs on the south coast of England
during September, October, or at other times.
ORDER VIII. NARCOMEDUSAE.
The Narcomedusae differ from the Trachomedusae in having the margin of the
umbrella divided into a number of lobes, and in bearing the gonads on the
sub-umbrellar wall of the gastral cavity instead of upon the radial canals.
The tentacles are situated at some little distance from the margin of the
umbrella at points on the aboral surface corresponding with the angles
between the umbrella lobes. Between the base of the tentacle and the
marginal angle there is a tract of modified epithelium called the
"peronium." The manubrium is usually short, and the mouth leads into an
expanded gastral chamber which is provided with lobular diverticula
reaching as far as the bases of the tentacles. The marginal sense-organs
are in the form of unprotected statorhabs. Very little is known concerning
the life-history of any of the Narcomedusae. In _Cunoctantha octonaria_ the
peculiar ciliated larva with two tentacles and a very long proboscis soon
develops two more tentacles and creeps into the bell of the Anthomedusan
_Turritopsis_, where, attached by its tentacles, it lives a parasitic life.
Before being converted into a Medusa it gives rise by gemmation to a number
of similar individuals, all of which become, in time, Medusae. The
parasitic stage is often regarded as the representative of the hydrosome
stage reduced and adapted to the oceanic habit of the adult.
{296}In _Cunina proboscidea_, and in some other species, a very remarkable
method of reproduction has been described by Metschnikoff, called by him
"sporogony." In these cases young sexual cells (male or female) wander from
the gonad of the parent into the mesogloea of the umbrella, where they
develop parthenogenetically into ciliated morulae. These escape by the
radial canals into the gastric cavity, and there form a stolon from which
young Medusae are formed by gemmation. In _C. proboscidea_ these young
Medusae are like the genus _Solmaris_, but in _C. rhododactyla_ they have
the form of the parent. In some cases the ciliated larvae leave the parent
altogether and become attached to a _Geryonia_ or some other Medusa, where
they form the stolon.
This very interesting method of reproduction cannot be regarded as a
primitive one, and throws no light on the origin of the order. It might be
regarded as a further stage in the degeneration of the hydrosome stage in
its adaptation to a parasitic existence.
The Narcomedusae have a wide geographical distribution. Species of
_Aeginopsis_ occur in the White Sea and Bering Strait, but the genera are
more characteristic of warmer waters. Some species occur in moderately deep
water, and _Cunarcha_ was found in 1675 fathoms off the Canaries, but they
are more usually found at or near the surface of the sea.
FAM. CUNANTHIDAE.—Narcomedusae with large gastral diverticula corresponding
in position with the bases of the tentacles. _Cunina_ and _Cunoctantha_,
occurring in the Mediterranean and in the Atlantic and Pacific Oceans,
belong to this family. In _Cunina_ the tentacles may be eight in number, or
some multiple of four between eight and twenty-four. In _Cunoctantha_ the
number of tentacles appears to be constantly eight.
FAM. PEGANTHIDAE.—There appear to be no gastral pouches in this family. The
species of _Pegantha_ are found at depths of about 80 fathoms in the Indian
and Pacific Oceans.
FAM. AEGINIDAE.—The large gastral pouches of this family alternate with the
bases of the tentacles. _Aegina_ occurs in the Atlantic and Pacific Oceans.
_Aeginopsis._
FAM. SOLMARIDAE.—In this family the gastral pouches are variable, sometimes
corresponding with, sometimes alternating with, the bases of the tentacles.
The circular canal is represented {297}in some genera by solid cords of
endoderm. _Solmaris_ sometimes appears in the English Channel, but it is
probably a wanderer from the warmer regions of the Atlantic Ocean. It is
found in abundance during November on the west coast of Ireland.
ORDER IX. SIPHONOPHORA.
In this order the naturalist finds collected together a number of very
beautiful, delicate transparent organisms to which the general term
"jelly-fish" may be applied, although their organisation is far more
complicated and difficult to describe than that of any of the Medusae. In
several of the Hydrozoa the phenomenon of dimorphism has already been
noticed. In these cases one set of individuals in a colony performs
functions of stinging and catching food and another the functions of
devouring and digesting it. In many of the Siphonophora there appears to be
a colony of individuals in which the division of labour is carried to a
much further extent than it is in the dimorphic Hydrozoa referred to above.
Not only are there specialised gastrozooids and dactylozooids, but also
gonozooids, zooids for propelling the colony through the water
("nectocalyces"), protective zooids ("hydrophyllia"), and in some cases a
specialised zooid for hydrostatic functions; the whole forming a swimming
or floating polymorphic colony. But this conception of the construction of
the Siphonophora is not the only one that has met with support. By some
zoologists the Siphonophoran body is regarded not as a colony of
individuals, but as a single individual in which the various organs have
become multiplied and dislocated.
The multiplication or repetition of organs that are usually single in each
individual is not unknown in other Hydrozoa. In the Medusa of the
Gymnoblast _Syncoryne_, usually known as _Sarsia_, for example, there is
sometimes a remarkable proliferation of the manubrium, and specimens have
been found with three or four long manubria attached by a tubular stalk to
the centre of the umbrella. Moreover, this complex of manubria may become
detached from the umbrella and live for a considerable time an independent
existence.[333]
If we regard the manubrium of a Medusa as an organ of the {298}animal's
body, it might be thought obvious that the phenomenon observed in the
Medusae of _Syncoryne_ is a case of a simple repetition of the parts of an
individual; but the power that the group of manubria possesses of leading
an independent existence renders its interpretation as a group of organs a
matter of some inconvenience. If we can conceive the idea that an organ may
become detached and lead an independent existence, there is no reason why
we should not regard the Medusa itself of _Syncoryne_ as an organ, and we
should be driven to the paradoxical conclusion that, as regards several
genera and families of Hydrozoa, we know nothing at present of the
individuals, but only of their free-swimming organs, and that in others the
individual has degenerated, although one of its organs remains.
There is, however, no convincing argument to support either the conception
that the Siphonophoran body is a colony of individuals, or that it is an
individual with disjointed organs. These two conceptions are sometimes
called the "Poly-person" and "Poly-organ" theories respectively. The
difficulty is caused by the impossibility of giving any satisfactory
definition in the case of the Hydrozoa of the biological terms "organ" and
"individual." In the higher animals, where the correlation of parts is far
more complex and essential than it is in Coelenterata, a defined limit to
the scope of these terms can be laid down, but in the lower animals the
conception of what is termed an organ merges into that which is called an
individual, and no definite boundary line between the two exists in Nature.
The difficulty is therefore a permanent one, and, in using the expression
"colony" for the Siphonophoran body, it must be understood that it is used
for convenience' sake rather than because it represents the only correct
conception of the organisation of these remarkable Coelenterates.
Regarding the Siphonophora as polymorphic colonies, then, the following
forms of zooids may be found.
_Nectocalyces._—The nectocalyces are in the form of the umbrella of a
medusa attached to the stolon of the colony by the aboral pole. They are
provided with a velum and, usually, four radial canals and a circular
canal. There is no manubrium, and the marginal tentacles and sense-organs
are rudimentary or absent. There may be one or more nectocalyces in each
colony, {299}and their function is, by rhythmic contractions, to propel the
colony through the water (Fig. 142, N).
_Gastrozooids._—These are tubular or saccular zooids provided with a mouth
and attached by their aboral extremity to the stolon (Fig. 142, G). In some
cases the aboral region of the zooid is differentiated as a stomach. It is
dilated and bears the digestive cells, the oral extremity or hypostome
being narrower and more transparent. In some cases the mouth is a simple
round aperture at the extremity of the hypostome, but in others it is
dilated to form a trumpet-like lip.
