colonies in the adult condition or remain independent, they exhibit certain
characters in common which distinguish them not only from the other
Coelenterata, but from all other animals. When an individual zooid is
examined in the living and fully expanded condition, it is seen to possess
a cylindrical {327}body, attached at one end (the aboral end) to the common
colonial matrix or to some foreign object. At the opposite or free
extremity it is provided with a mouth surrounded by a crown of tentacles.
In these respects, however, they resemble in a general way some of the
Hydrozoa. It is only when the internal anatomy is examined that we find the
characters which are absolutely diagnostic of the group.
In the Hydrozoa the mouth leads directly into the coelenteric cavity; in
the Anthozoa, however, the mouth leads into a short tube or throat, called
the "stomodaeum," which opens into the coelenteric cavity. Moreover, this
tube is connected with the body-wall, and is supported by a series of
fleshy vertical bands called the mesenteries (Fig. 146). The mesenteries
not only support the stomodaeum, but extend some distance below it. Where
the mesenteries are free from the stomodaeum their edges are thickened to
form the important digestive organs known as the mesenteric filaments
(_mf_). It is in the possession of a stomodaeum, mesenteries, and
mesenteric filaments that the Anthozoa differ from all the other
Coelenterata. There is one character that the Anthozoa share with the
Scyphozoa, and that is, that the gonads or sexual cells (G) are derived
from the endoderm. They are discharged first into the coelenteric cavity,
and then by way of the mouth to the exterior. In the Anthozoa the gonads
are situated on the mesenteries.
[Illustration: FIG. 146.—Diagram of a vertical section through an Anthozoan
zooid. B, Body-wall; G, gonads; M, mesentery; MF, mesenteric filament; ST,
stomodaeum; T, tentacle.]
Nearly all the Anthozoa are sedentary in habit. They begin life as ciliated
free-swimming larvae, and then, in a few hours or days, they become
attached to some rock or shell at the bottom and immediately (if colonial)
start the process of budding, which gives rise to the colonies of the adult
stage. Many of the Sea-anemones, however, move considerable distances by
gliding {328}over the rocks or seaweeds, others habitually burrow in the
sand (_Edwardsia_, _Cerianthus_), and one family (the Minyadidae) are
supported by a gas bladder, and float at the surface of the sea. The
Sea-pens, too, although usually partly buried in the sand or mud, are
capable of shifting their position by alternate distension and contraction
of the stalk.[362] The Anthozoa are exclusively marine. With the exception
of a few Sea-anemones that are found in brackish or almost fresh water in
river estuaries, they only occur in salt sea water. The presence of a
considerable admixture of fresh water, such as we find at the mouths of
rivers, seems to interfere very materially with the development and growth
of all the reef-forming Corals, as will be noticed again in the chapter on
coral reefs. A few genera descend into the greatest depths of the ocean,
but the home of the Anthozoa is pre-eminently the shallow seas, and they
are usually found in great abundance in depths of 0-40 fathoms from the
shores of the Arctic and Antarctic lands to the equatorial belt.
The only Anthozoa of any commercial importance are the Precious Corals
belonging to the Alcyonarian family Coralliidae. The hard pink axis of
these corals has been used extensively from remote times in the manufacture
of jewellery and ornaments. Until quite recently the only considerable and
systematic fishery for the Precious Corals was carried on in the
Mediterranean Sea, and this practically supplied the markets of the world.
In more recent times, however, an important industry in corals has been
developed in Japan. In 1901 the value of the coral obtained on the coasts
of Japan was over £50,000, the greater part of which was exported to Italy,
a smaller part to China, and a fraction only retained for home consumption.
The history of the coral fishery in Japan is of considerable interest.
Coral was occasionally taken off the coast of Tsukinada in early times. But
in the time of the Daimyos the collection and sale of coral was prohibited,
for fear, it is said, that the Daimyo of Tosa might be compelled to present
such precious treasure to the Shogun. After the Meiji reform, however
(1868), the industry revived, new grounds were discovered, improved methods
employed, and a large export trade developed.