_Dactylozooids_.—In _Velella_ and _Porpita_ the dactylozooids are similar
in general characters to the tentacles of many Medusae. They are arranged
as a frill round the margin of the colony, and each consists of a simple
tube of ectoderm and endoderm terminating in a knobbed extremity richly
provided with nematocysts.
In many other Siphonophora, however, the dactylozooids are very long and
elaborate filaments, which extend for a great distance from the colony into
the sea. They reach their most elaborate condition in the Calycophorae.
[Illustration: FIG. 141.—A small Crustacean (_Rhinocalanus_) caught by a
terminal filament (_f.t_) of a battery of _Stephanophyes_. _b_, The
proximal end of the battery with the most powerful nematocysts; _e_,
elastic band; _S_, stalk supporting the battery on the dactylozooid. (After
Chun.)]
The dactylozooid in these forms has a hollow axis, and the lumen is
continuous with the cavity of the neighbouring gastrozooid. Arranged at
regular intervals on the axis is a series of tentacles ("tentilla"), and
each of these supports {300}a kidney-shaped swelling, the "cnidosac," or
battery, which is sometimes protected by a hood. Each battery contains an
enormous number of nematocysts. In _Stephanophyes_, for example, there are
about 1700 nematocysts of four different kinds in each battery. At the
extremity of the battery there is a delicate terminal filament. The action
of the battery in _Stephanophyes_ is, according to Chun,[334] a very
complicated one. The terminal filament lassos the prey and discharges its
somewhat feeble nematocysts at it (Fig. 141). If this kills it, the
dactylozooid contracts and passes the prey to a gastrozooid. If the animal
continues its struggles, it is drawn up to the distal end of the battery
and receives the discharge of a large number of nematocysts; and if this
also fails to put an end to its life, a membrane covering the largest and
most powerful nematocysts at the proximal end of the whole battery is
ruptured, and a final broadside of stinging threads is shot at it.
The larger nematocysts of these batteries in the Siphonophora are among the
largest found in Coelenterata, being from 0.5 to 0.1 mm. in length, and
they are frequently capable of inflicting painful stings on the human skin.
The species of _Physalia_, commonly called "Portuguese Men-of-War," have
perhaps the worst reputation in this respect, the pain being not only
intense but lasting a long time.
_Hydrophyllia._—In many Siphonophora a number of short, mouthless,
non-sexual zooids occur, which appear to have no other function than that
of shielding or protecting other and more vital parts of the colony. They
consist of an axis of firm mesogloea, covered by a layer of flattened
ectoderm, and they may be finger-shaped or triangular in form. In _Agalma_
and _Praya_ an endoderm canal perforates the mesogloea and terminates in a
little mouth at the free extremity. In _Athoria_ and _Rhodophysa_ the
hydrophyllium terminates in a little nectocalyx.
_Pneumatophore._—In all the Siphonophora, with the exception of the
Calycophorae, there is found on one side or at one extremity of the colony
a vesicle or bladder containing a gas,[335] which serves as a float to
support the colony in the water. {301}This bladder or pneumatophore is
probably in all cases a much modified nectocalyx. It shows great variations
in size and structure in the group. It is sometimes relatively very large,
as in _Physalia_ and _Velella_, sometimes very small, as in _Physophora_.
It is provided with an apical pore in some genera (_Rhizophysa_), or a
basal pore in others (Auronectidae), but it is generally closed. In the
many chambered pneumatophore of the Chondrophoridae there are several
pores.
In many forms two distinct parts of the pneumatophore can be recognised—a
distal region lined by chitin,[336] probably representing the sub-umbrellar
cavity of the nectocalyx, and a small funnel-shaped region lined by an
epithelium, the homology of which is a matter of dispute. It is believed
that the gas is secreted by this epithelium. In the Auronectidae the region
with secretory epithelium is relatively large and of a more complicated
histological character. It is remarkable also that in this family the pore
communicates, not with the chitin-lined region, but directly with the
epithelium-lined region.
There is no pneumatophore in the Calycophorae, but in this sub-order a
diverticulum of an endoderm canal secretes a globule of oil which may serve
the same hydrostatic function.
The _stolon_ is the common stem which supports the different zooids of the
colony. In the Calycophorae the stolon is a long, delicate, and extremely
contractile thread attached at one end to a nectocalyx, and bearing the
zooids in discontinuous groups. These groups of zooids arranged at
intervals on the stolon are called the "cormidia." The stolon is a tube
with very thick walls. Its lumen is lined by a ciliated endoderm with
circular muscular processes, and the surface is covered with an ectoderm,
also provided with circular muscular processes. Between these two layers
there is a relatively thick mesogloea showing on the outer side deep and
compound folds and grooves supporting an elaborate system of longitudinal
muscular fibres. In many Physonectidae the stolon is long and filamentous,
but not so contractile as it is in Calycophorae, but in others it is much
reduced in length and relatively stouter. The reduction {302}in length of
the stolon is accompanied by a complication of structure, the simple
tubular condition being replaced by a spongy complex of tubes covered by a
common sheath of ectoderm. In the Auronectidae the stolon is represented by
a conical or hemispherical spongy mass bearing the zooids, and in the
Rhizophysaliidae and Chondrophoridae it becomes a disc or ribbon-shaped pad
spreading over the under side of the pneumatophore.
_Gonozooids._—The gonozooids are simple tubular processes attached to the
stolon which bear the Medusae or the degenerate medusiform gonophores. In
the Chondrophoridae the gonozooids possess a mouth, but in most
Siphonophora they have neither mouth nor tentacles. In some cases, such as
_Anthophysa_, the colonies are bisexual—the male and female gonophores
being borne by separate gonozooids—but in others (e.g. _Physalia_) the
colonies appear to be unisexual.
As a general rule the gonophores of Siphonophora do not escape from the
parent colony as free-swimming Medusae, but an exception occurs in
_Velella_, which produces a number of small free-swimming Medusae formerly
described by Gegenbaur under the generic name _Chrysomitra_. This Medusa
has a velum, a single tentacle, eight to sixteen radial canals, and it
bears the gonads on the short manubrium. The Medusa of _Velella_ has, in
fact, the essential characters of the Anthomedusae.
Our knowledge of the life-history of the Siphonophora is very incomplete,
but there are indications, from scattered observations, that in some
genera, at least, it may be very complicated.
The fertilised ovum of _Velella_ gives rise to a planula which sinks to the
bottom of the sea, and changes into a remarkable larva known as the
_Conaria_ larva. This larva was discovered by Woltereck[337] at depths of
600-1000 metres in great numbers. It is very delicate and transparent, but
the endoderm is red (the colour so characteristic of animals inhabiting
deep water), and it may be regarded as essentially a deep-sea larva. The
larva rises to the surface and changes into the form known as the
_Ratarula_ larva, which has a simple one-chambered pneumatophore containing
a gas, and a rudiment of the sail. In contrast to the _Conaria_, the
_Ratarula_ is blue in colour. With the development of the zooids on the
under side of this {303}larva (_i.e._ the side opposite to the
pneumatophore), a definite octoradial symmetry is shown, there being for
some time eight dactylozooids and eight definite folds in the wall of the
pneumatophore. This octoradial symmetry, however, is soon lost as the
number of folds in the pneumatophore and the number of tentacles increase.
It is probable that in the Siphonophora, as in many other Coelenterata, the
production of sexual cells by an individual is no sign that its
life-history is completed. There may possibly be two or more phases of life
in which sexual maturity is reached.