There is evidence, however, in the art of Japan, of another {329}coral
fishery in ancient times, of which the history is lost. Coral was imported
into Japan at least two hundred years ago, and used largely in the
manufacture of those exquisite pieces of handicraft for which that country
is so justly famous. On many of the carved "Netsukes" and other ornaments,
however, the coral branches are represented as the booty of dark-skinned,
curly-headed fishermen, "kurombo," and never of Japanese fishermen. The
coral used in this art-work can hardly be distinguished from Mediterranean
coral, and there are some grounds for believing that Japan imported coral
from the far West in very early times. But this does not account for the
"kurombo." The only coast-dwelling people of the type that is so clearly
carved on these ornaments within the area of the Pacific Ocean at the
present time are the Melanesians and Papuans, and the suggestion occurs
that a coral fishery existed at one time in the Southern Pacific, which has
since been lost.[363]
The class ANTHOZOA is divided into two sub-classes:—I. ALCYONARIA; II.
ZOANTHARIA.
In the Alcyonaria the fully developed zooids have always eight tentacles
and eight mesenteries. In the Zoantharia the number of tentacles and the
number of mesenteries in the fully developed zooids may be six, twelve,
twenty-four, or an indefinite number, but individuals with eight
mesenteries and only eight tentacles are not known to occur.
SUB-CLASS I. ALCYONARIA.
This sub-class includes a large number of genera living in shallow
sea-water and a few genera that extend down into deep water. With a few
doubtful exceptions (Protoalcyonacea) they all form colonies composed of a
large number of zooids. These zooids may be connected together by basal
plates or a network of basal strands (stolons), or by stolons with
additional connecting bars (CLAVULARIA VIRIDIS, SYRINGOPORA) or by plates
(TUBIPORA). In the majority of the genera the individual zooids are for the
greater part of their length, from the base upwards, united together to
form a continuous spongy, colonial mass, which determines the shape of the
colony as a whole.
In this last-named group of genera there may be {330}distinguished the free
distal portions of the zooids bearing the mouths and tentacles (the
"anthocodiae") from the common colonial mass perforated by the coelenteric
cavities of the individual zooids. The coelenteric cavities are separated
by a considerable amount of a substance called the "mesogloea," usually
gelatinous in consistency but chemically more closely related to mucin than
to gelatin, which is traversed by endodermal canals, rods of endoderm cells
and a number of free amoeboid cells. In this substance, moreover, there are
found in nearly all cases numerous spicules of carbonate of lime formed by
the "scleroblasts" (spicule-forming cells) which have wandered from the
superficial ectoderm of the common colonial mass. This common colonial
mesogloea with its spicules, endoderm cells, and superficial covering of
ectoderm is called the "coenenchym." The form assumed by the colonies is
very varied. In some species of _Clavularia_ they form encrusting plates
following the irregularity of the rock or stones on which they grow, in
_Alcyonium_ they construct lobed masses of irregular form, in _Sarcophytum_
they are usually shaped like a mushroom, in _Juncella_ they are long
whip-like rods, in most of the Gorgonacea they are branched in all
directions like shrubs or in one plane to form fan-shaped growths, and in
many of the Pennatulacea they assume that graceful feather form which gives
the order its name.
The consistency and texture of the colonies also varies considerably. In
some cases where the spicules are few or very small, the substance of the
colony is soft to the touch, and frequently slimy at the surface, in other
cases the great number of the spicules makes the colony hard but brittle,
whilst in a few genera (_Sclerophytum_, _Heliopora_) the colony is so hard
that it can only be broken by the hand with difficulty. In some genera
(_Spongodes_ and the Muriceidae) projecting spicules cause the surface to
be rough or thorny, and in the Primnoidae the zooids and the surface of the
general coenenchym are protected by a series of overlapping scales or
plates.
In all the Alcyonaria the nematocysts are very minute, and although they
can undoubtedly paralyse minute organisms they are unable to penetrate the
human skin. None of the Alcyonaria have been described as stinging-corals
except the Pennatulid _Virgularia rumphii_.
ZOOIDS.—The fully formed zooids of the Alcyonaria exhibit {331}a remarkable
uniformity of structure. They have eight intermesenteric tentacles
containing a cavity continuous with the coelenteron. Each of these
tentacles bears at least two rows of simple pinnules, and they are
therefore said to be "pinnate" tentacles. In some species of _Xenia_ the
tentacles may have three or four rows of pinnules, which give them a much
more feathery appearance than is usually the case. In the great majority of
species a single row of from eight to fourteen pinnules is found disposed
laterally on each side of the tentacle. The mouth is usually small and
slit-like with a slight rounded gape at the ventral extremity. The
stomodaeum is usually very short, but in _Xenia_ and in the autozooids of
some Pennatulids it is relatively much longer. It is not known how far the
stomodaeum is of importance in the digestion of the food. In _Xenia_[364]
it has probably some importance, as shown by its unusual length and the
numerous large goblet cells (mucus cells) which it exhibits, associated
with the fact that the mesenteric filaments are relatively very small. In
_Alcyonium_ and other Alcyonaria gland cells also occur in the stomodaeum,
and it is probable that they secrete a fluid capable of digesting to some
extent the food as it passes through. The most important part of the
digestion, however, is performed by the six "ventral" mesenteric filaments.
Attention has already been drawn to the fact (p. 330) that two regions of
the zooids of the colonial Alcyonaria can be recognised. At the oral end
there is a region, which in the fully expanded condition consists of a
crown of eight tentacles surrounding the mouth, and a body-wall free from
its immediate neighbours. This region is called the "anthocodia." The
anthocodia is continuous with a region which forms a part of the common
colonial mass. Some genera seem to have very little power of contracting
the tentacles or of withdrawing the anthocodiae. The zooids of
_Stereosoma_, of _Xenia_, of _Umbellula_, and of a few other genera may be
described as non-retractile. In many cases, however, the tentacles can be
considerably contracted, bent over the mouth, and withdrawn into the
shelter of the subjacent body-wall. In such a condition the surface of the
colony exhibits a number of tubular, conical, or convex protuberances,
called "verrucae," and the colony is said to be partially retractile. In
many genera, however, the whole of the {332}anthocodiae can be withdrawn
below the general surface of the coenenchym, so that the position of the
zooids in the colony is indicated only by star-like holes, or simple
key-hole slits in the superficial coenenchym. Such colonies are said to be
completely retractile (Fig. 147).
It is often very difficult to determine whether a particular species is or
is not completely retractile, unless observations can be made upon the
living colony; and there are many instances of confusion in the work of
systematists due to a species being described as partially retractile in
one instance, and completely retractile in another. The complete retraction
of the anthocodiae may be effected very slowly, and after continuous
irritation only. If the colony is killed too quickly, the anthocodiae
remain in a state of partial retraction. An example of this may be found in
the common British _Alcyonium digitatum_. Specimens of this species which
are put into a bucket of sea water and allowed to roll about with the
movements of a small boat in a rough sea, undergo complete retraction; but
if the same specimens be allowed to expand in the aquarium, and then
plunged into spirit, or allowed to dry in the sun, they will die in a
condition of partial retraction.
[Illustration: FIG. 147.—Diagram of a vertical section of a portion of a
lobe of _Alcyonium_ to show the mode of retraction of the anthocodiae. 1,
Anthocodia of a zooid fully expanded; 2, in the first stage of retraction;
3, in the second stage; 4, in the third stage, leaving a shallow prominence
or "verruca" on the surface; 5, final stage, the verruca flattened down and
the coenenchym closed. _can_, Canal system; _d.m.f_, dorsal mesenteric
filament of a zooid; _si_, siphonoglyph.]
The phenomenon of dimorphism occurs in some Alcyonaria. A certain number of
the zooids of a colony are arrested in their development, and are known as
the "siphonozooids." They may be distinguished from the fully formed
zooids, which, in these {333}cases, are called the "autozooids," by the
absence of tentacles, by the absence of the six ventral and lateral
mesenteric filaments, and by the incomplete development of the muscles on
the mesenteries, and of the mesenteries themselves. They are, moreover,
frequently distinguished by the greater development and extent of the
ciliated groove or siphonoglyph on the ventral side of the stomodaeum.
It is often difficult to distinguish between true siphonozooids and young
autozooids, and consequently dimorphism has been attributed to some genera
in which it almost certainly does not occur. Simple dimorphism undoubtedly
occurs in the genera _Heteroxenia_, _Sarcophytum_, _Anthomastus_,
_Lobophytum_, _Acrophytum_, and _Paragorgia_. It has also been said to
occur in _Corallium_ (Moseley and Kishinouye), _Melitodes_ (Ridley), and
some species of Dasygorgiidae.
The Pennatulacea are trimorphic. The main shaft of these colonies is the
much modified first formed or axial zooid, adapted for the support of all
the other zooids. It usually exhibits no mouth, no tentacles, and only four
of the original eight mesenteries. It has no mesenteric filaments and no
stomodaeum, and bears no sexual cells. The other zooids of the colony are
similar in structure to the autozooids and siphonozooids of the dimorphic
Alcyonaria.