An example of a complicated life-history is found in the Calycophoran
species _Muggiaea kochii_. The embryo gives rise to a form with a single
nectocalyx which is like a _Monophyes_, and this by the budding of a second
nectocalyx produces a form that has a remarkable resemblance to a
_Diphyes_, but the primary nectocalyx degenerates and is cast off, while
the secondary one assumes the characters of the single _Muggiaea_
nectocalyx. The stolon of the _Muggiaea_ produces a series of cormidia, and
as the sexual cells of the cormidia develop, a special nectocalyx is formed
at the base of each one of them, and the group of zooids is detached as an
independent colony, formerly known as _Eudoxia eschscholtzii_. In a similar
manner the cormidia of _Doramasia picta_ give rise to the sexual
free-swimming monogastric forms, known by the name _Ersaea picta_ (Fig.
142). In these cases it seems possible that the production of ripe sexual
cells is confined to the _Eudoxia_ and _Ersaea_ stages respectively, but it
is probable that in other species the cormidia do not break off from the
stolon, or may escape only from the older colonies.
[Illustration: FIG. 142.—Free-swimming _Ersaea_ group of _Doramasia picta_.
_B_, _B_, batteries of nematocysts borne by the tentilla; _D_,
dactylozooid; _G_, gastrozooid; _H_, hydrophyllium; _N_, nectocalyx; _O_,
oleocyst; _f.t_, terminal filament of a battery; _t_, _t_, tentilla. The
gonozooid is hidden by the gastrozooid. × 10. (After Chun.)]
The Siphonophora are essentially free-swimming pelagic {304}organisms. Some
of them (Auronectidae) appear to have become adapted to a deep-sea habit,
others are usually found in intermediate waters, but the majority occur
with the pelagic plankton at or very near the surface of the open sea.
Although the order may be said to be cosmopolitan in its distribution, the
Siphonophora are only found in great numbers and variety in the
sub-tropical and tropical zones. In the temperate and arctic zones they are
relatively rare, but _Galeolaria biloba_ and _Physophora borealis_ appear
to be true northern forms. The only British species are _Muggiaea
atlantica_ and _Cupulita sarsii_. _Velella spirans_ occasionally drifts
from the Atlantic on to our western shores, and sometimes great numbers of
the pneumatophores of this species may be found cast up on the beach.
_Diphyes_ sp., _Physalia_ sp., and _Physophora borealis_ are also
occasionally brought to the British shores by the Gulf Stream.
The Calycophorae are usually perfectly colourless and transparent, with the
exception of the oil-globule in the oleocyst, which is yellow or orange in
colour. Many of the other Siphonophora, however, are of a transparent, deep
indigo blue colour, similar to that of many other components of the
plankton.
Most of the Siphonophora, although, strictly speaking, surface animals, are
habitually submerged; the large pneumatophores of _Velella_ and _Physalia_,
however, project above the surface, and these animals are therefore
frequently drifted by the prevailing wind into large shoals, or blown
ashore. At Mentone, on the Mediterranean, _Velella_ is sometimes drifted
into the harbour in countless numbers. Agassiz mentions the lines of deep
blue Velellas drifted ashore on the coast of Florida; and a small species
of blue _Physalia_ may often be seen in long lines on the shore of some of
the islands of the Malay Archipelago.
The food of most of the Siphonophora consists of small Crustacea and other
minute organisms, but some of the larger forms are capable of catching and
devouring fish. It is stated by Bigelow[338] that a big _Physalia_ will
capture and devour a full-grown Mackerel. The manner in which it feeds is
described as follows:—"It floats on the sea, quietly waiting for some
heedless individual to bump its head against one of the tentacles. The
fish, on striking, is stung by the nettle-cells, and fastened probably by
them to the tentacle. Trying to run away the fish pulls on the
{305}tentacle. The tension on its peduncle thus produced acts as a stimulus
on apparently some centre there which causes it to contract. The fish in
this way is drawn up so that it touches the sticky mouths of the squirming
siphons [_i.e._ gastrozooids]. As soon as the mouths, covered as they are
with a gluey substance and provided with nettle-cells, touch the fish they
stick fast, a few at first, and gradually more. The mouths open, and their
lips are spread out over the fish until they touch, so that by the time he
is dead the fish is enclosed in a tight bag composed of the lips of a dozen
or more siphon mouths. Here the fish is digested. As it begins to
disintegrate partially digested fragments are taken into the stomachs of
the attached siphons (gastrozooids). When they have become gorged they
detach themselves from the remains of the fish, the process of digestion is
completed in the stomachs, and the nutrient fluid is distributed...."
In consequence of the very unsatisfactory state of our knowledge of the
life-history of the Siphonophora the classification of the order is a
matter of unusual difficulty.
SUB-ORDER I. CALYCOPHORAE.
The character which distinguishes this sub-order is the absence of a
pneumatophore.
The colony usually consists of a long, slender, contractile stolon,
provided at one end with one, two, or several nectocalyces. Upon the stolon
are arranged several groups ("cormidia") of polymorphic zooids.
The nectocalyces have a well-developed velum, four radial canals, and a
muscular umbrella-wall. A special peculiarity of the nectocalyx of this
sub-order is a diverticulum (oleocyst) from one of the radial canals,
containing a coloured globule of oil. The function of this oil-globule is
probably similar to that of the pneumatophore, and assists the muscular
efforts of the nectocalyces in keeping the colony afloat. One of the
nectocalyces of each colony exhibits on one side a deep ectodermic fold,
which is frequently converted into a pit. At the bottom of this pit is
attached the end of the stolon, the whole of which with its numerous
cormidia can be withdrawn into the shelter of the pit when danger
threatens. The cormidia consist of at least four {306}kinds of zooids: a
gastrozooid with a trumpet-shaped mouth armed with nematocysts, a long
dactylozooid provided with a series of tentilla, and a rudimentary
gonozooid bearing numbers of male or female medusiform gonophores. These
three kinds of zooids are partially covered and protected by a bent
shield-shaped phyllozooid or hydrophyllium.
Each of the cormidia is unisexual, but the colony as a whole is usually
hermaphrodite, the male and female cormidia regularly alternating, or the
male cormidia being arranged on the nectocalycine half and the female
cormidia on the opposite half of the stolon.
The families of the Calycophorae are:—
FAM. 1. MONOPHYIDAE.—In this family there is a single conical or
mitre-shaped nectocalyx. The cormidia become detached as free-swimming
_Eudoxia_ or _Ersaea_ forms.
Sub-Fam. 1. SPHAERONECTINAE.—The primary nectocalyx persists throughout
life—_Monophyes_ and _Sphaeronectes_.
Sub-Fam. 2. CYMBONECTINAE.—The primary nectocalyx is thrown off, and is
replaced by a secondary and permanent nectocalyx—_Cymbonectes_, _Muggiaea_,
and _Doramasia_.
FAM. 2. DIPHYIDAE.—The primary mitre-shaped nectocalyx is thrown off and
replaced by two secondary rounded, prismatic, or pyramidal, heteromorphic
nectocalyces.
This family contains several sub-families, which are arranged in two
groups: the Diphyidae Oppositae, in which the two secondary bells are
opposite one another, and do not exhibit pronounced ridges; and the
Diphyidae Superpositae, in which one of the two secondary nectocalyces is
situated in front of the other, and each nectocalyx is provided externally
with very definite and often wing-like ridges. In all the Diphyidae
Oppositae the cormidia remain attached, whereas in most of the Diphyidae
Superpositae they become free-swimming, as in the Monophyidae.
The sub-families of the DIPHYIDAE OPPOSITAE are:—
Sub-Fam. 1. AMPHICARYONINAE.—One of the two secondary nectocalyces becomes
flattened above to form a shield, and at the same time its sub-umbrellar
cavity is atrophied, and its radial canals reduced. _Mitrophyes_, Atlantic
Ocean.