There are eight MESENTERIC FILAMENTS in all Alcyonarian zooids. They have
the appearance of thickenings of the free edges of the mesenteries. Two of
them, called the "dorsal" mesenteric filaments, are straight when the
anthocodia is expanded, and extend from the edge of the stomodaeum for a
long distance down into the coelenteron of the zooid; the other six, called
the "ventral" mesenteric filaments (_i.e._ the ventral and ventro-lateral
and dorso-lateral), are usually short and are almost invariably slightly
convoluted. The dorsal filaments are built up of columnar cells provided
with long cilia, and have usually no gland cells, the others may show a few
cilia but are principally composed of non-ciliated gland cells. When the
bolus of food has passed through the stomodaeum it is seized by these
ventral filaments and rapidly disintegrated by the secretion of its cells.
The function of the dorsal mesenteric filaments is mainly respiratory.
During life their cilia produce a current which flows towards the
stomodaeum. On the ventral side of the {334}stomodaeum itself there is a
groove called the "siphonoglyph" composed of a specialised epithelium
bearing long powerful cilia. But the current produced by the siphonoglyph
flows from the mouth downwards into the coelenteric cavity and is thus in
the opposite direction to that produced by the dorsal mesenteric filaments.
It is very probable that these two currents on the opposite sides of the
zooids maintain the circulation of water in the deep-seated parts of the
colony which is necessary for the respiration of the tissues.
On each of the eight mesenteries there is a longitudinal ridge due to the
presence of a band of retractor muscles. The position of these muscles on
the ventral surfaces of the mesenteries only is one of the characteristic
features of the sub-class (Fig. 148, and p. 329). They vary considerably in
thickness and extent according to the power of retractility possessed by
the zooids, but they never vary in their position on the mesenteries.
[Illustration: FIG. 148.—Diagrammatic transverse sections of an
Alcyonarian. A, through the stomodaeum; B, below the level of the
stomodaeum. _DD_, Dorsal directive; _dlmf_, dorso-lateral mesenteric
filament; _dmf_, dorsal mesenteric filament; _gon_, gonad; _Si_,
siphonoglyph; _V.D_, ventral mesentery; _V.L_, ventro-lateral mesentery.
The upper half of the section in B is taken at a higher level than the
lower half.]
The SKELETON of Alcyonaria may consist of spicules of calcium carbonate, of
a horny substance frequently impregnated with calcium carbonate and
associated with spicules of the same substance, or in _Heliopora_ alone,
among recent forms, of a continuous crystalline corallum of calcium
carbonate.
The spicules constitute one of the most characteristic features of the
Alcyonaria. They are not found in _Cornularia_, _Stereosoma_, in a recently
discovered genus of Gorgoniidae (_Malacogorgia_), in certain Pennatulacea
and in _Heliopora_; and it is probable that they may be absent in some
local varieties of certain species of _Clavularia_.
The spicules of Alcyonaria consist of an organic matrix {335}supporting a
quantity of crystalline calcium carbonate. In some cases (_Xenia_) the
amount of inorganic salt is so small that the spicule retains its shape
after prolonged immersion in an acid; but generally speaking the relative
amount of calcium carbonate is so great that it is only by the careful
decalcification of the spicules in weak acetic acid that the delicate
fibrous organic matrix can be demonstrated.
The spicules vary in size from minute granules to long spindles 9 mm. in
length (_Spongodes_, sp.). They exhibit so many varieties of shape that an
attempt must be made to place them in groups. The most prevalent type
perhaps is that called the spindle. This is a rod-shaped spicule with more
or less pointed extremities. They are usually ornamented with short simple
or compound wart-like tubercles (Fig. 149, 5). Spicules belonging to this
type are found in all the principal subdivisions of the group except the
Pennatulacea.
In the Pennatulacea a very characteristic form of spicule is a long rod or
needle marked with two or three slightly twisted ridges, frequently a
little knobbed or swollen at the extremities. In the same group, in _Xenia_
and _Heteroxenia_ among the Alcyonacea, and in the family Chrysogorgiidae
the spicules are in the form of minute discs or spheres, and in some genera
the discs may be united in couples (twins) or in threes (triplets) by short
connecting bars (Fig. 149, 10). More irregular calcareous corpuscles of
minute size are found in some genera of Pennatulacea.