Sub-Fam. 2. PRAYINAE.—The colony exhibits a pair of large, obtuse
nectocalyces, with a relatively small sub-umbrellar cavity. _Praya_,
Mediterranean and Atlantic.
{307}Sub-Fam. 3. DESMOPHYINAE.—The colony bears a large number of reserve
or tertiary nectocalyces arranged in two rows. _Desmophyes_, Indian Ocean.
Sub-Fam. 4. STEPHANOPHYINAE.—There are four nectocalyces arranged in a
horizontal plane. Each one of the cormidia bears a nectocalyx, which is
periodically replaced. This sub-family is constituted for _Stephanophyes
superba_ from the Canary Islands. It attains a length of 25 cm., and is
probably the largest and most beautiful of all the Calycophoridae.[339]
The group DIPHYIDAE SUPERPOSITAE contains the following:—
Sub-Fam. 1. GALEOLARINAE.—_Galeolaria._
Sub-Fam. 2. DIPHYOPSINAE.—_Diphyes._
Sub-Fam. 3. ABYLINAE.—_Abyla._
These sub-families differ from one another in the character and shape of
the nectocalyces and in other characters. They have a world-wide
distribution, _Diphyes_ and _Galeolaria_ extending north into the Arctic
Seas. _Diphyes_ is British.
FAM. 3. POLYPHYIDAE.—The nectocalyces are numerous, and superposed in two
rows. The cormidia remain attached.
The family contains the genera _Polyphyes_ and _Hippopodius_, both probably
cosmopolitan in warm waters.
SUB-ORDER II. PHYSOPHORAE.
In this sub-order the primary nectocalyx gives rise to a definite
pneumatophore. There are four families.
FAM. 1. PHYSONECTIDAE.—In this, the largest family of the sub-order, there
is a monothalamic pneumatophore supporting a stolon, which in some forms is
of great length, but in others is reduced to a stump or pad, on which there
are usually found several nectocalyces, hydrophyllia, gastrozooids,
gonozooids, and tentilla.
The principal sub-families are:—
AGALMINAE.—With a long stolon, bearing at the upper end (_i.e._ the end
next to the pneumatophore) two rows of nectocalyces. The other zooids are
arranged in cormidia on the stolon, each covered by a hydrophyllium.
Dactylozooids with tentilla. _Agalma_ and _Cupulita_, Mediterranean Sea.
APOLEMINAE.—Similar to the above, but without tentilla.
{308}_Apolemia_—this genus attains a length of two or three metres.
Mediterranean Sea. _Dicymba_, Indian Ocean.
PHYSOPHORINAE.—The pneumatophore larger in proportion than it is in the
preceding families. The stolon is short, and bears rows of nectocalyces at
the upper end. The gastrozooids, dactylozooids, and gonozooids are arranged
in verticils on the lower expanded part of the stolon. Hydrophyllia absent.
_Physophora_, cosmopolitan in the areas of warm sea water.
FAM. 2. AURONECTIDAE.—The pneumatophore is large. The stolon is reduced to
a spongy mass of tissue on the under side of the pneumatophore, and this
bears numerous cormidia arranged in a helicoid spiral. Projecting from the
base of the pneumatophore there is a peculiar organ called the "aurophore,"
provided with an apical pore. This organ has been described as a specially
modified nectocalyx, but it is probably a specialised development of the
epithelium-lined portion of the pneumatophore of other Physophorae. The
Auronectidae are found only at considerable depths, 300 to 1400 fathoms,
and are probably specially adapted to that habitat. _Rhodalia_,
_Stephalia_, Atlantic Ocean.
FAM. 3. RHIZOPHYSALIIDAE.—The pneumatophore is large, or very large, in
this family. The zooids are arranged in horizontal rows on the under side
of the pneumatophore (_Physalia_), or in a helicoid spiral on a short
stolon (_Epibulia_). There are no nectocalyces nor hydrophyllia.
The genus _Physalia_ is the notorious "Portuguese Man-of-War." The
pneumatophore is a large bladder-like vesicle, sometimes attaining a length
of 12 cm. One species described by Haeckel under the generic name
_Caravella_ has a pneumatophore 30 cm. and more in length, and
dactylozooids attaining a length of 20 metres. It is a curious fact that
only the male colonies of _Physalia_ are known, and it is suggested that
the female may have quite a different form.[340] _Epibulia_ has a much
smaller bladder than _Physalia_. Both genera have a cosmopolitan
distribution at the surface of the warm seas.
FAM. 4. CHONDROPHORIDAE.—This family stands quite by itself in the
sub-order Physophorae, and is placed in a separate division of the
sub-order by Chun, who gives it the name TRACHEOPHYSA. The essential
distinguishing characters of the family are {309}the large polythalamic
pneumatophore and the single large central gastrozooid.
The colony is disc-shaped, and has a superficial resemblance to a Medusa.
On the upper side is the flattened pneumatophore, covered by a fold of
tissue continuous with that at the edge of the disc. In _Velella_ a
vertical triangular sail or crest rises from the upper side, but this is
absent in _Porpita_.
The mouth of the gastrozooid opens into a large digestive cavity, and
between this and the under surface of the pneumatophore there is a
glandular spongy tissue called the liver. The liver extends over the whole
of the under side of the pneumatophore, and sends processes round the edge
of the disc into the tissues of its upper surface. Intimately associated
with the liver, and penetrating its interstices, is an organ which appears
to be entirely composed of nematocysts, derived from the ectoderm, and
called the central organ. At the margin of the disc there is a fringe of
simple digitiform dactylozooids, and between the dactylozooids and the
centrally placed gastrozooid are numerous gonozooids. Each of the
gonozooids is provided with a distinct mouth, and bears the gonophores,
which escape before the ripening of the gonads as the free-swimming Medusae
called _Chrysomitra_. The pneumatophore consists of a number of annular
chambers arranged in a concentric manner round the central original chamber
formed from a modified zooid. These annular chambers are in communication
with one another, and have each two pores (pneumatopyles) opening above to
the exterior. The most remarkable feature, however, of the system is a
series of fine branching tubes ("tracheae"), which pass from the annular
chambers of the pneumatophore downwards into the hepatic mass and ramify
there.
There are two well-known genera: _Velella_ with a sail, and _Porpita_
without a sail. They are both found at the surface of the warmer regions of
the great oceans and in the Mediterranean. _Velella_ sometimes drifts on to
British coasts from the Atlantic.
The genus _Discalia_ has a much more simple octoradial structure. It was
found at depths of 2600 and 2750 fathoms in the Pacific Ocean.
{310}CHAPTER XII
COELENTERATA (_CONTINUED_): SCYPHOZOA = SCYPHOMEDUSAE
CLASS II. SCYPHOZOA = SCYPHOMEDUSAE
The Scyphozoa are jelly-fishes, usually found floating at or near the
surface of the sea. A few forms (Stauromedusae) are attached to rocks and
weeds by a stalked prolongation of the aboral region of the umbrella. With
this exception, however, they are all, in the adult stage, of the Medusa
type of structure, having a bell-shaped or discoid umbrella, from the under
surface of which depends a manubrium bearing the mouth or (in Rhizostomata)
the numerous mouths.
Although many of the species do not exceed an inch or a few inches in
diameter, others attain a very great size, and it is among the Scyphozoa
that we find the largest individual zooids of the Coelenterata. Some
Discophora have a disc three or four feet in diameter, and one specimen
obtained by the Antarctic Expedition of 1898-1900 weighed 90 lbs.[341] The
common jelly-fish, _Aurelia_, of our coasts belongs to a species that
appears to be very variable in general characters as well as in size.