Other characteristic spicules are the warted clubs of _Juncella_, the
torch-like spicules of _Eunicella_ (Fig. 149, 3), the clubs with irregular
leaf-like expansions at one extremity ("Blattkeulen") of _Eunicea_, and the
flat but very irregular scales of the Primnoidae. There are also many
genera exhibiting spicules of quite irregular form (Fig. 149, 8).
In the greater number of cases the spicules lie loosely in the mesogloea
and readily separate when the soft tissues of the colony decay or are
dissolved in a solution of potash. In a few noteworthy examples the
spicules become in their growth tightly wedged together to form a compact
skeleton, which cannot subsequently be disintegrated into its constituent
elements. In the Precious corals (Coralliidae) the spicules of the axial
region fuse together to form a solid mass of lime almost as hard and
compact as the substance of a pearl.
{336}[Illustration: FIG. 149.—Spicules of Alcyonaria. 1, Club of
_Juncella_; 2, warted cross of _Plexaurella_; 3, torch of _Eunicella_; 4,
needle of _Renilla_; 5, warted spindle of _Gorgonella_; 6, spicule of
_Pennatula_; 7, foliate club of _Eunicea_; 8, irregular spicule of
_Paramuricea_; 9, scale of _Primnoa_; 10, spicules of _Trichogorgia_. (5
and 10 original, the remainder after Kölliker.)]
In _Paragorgia_ and some other closely related genera the spicules of the
axis of the colony also become tightly wedged together, but the core thus
formed is far more porous and brittle than it is in the Coralliidae. In
_Tubipora_ (the organ-pipe coral) and in _Telesto rubra_ the spicules of
the body-walls of the zooids fuse to form perforated calcareous tubes. In
some species of _Sclerophytum_ the large spicules of the coenenchym become
so closely packed that they form dense stony masses, almost as hard as a
Perforate Madreporarian coral. The horny substance, allied chemically to
keratin, plays an {337}important part in the building up of skeletal
structures in many Alcyonaria. In _Clavularia viridis_ and in _Stereosoma_
a change in the chemical character of the mesogloea of the body-walls of
the polyps leads to the formation of a horny tube, which in the former case
is built up of interlacing fibres, and in the latter is formed as a
homogeneous sheath. In many of the Alcyonacea which have a compact axial
skeleton the spicules are cemented together by a horny matrix.
In the Gorgonellidae and some others the hard axis is formed of a horny
substance impregnated with a crystalline form of calcium carbonate; but in
the Gorgoniidae, many of the Pennatulacea and some other genera very little
or no carbonate of lime is found in the horny axis.
The skeleton of the genus _Heliopora_ differs from that of all the other
Alcyonaria in its development, structure, and form. In the words of Dr. G.
C. Bourne,[365] "the calcareous skeleton of _Heliopora_ is not formed from
spicules developed within cells but is a crystalline structure formed by
crystallisation of carbonate of lime, probably in the form of aragonite, in
an organic matrix produced by the disintegration of cells which I have
described as calicoblasts." It is further characterised by its blue colour.
A peculiar form of the axial skeleton (Fig. 155), consisting of alternate
nodes mainly composed of keratin, and internodes mainly composed of calcium
carbonate, is seen in the families Isidae and Melitodidae. In the
Melitodidae the nodes contain a considerable number of loose spicules, and
the internodes are mainly composed of spicules in close contact but firmly
cemented together by a sparse horny matrix. In the Isidae the scanty
calcareous substance of the nodes, and the bulk of the substance of the
internodes, is formed of amorphous crystalline limestone.
The Alcyonaria exhibit a great variety of COLOUR. Very little is known at
present of the chemistry of the various pigments found in the group, but
they may conveniently be arranged in two sections, the soluble pigments and
the insoluble pigments. To the former section belong various green and
brown pigments found in the anthocodiae and superficial coenenchym of many
genera. These are related to chlorophyll, and may be very largely the
product, not of the Alcyonarians themselves, but of the {338}symbiotic
"Algae" (cf. p. 261) they carry. A diffuse salmon-pink colour soluble in
spirit occurs in the living _Primnoa lepadifera_ of the Norwegian fjords,
and a similar but paler pink colour occurs in some varieties of the common
_Alcyonium digitatum_. Gilchrist[366] states that when he was preserving
specimens of _Alcyonium purpureum_ from Cape waters a considerable quantity
of a soluble purple pigment escaped.