Specimens obtained by the "Siboga" in the Malay Archipelago ranged from 6
to 64 cm. in diameter. The colour is very variable, shades of green, blue,
brown, and purple being conspicuous in many species; but a pale milky-blue
tint is perhaps the most prevalent, the tissues being generally less
transparent than they are in the Medusae of the Hydrozoa. The colour of the
Cubomedusae is usually yellow or brown, but _Charybdea xaymacana_ is
colourless and transparent. The deep-sea species, particularly the
Periphyllidae, have usually an opaque brown or dark red colour. The
surface-swimming {311}forms, such as the common _Aurelia_, _Pelagia_,
_Cyanaea_, are usually of a uniform pale milky-blue or green colour.
Generally the colour is uniformly distributed, but sometimes the surface of
the umbrella is freckled with irregular brown or yellow patches, as in
_Dactylometra_ and many others. There is frequently a special colour in the
statorhabs which renders them conspicuous in the living jelly-fish, and the
lips, or parts of the lips, of the manubrium have usually a different
colour or tone to that of the umbrella.
There is no reason to believe that the general colour of any of these
jelly-fishes has either a protective or a warning significance. Nearly all
the larger species, whether blue, green, or brown in colour, can be easily
seen from a considerable distance, and the colours are not sufficiently
bright or alarming to support the belief that they can serve the purpose of
warning either fish or birds of the presence of a dangerous stinging
animal. It is possible, however, that the brighter spots of colour that are
often noticed on the tips of the tentacles and on the lips may act as a
lure or bait in attracting small fish and Crustacea.
Some of the Scyphozoa are phosphorescent, but it is a singular fact that
there are very few recorded observations concerning the phosphorescence or
the absence of it in most of the species. The pale blue light of _Pelagia
noctiluca_ or _P. phosphora_ can be recognised from the deck of a ship in
the open ocean, and they are often the most brilliant and conspicuous of
the phosphorescent organisms.
The food of the Scyphozoa varies a good deal. _Charybdea_ and _Periphylla_,
and probably many others with large mouths, will capture and ingest
relatively large fish and Crustacea; but _Chrysaora isosceles_[342]
apparently makes no attempt to capture either Copepoda or small fish, but
preys voraciously upon Anthomedusae, Leptomedusae, Siphonophora,
Ctenophora, and pelagic worms. Very little is known about the food of the
Rhizostomata, but the small size of the mouths of these forms suggests that
their food must also be of minute size. The frequent association of small
fish with the larger jelly-fish is a matter of some interest that requires
further investigation. In the North Sea young whiting are the constant
guests of _Cyanaea capillata_.[343] Over a {312}hundred young
horse-mackerel (_Caranx trachurus_) may be found sheltering under the
umbrella of _Rhizostoma pulmo_. As the animal floats through the water the
little fishes hover round the margin, but on the slightest alarm dart into
the sub-umbrella cavity, and ultimately seek shelter in the sub-genital
pits.[344]
Two species of fish accompany the American Medusa _Dactylometra lactea_,
one a Clupeoid, the other the young of the Butter-fish (_Stromateus
triacanthus_). According to Agassiz and Mayer[345] this is not an ordinary
case of mutualism, as the fish will tear off and devour fragments of the
tentacles and fringe of the Medusa, whilst the Medusa will in its turn
occasionally capture and devour one of the fish.
A great many of the Scyphozoa, particularly the larger kinds, have the
reputation of being able to sting the human skin, and in consequence the
name Acalephae[346] was formerly used to designate the order. Of the
British species _Aurelia aurita_ is almost harmless, and so is the rarer
_Rhizostoma pulmo_; but the nematocysts on the tentacles of _Cyanaea_,
_Chrysaora_, and _Pelagia_ can inflict stings on the more delicate parts of
the skin which are very painful for several hours, although the pain has
been undoubtedly greatly exaggerated in many popular works.
The soft structure of the Medusae does not favour their preservation in the
rocks, but the impressions left by several genera, all belonging apparently
to the Rhizostomata, have been found in Cambrian, Liassic, and Cretaceous
deposits.
There is reason to believe that many Scyphozoa exhibit a considerable range
of variation in the symmetry of the most important organs of the body. Very
little information is, however, at hand concerning the variation of any
species except _Aurelia aurita_, which has been the subject of several
investigations. Browne[347] has found that in a local race of this species
about 20 per cent exhibit variations from the normal in the number of the
statorhabs, and about 2 per cent in the number of gastric pouches.
The Scyphozoa are not usually regarded as of any commercial or other value,
but in China and Japan two species of Rhizostomata (_Rhopilema esculenta_
and _R. verrucosa_) are used as food. {313}The jelly-fish is preserved with
a mixture of alum and salt or between the steamed leaves of a kind of oak.
To prepare the preserved food for the table it is soaked in water, cut into
small pieces, and flavoured. It is also stated that these Medusae are used
by fishermen as bait for file-fish and sea-bream.[348]
In general structure the Scyphozoa occupy an intermediate position between
the Hydrozoa and the Anthozoa. The very striking resemblance of the
body-form to the Medusa of the Hydrozoa, and the discovery of a fixed
hydriform stage in the life-history of some species, led the older
zoologists to the conclusion that they should be included in the class
Hydrozoa. Recently the finer details of development have been invoked to
support the view that they are Anthozoa specially adapted for a
free-swimming existence, but the evidence for this does not appear to us to
be conclusive.
They differ from the Hydrozoa and resemble the Anthozoa in the character
that the sexual cells are matured in the endoderm, and escape to the
exterior by way of the coelenteric cavity, and not directly to the exterior
by the rupture of the ectoderm as in all Hydrozoa. They differ, on the
other hand, from the Anthozoa in the absence of a stomodaeum and of
mesenteries.
The view that the Scyphozoa are Anthozoa is based on the belief that the
manubrium of the former is lined by ectoderm, and is homologous with the
stomodaeum of the latter; and that the folds of mesogloea between the
gastric pouches are homologous with the septa.[349]
The Scyphozoa, notwithstanding their general resemblance to the Medusae of
Hydrozoa, can be readily distinguished from them by several important
characters. The absence of a velum in all of them (except the Cubomedusae)
is an important and conspicuous character which gave to the class the name
of Acraspeda. The velum of the Cubomedusae can, however, be distinguished
from that of the Craspedote Medusae (_i.e._ the Medusae of the Hydrozoa) by
the fact that it contains endodermal canals.
Sense-organs are present in all Scyphozoa except some of the Stauromedusae,
and they are in the form of statorhabs (tentaculocysts), bearing statoliths
at the extremity, and in many species, {314}at the base or between the base
and the extremity, one or more eyes. These organs differ from the
statorhabs of the Hydrozoa in having, usually, a cavity in the axial
endoderm; but as they are undoubtedly specially modified marginal
tentacles, they are strictly homologous in the two classes. In nearly all
the Scyphozoa these organs are protected by a hood or fold formed from the
free margin of the umbrella, and this character, although not of great
morphological importance, serves to distinguish the common species from the
Craspedote Medusae. It was owing to this character that Forbes gave the
name STEGANOPHTHALMATA, or "covered-eyed Medusae," to the class.
Another character of some importance is the presence in the coelenteric
cavity of all Scyphozoa of clusters or rows of delicate filaments called
the "phacellae." These filaments are covered with a glandular epithelium,
and are usually provided with numerous nematocysts. They have a
considerable resemblance to the acontia of certain Anthozoa, and are
probably mainly digestive in function. These three characters, in addition
to the very important character of the position and method of discharge of
the sexual cells already referred to, justify the separation of the
Scyphozoa from the Medusae of the Hydrozoa as a distinct class of
Coelenterata.
The umbrella of the Scyphozoa varies a good deal in shape. It is usually
flattened and disc-like (Discophora), but it may be almost globular
(_Atorella_), conical (some species of _Periphylla_), or cubical
(Cubomedusae). It is divided into an aboral and a marginal region by a
circular groove in the Coronata. The margin may be almost entire, marked
only by notches where the statorhabs occur, or deeply lobed as in the
Coronata and many Discophora. Marginal tentacles are present in all but the
Rhizostomata, and may be few in number, four in _Charybdea_, eight in
_Ulmaris_ (Fig. 143), or very numerous in _Aurelia_ and many others. The
tentacles may be short (_Aurelia_), or very long as in _Chrysaora
isosceles_, in which they extend for a length of twenty yards from the
disc.
The manubrium of the Scyphozoa is usually quadrangular in section, and in
those forms in which the shape is modified in the adult Medusa the
quadrangular shape can be recognised in the earlier stages of development.
The four angles of the manubrium are of importance in descriptive anatomy,
as the planes drawn {315}through the angles to the centre of the manubrium
are called "perradial," while those bisecting the perradial planes and
passing therefore through the middle line of the flat sides of the
manubrium are called "interradial."
The free extremity of the manubrium in many Scyphozoa is provided with four
triangular perradial lips, which may be simple or may become bifurcated or
branched, and have frequently very elaborate crenate edges beset with
batteries of nematocysts. In _Pelagia_ and _Chrysaora_ and other genera
these lips hang down from the manubrium as long, ribbon-like, folded bands,
and according to the size of the specimen may be a foot or more in length,
or twice the diameter of the disc.
In the Rhizostomata a peculiar modification of structure takes place in the
fusion of the free edges of the lips to form a suture perforated by a row
of small apertures, so that the lips have the appearance of long
cylindrical rods or tubes attached to the manubrium, and then frequently
called the "oral arms." The oral arms may be further provided with
tentacles of varying size and importance. In many Rhizostomata branched or
knobbed processes project from the outer side of the upper part of the oral
arms. These are called the "epaulettes."
[Illustration: FIG. 143.—_Ulmaris prototypus._ _g_, Gonad; _I_, interradial
canal; _M_, the fringed lip of the manubrium; _P_, perradial canal; _S_,
marginal sense-organ; _t_, tentacle. × 1. (After Haeckel.)]
The lumen of the manubrium leads into a large cavity in the disc, which is
usually called the gastric cavity, and this is extended into four or more
interradial or perradial gastric pouches. The number of these pouches is
usually four, but in this, as in {316}other features of their radial
symmetry, the jelly-fish frequently exhibit duplication or irregular
variation of the radii.[350]
The gastric pouches may extend to the margin of the disc, where they are
united to form a large ring sinus, or they may be in communication at the
periphery by only a very narrow passage (Cubomedusae). In the Discophora
the gastric pouches, however, do not extend more than half-way to the
margin, and they may be connected with the marginal ring-canal by a series
of branched interradial canals. Between the gastric pouches in these forms
branched perradial canals pass from the gastric cavity to the marginal ring
canal, and the system of canals is completed by unbranched "adradial"
canals passing between the perradials and interradials from the sides of
the gastric pouches to the ring-canal (Fig. 143).
In the Discophora there are four shallow interradial pits or pouches lined
by ectoderm on the under side of the umbrella-wall. As these pits
correspond with the position of the gonads in the gastric pouches they are
frequently called the "sub-genital pits." In the Stauromedusae and
Cubomedusae they are continued through the interradial gastric septa to the
aboral side of the disc, and they are generally known in these cases by the
name "interradial funnels." The functions and homologies of these
ectodermic pits and funnels are still uncertain.
The Scyphozoa are usually dioecious, but _Chrysaora_ and _Linerges_ are
sometimes hermaphrodite. The female Medusae can usually be distinguished
from the male by the darker or brighter colour of the gonads, which are
band-shaped, horseshoe-shaped, or circular organs, situated on the endoderm
of the interradial gastric pouches. They are, when nearly ripe, conspicuous
and brightly coloured organs, and in nearly all species can be clearly seen
through the transparent or semi-transparent tissues of the disc. The
reproductive cells are discharged into the gastric cavity and escape by the
mouth. The eggs are probably fertilised in the water, and may be retained
in special pouches on the lips of the manubrium until the segmentation is
completed.[351] Asexual reproduction does not occur in the free-swimming or
adult stage of any Scyphozoa. In some cases (probably exceptional) the
development is direct. In _Pelagia_, for example, it is known that the
fertilised egg gives {317}rise to a free-swimming Medusa similar in all
essential features to the parent.
In many species, however, the planula larva sinks to the bottom of the sea,
develops tentacles, and becomes attached by its aboral extremity to a rock
or weed, forming a sedentary asexual stage of development with a
superficial resemblance to a _Hydra_. This stage is the "Scyphistoma," and
notwithstanding its simple external features it is already in all essential
anatomical characters a Scyphozoon.
The Scyphistoma may remain as such for some time, during which it
reproduces by budding, and in some localities it may be found in great
numbers on seaweeds and stones.[352]
In the course of time, however, the Scyphistoma exhibits a ring-like
constriction of the body just below the crown of tentacles, and as this
deepens the general features of a Scyphomedusa are developed in the free
part above the constriction. In time this free part escapes as a small
free-swimming jelly-fish, called an "Ephyra," while the attached part
remains to repeat the process. In many species the first constriction is
followed by a second immediately below it, then a third, a fourth, and so
on, until the Scyphistoma is transformed into a long series of narrow
discs, each one acquiring, as it grows, the Ephyra characters. Such a stage
has been compared in form to a pile of saucers, and is known as the
"Strobila."
The Ephyra differs from the adult in many respects. The disc is thin and
flat, the manubrium short, the margin of the umbrella deeply grooved, while
the statorhabs are mounted on bifid lobes which project outwards from the
margin. The stabilisation of the Scyphistoma is a process of reproduction
by transverse fission, and in some cases this is supplemented by gemmation,
the Scyphistoma giving rise to a number of buds which become detached from
the parent and subsequently undergo the process of strobilisation.
[Illustration: FIG. 144.—The perisarc tubes of a specimen of _Spongicola
fistularis_ (_N_) ramifying in the skeleton of the Sponge _Esperella
bauriana_ (Sp.), as seen in a macerated specimen, × 1. (After Schulze.)]
The Scyphistoma of _Nausithoe_ presents us with the most {318}remarkable
example of this mode of reproduction (Fig. 144), as it forms an elaborate
branching colony in the substance of certain species of sponges. The
ectoderm secretes a chitinous perisarc, similar to that of the hydrosome
stage of many of the Hydrozoa, and consequently _Stephanoscyphus_
(_Spongicola_), as this Scyphistoma was called, was formerly placed among
the Gymnoblastea. It is remarkable that, although the Scyphozoan characters
of _Spongicola_ were proved by Schulze[353] in 1877, a similar Scyphistoma
stage has not been discovered in any other genus.
ORDER I. CUBOMEDUSAE.
Scyphozoa provided with four perradial statorhabs, each of which bears a
statolith and one or several eyes. There are four interradial tentacles or
groups of tentacles. The stomach is a large cavity bearing four tufts of
phacellae (Fig. 145, _Ph_), situated interradially. There are four
flattened perradial gastric pouches in the wall of the umbrella which
communicate with the stomach by the gastric ostia (_Go_). These pouches are
separated from one another by four interradial septa; and the long
leaf-like gonads are attached by one edge to each side of the septa. In
many respects the Cubomedusae appear to be of simple structure, but the
remarkable differentiation of the eyes and the occurrence of a velum (p.
313) suggest that the order is a highly specialised offshoot from a
primitive stock.
[Illustration: FIG. 145.—Vertical section in the interradial plane of
_Tripedalia cystophora_. _Go_, Gastric ostia; _Man_, manubrium; _Ph_, group
of phacellae; _T_, tentacles in four groups of three; _tent_, perradial
sense-organs; _V_, velum. (After Conant.)]
FAM. 1. CHARYBDEIDAE.—Cubomedusae with four interradial tentacles.
{319}_Charybdea_ appears to have a very wide geographical distribution.
Some of the species are usually found in deep water and come to the surface
only occasionally, but others (_C. xaymacana_) are only found at the
surface of shallow water near the shore. The genus can be easily recognised
by the four-sided prismatic shape of the bell and the oral flattened
expansion of the base of the tentacles. The bell varies from 2-6 cm. in
length (or height) in _C. marsupialis_, but a giant form, _C.
grandis_,[354] has recently been discovered off Paumotu Island which is as
much as 23 cm. in height. The colour is usually yellow or brown, but _C.
grandis_ is white and _C. xaymacana_ perfectly transparent.
"_Charybdea_ is a strong and active swimmer, and presents a very beautiful
appearance in its movements through the water; the quick, vigorous
pulsations contrasting sharply with the sluggish contractions seen in most
Scyphomedusae." It appears to be a voracious feeder. "Some of the specimens
taken contained in the stomach small fish, so disproportionately large in
comparison with the stomach that they lay coiled up, head overlapping
tail."[355]
Very little is known of the development, but it is possible that _Tamoya
punctata_, which lacks gonads, phacellae, and canals in the velum, may be a
young form of a species of _Charybdea_.
FAM. 2. CHIRODROPIDAE.—Cubomedusae with four interradial groups of
tentacles.
This family is represented by the genera _Chirodropus_ from the Atlantic
and _Chiropsalmus_ from the Indian Ocean and the coast of North Carolina.
FAM. 3. TRIPEDALIIDAE.—Cubomedusae with four interradial groups of three
tentacles.
The single genus and species _Tripedalia cystophora_ has only been found in
shallow water off the coast of Jamaica. Specimens of this species were kept
for some time by Conant in an aquarium, and produced a number of
free-swimming planulae which settled on the glass, and quickly developed
into small hydras with a mouth and four tentacles. The further development
of this sedentary stage is unfortunately not known.
{320}ORDER II. STAUROMEDUSAE.
This order contains several genera provided with an aboral stalk which
usually terminates in a sucker, by means of which the animal is temporarily
fixed to some foreign object. There can be little doubt that this sedentary
habit is recently acquired, and the wide range of the characteristic
features of the order may be accounted for as a series of adaptations to
the change from a free-swimming to a sedentary habit.
It is difficult to give in a few words the characters of the order, but the
Stauromedusae differ from other Scyphozoa in the absence or profound
modification in structure and function of the statorhabs. They are absent
in _Lucernaria_ and the Depastridae, and very variable in number in
_Haliclystus_.
The statorhab of _Haliclystus_ terminates in a spherical knob, which is
succeeded by a large annular pad or collar bearing a number of glandular
cells which secrete a sticky fluid. At the base of the organ there is a
rudimentary ocellus. The number is very variable, and sometimes they are
abnormal in character, being "crowned with tentacles." There can be little
doubt that the principal function of these organs is not sensory but
adhesive, and hence they have received the names "colletocystophores" and
"marginal anchors," but they are undoubtedly homologous with the statorhabs
of other Scyphozoa.
The tentacles are short and numerous, and are frequently mounted in groups
on the summit of digitate outgrowths from the margin of the umbrella. They
are capitate, except in _Tessera_, the terminal swelling containing a
battery of nematocysts.
Very little is known concerning the life-history and development of the
Stauromedusae.
FAM. 1. LUCERNARIIDAE.—Marginal lobes digitate, bearing the capitate
tentacles in groups. _Haliclystus auricula_ is a common form on the shores
of the Channel Islands, at Plymouth, and other localities on the British
coast. It may be recognised by the prominent statorhabs situated in the
bays between the digitate lobes of the margin of the umbrella. Each of the
marginal lobes bears from 15 to 20 capitate tentacles. It is from 2 to 3
cm. in length. The genus occurs in shallow water {321}off the coasts of
Europe and North America, extending south into the Antarctic region.
_Lucernaria_ differs from _Haliclystus_ in the absence of statorhabs. It
has the same habit as _Haliclystus_, and is often found associated with it.
_L. campanulata_ is British.
_Halicyathus_ is similar in external features to _Haliclystus_, but differs
from it in certain important characters of the coelenteric cavities. It is
found off the coasts of Norway, Greenland, and the Atlantic side of North
America.
In _Capria_, from the Mediterranean, the tentacles are replaced by a
denticulated membrane bearing nematocysts.
The rare genus _Tessera_, from the Antarctic Ocean, differs from all the
other Stauromedusae in having no stalk and in having only a few relatively
long non-capitate tentacles. If _Tessera_ is really an adult form it should
be placed in a separate family, but, notwithstanding the presence of
gonads, it may prove to be but a free-swimming stage in the history of a
normally stalked genus.
FAM. 2. DEPASTRIDAE.—The margin of the umbrella is provided with eight
shallow lobes bearing one or more rows of tentacles. Statorhabs absent.
_Depastrum cyathiforme_ occurs in shallow water at Plymouth, Port Erin, and
in other localities on the coasts of Britain and Norway. The tentacles are
arranged in several rows on the margin of the umbrella. In _Depastrella_
from the Canaries there is only one row of marginal tentacles.
FAM. 3. STENOSCYPHIDAE.[356]—Stauromedusae with simple undivided umbrella
margin. The eight principal tentacles are converted into adhesive anchors.
Secondary tentacles arranged in eight adradial groups. _Stenoscyphus
inabai_, 25 cm., Japan.
ORDER III. CORONATA.[357]
The external surface of the umbrella is divided into two regions, an aboral
region and a marginal region, by a well-marked circular groove (the coronal
groove). The aboral region is usually smooth and undivided, but it is an
elongated dome, {322}thimble- or cone-shaped, in marked contrast to the
flattened umbrella of the Discophora. The margin is divided into a number
of triangular or rounded lobes, and these are continued as far as the
coronal groove as distinct areas delimited by shallow grooves on the
surface of the umbrella. The tentacles arise from the grooves between the
marginal areas, and are provided with expanded bases called the pedalia.
The manubrium may be short or moderately long, but it is never provided
with long lips.
FAM. 1. PERIPHYLLIDAE.[358]—Coronata with four or six statorhabs.
In _Pericolpa_ (Kerguelen) there are only four tentacles and four
statorhabs. In _Periphylla_, a remarkable deep-sea genus from 700 to 2000
fathoms in all seas, but occasionally found at the surface, there are
twelve tentacles and four statorhabs. The specimens from deep water have a
characteristic dark red-brown or violet-brown colour. They are usually
small Medusae, but the umbrella of _P. regina_ is over 21 cm. in diameter.
_Atorella_ has six tentacles and six statorhabs.
FAM. 2. EPHYROPSIDAE.—Coronata with eight or more than eight statorhabs.
_Nausithoe punctata_ is a small, transparent jelly-fish, not exceeding 10
mm. in diameter, of world-wide distribution. Its Scyphistoma stage is
described on p. 317. _N. rubra_, a species of a reddish colour found at a
considerable depth in the South Atlantic and Indian Oceans, is probably an
abysmal form. _Palephyra_ differs from _Nausithoe_ in having elongated
instead of rounded gonads. _Linantha_ and _Linuche_ differ from the others
in having subdivided marginal lobes.
FAM. 3. ATOLLIDAE.—_Atolla_ is a deep-sea jelly-fish of very wide
geographical distribution. It is characterised by the multiplication of the
marginal appendages, but the number is very irregular. There may be double
or quadruple the usual number of marginal lobes, or an indefinite number.
There may be sixteen to thirty-two statorhabs, and the number of tentacles
is quite irregular. Some of the species attain a considerable size, the
diameter of the umbrella of _A. gigantea_ being 150 mm., of _A. valdiviae_
sometimes 130 mm., and of _A. bairdi_ 110 mm.
{323}ORDER IV. DISCOPHORA.
This order contains not only by far the greater number of the species of
Scyphozoa, but those of the largest size, and all those that are familiar
to the seaside visitor and the mariner under the general term jelly-fish.
They may be distinguished from the other Scyphozoa by several well-marked
characters. The umbrella is flattened and disc-shaped or slightly domed,
but not divided by a coronary groove. The perradial angles of the mouth are
prolonged into long lips, which may remain free (Semaeostomata) or fuse to
form an elaborate proboscis (Rhizostomata).
SUB-ORDER I. SEMAEOSTOMATA.
In this sub-order the mouth is a large aperture leading into the cavity of
the manubrium, and is guarded by four long grooved and often tuberculated
lips. The margin of the umbrella is provided with long tentacles.
FAM. 1. PELAGIIDAE.—Semaeostomata with wide gastric pouches, which are not
united by a marginal ring sinus. _Pelagia_, which forms the type of this
family, has eight long marginal tentacles. It develops directly from the
egg, the fixed Scyphistoma stage being eliminated.[359] It is probably in
consequence of this peculiarity of its development and independence of a
shore for fixation that _Pelagia_ has become a common and widespread
inhabitant of the high seas. In the Atlantic and Indian Oceans _P.
phosphora_ occurs in swarms or in long narrow lines many miles in length.
It is remarkable for its power of emitting phosphorescent light. In the
Atlantic it extends from 50° N. to 40° S., but is rare or absent from the
colder regions. _P. perla_ is found occasionally on the west coast of
Ireland. _Chrysaora_ differs from _Pelagia_ in the larger number of
tentacles. There are, in all, 24 tentacles and 8 statorhabs, separated by
32 lobes of the margin of the umbrella. _C. isosceles_ is occasionally
found off the British coast. It passes through a typical Scyphistoma stage
in development. _Dactylometra_, a very {324}common jelly-fish of the
American Atlantic shores, differs from _Chrysaora_ in having sixteen
additional but small tentacles arranged in pairs at the sides of the
statorhabs.
FAM. 2. CYANAEIDAE.—Semaeostomata with eight radial and eight adradial
pouches, which give off ramifying canals to the margin of the umbrella; but
these canals are not united by a ring-canal. The tentacles are arranged in
bundles on the margin of the deeply lobed umbrella.
The yellow _Cyanaea capillata_ and the blue _C. lamarcki_ are commonly
found on the British coasts.
FAM. 3. ULMARIDAE.—The gastric pouches are relatively small, and
communicate with a marginal ring-canal by branching perradial and
interradial canals and unbranched adradial canals.
In _Ulmaris prototypus_ (Fig. 143, p. 315) there are only eight long
adradial tentacles, and the lips of the manubrium are relatively short. It
is found in the South Atlantic.
_Aurelia_ is a well-known and cosmopolitan genus, which may be recognised
by the eight shallow lobes of the umbrella-margin beset with a fringe of
numerous small tentacles.
SUB-ORDER II. RHIZOSTOMATA.
In this sub-order the lips are very much exaggerated in size, and are fused
together by their margin in such a manner that the mouth of the animal is
reduced to a number of small apertures situated along the lines of suture.
Tentacles are absent on the margin of the umbrella. This sub-order contains
some of the largest known jelly-fishes, and exhibits a considerable range
of structure. The families are arranged by Maas[360] in three groups.
Group I. ARCADOMYARIA.—Musculature of the disc arranged in feather-like
arcades. Oral arms pinnate.
FAM. CASSIOPEIDAE.—There are no epaulettes on the arms. Labial tentacles
present. _Cassiopea_ is common in the Indo-Pacific seas, and extends into
the Red Sea. It includes a great many species varying in size from 4 to
about 12 cm. in diameter.
Group II. RADIOMYARIA.—Musculature arranged in radial tracts. Oral arms
bifid.
FAM. CEPHEIDAE.—The genera included in this family differ {325}from the
Cassiopeidae in the characters of the group. _Cephea_ is found in the
Indo-Pacific Oceans and Red Sea. _Cotylorhiza_ is common in the
Mediterranean Sea and extends into the Atlantic Ocean.
Group III. CYCLOMYARIA.—The group contains the majority of the
Rhizostomata. Musculature arranged in circular bands round the disc. Oral
arms primarily trifid, but becoming in some cases very complicated. The
principal families are:—
FAM. RHIZOSTOMATIDAE.—With well-marked epaulettes, and sixteen radial
canals passing to the margin of the umbrella.
_Rhizostoma pulmo_ (= _Pilema octopus_), a widely distributed species, is
often found floating at the surface off the western coasts of Scotland and
Ireland, and sometimes drifts up the English Channel into the German Ocean
in the autumn. The umbrella is about two feet in diameter, and the combined
length of the umbrella and arms is four feet. The colour varies
considerably, but that of a specimen obtained off Valencia in 1895 was
described as follows: "The colour of the umbrella was pale green, with a
deep reddish margin. Arms bright blue."[361]
The family includes _Stomolophus_, of the Pacific and Atlantic coasts of
America, in which the oral arms are united at the base, and _Rhopilema_,
the edible Medusa of Japan and China.
FAM. LYCHNORHIZIDAE.—Here there are only eight radial canals reaching as
far as the margin of the umbrella, and eight terminating in the ring-canal.
There are no epaulettes, and the oral tentacles are often very long. The
family includes _Lychnorhiza_ from the coast of Brazil, _Crambione_ from
the Malay Archipelago, and _Crambessa_ from the Atlantic shores of France
and Spain and from Brazil and Australia. The last-named genus has been
found in brackish water at the mouth of the Loire.
In the families LEPTOBRACHIIDAE and CATOSTYLIDAE there are eight radial
canals reaching the margin of the umbrella, and between them a network of
canals with many openings into the ring-canal. In a few of the
Leptobrachiidae the intermediate canal-network has only eight openings into
the ring-canal, as in the Lychnorhizidae.
{326}CHAPTER XIII
COELENTERATA (_CONTINUED_): ANTHOZOA = ACTINOZOA—GENERAL
CHARACTERS—ALCYONARIA
CLASS III. ANTHOZOA = ACTINOZOA
Among the familiar objects included in this class are the Sea-anemones, the
Stony Corals (Madrepores), the Flexible Corals, the Precious Coral, and the
Sea-pens. With the exception of a few species of Sea-anemone, Anthozoa are
not commonly found on British sea-shores; but in those parts of the
tropical world where coral reefs occur, the shore at low tide is carpeted
with various forms of this class, and the sands and beaches are almost
entirely composed of their broken-down skeletons.
The majority of the Anthozoa are colonial in habit, a large number of
individuals, or zooids as they are called, being organically connected
together by a network of nutritive canals, and forming a communal
gelatinous or stony matrix for their protection and support. Whilst the
individuals are usually small or minute, the colonial masses they form are
frequently large. Single colonies of the stony corals form blocks of stone
which are sometimes five feet in diameter, and reach a height of two or
three feet from the ground. From the tree or shrub-like form assumed by
many of the colonies they were formerly included in a class Zoophyta or
animal-plants.