But the predominant colour of Alcyonarians is usually due to the insoluble
pigments of the calcareous spicules. These may be of varying shades of
purple, red, orange, and yellow. The colours may be constant for a species
or genus, or they may vary in different specimens of one species, or even
in different parts of a single colony. Thus the skeletons of _Tubipora
musica_ from all parts of the world have a red colour, the species of the
genus _Anthomastus_ have always red spicules. On the other hand, we find in
_Melitodes dichotoma_ red and yellow varieties in the same locality, and in
_M. chamaeleon_ some of the branches of a colony are red and others yellow.
In _Chironephthya variabilis_ the colour of the spicules in any one
specimen varies considerably, but in a collection of several specimens from
a single locality a kaleidoscopic play of colours may be seen, no two
specimens being exactly the same in the arrangement of their colour
pattern. The influences that determine the colour of the spicules is at
present quite unknown, and in view of the great variability that occurs in
this respect, colour must be regarded as a most uncertain guide for the
determination of species. The blue colour of the genus _Heliopora_ is due
to a peculiar pigment which shows characteristic bands in the
spectrum.[367]
PHOSPHORESCENCE.—A great many Alcyonaria are known to be phosphorescent.
Moseley says that "All the Alcyonarians dredged by the 'Challenger' in deep
water were found to be brilliantly phosphorescent when brought to the
surface." The phosphorescence of the common British _Pennatula phosphorea_
has attracted more attention than that of any other species, and has been
well described by Panceri, Forbes, and others. Forbes[368] says, "The pen
is phosphorescent only when irritated by touch; the phosphorescence appears
at the place touched, and {339}proceeds thence in an undulating wave to the
extremity of the rachis, but never in the opposite direction; it is only
the parts at and above the point of stimulation that show phosphorescence,
the light is emitted for a longer time from the point of stimulation than
from the other luminous parts; detached portions may show phosphorescence.
When plunged in fresh water, the _Pennatula_ scatters sparks about in all
directions—a most beautiful sight."
Panceri was of opinion that the mesenteric filaments were the organs of
phosphorescence, but the whole question of the cause and localisation of
the light in these colonies requires further investigation.
FOOD.—Very little is known about the food of Alcyonaria, but it is very
probable that it consists entirely of minute larvae and other living
organisms. When the coelenteric cavities of preserved Alcyonaria are
examined, food is very rarely found in them, although fragments of
Crustacean appendages have occasionally been seen in the neighbourhood of
the mesenteric filaments. Experimenting upon _Alcyonium digitatum_, Miss
Pratt[369] has found that the zooids seize and swallow various small
organisms of a surface-net gathering, and that they will also swallow
finely minced fragments of the muscle of fish, but that they reject many
kinds of fish ova. In many tropical and some extra-tropical species the
superficial canal systems and the inter-mesenterial spaces of the zooids
contain a large number of Zooxanthellae, and their presence seems to be
associated in some cases with a decided degeneration of the digestive
organs. It has been suggested that these symbiotic "Algae" prepare food
materials after the manner of plants, and that these are absorbed by the
hosts, but it appears improbable that in any case this source of food
supply is sufficient. It must probably be supplemented in some degree by
food obtained by the mouth, and digested in the coelenteric cavity.
The question whether the Alcyonaria can form an important part of the
dietary of fish or other carnivorous animals may be economically important.
Fragments of the Pennatulid _Virgularia_ have been found in the stomachs of
cod and other fish, but with this exception there is no evidence that any
genus is systematically or even occasionally preyed upon by any animal.
With a very {340}few exceptions Alcyonaria show no signs of having been
torn, bitten, or wounded by carnivorous animals. It is improbable that the
presence of nematocysts in the tentacles can account for this immunity, as
it is known that some predaceous animals do feed upon Coelenterates
provided with much larger nematocysts than any Alcyonarian possesses. All
Alcyonaria, however, have a characteristic disagreeable odour, and it is
possible, as in many other cases, that this is accompanied by an unpleasant
taste. But if the Alcyonaria themselves are immune, it is possible that
their large yolk-laden eggs may form a not unimportant source of food
supply. In places where large colonies flourish, an immense number of eggs
or embryos must be discharged into the water during the spawning season,
and of these only a minute fraction can survive long enough to found a new
colony.
REPRODUCTION.—The formation of colonies by gemmation has frequently been
mentioned above. The young buds of a colony arise from the endoderm canals
in the body-wall of the zooids, in the general coenenchym, or in the
stolon. They never arise from evagination of the coelenteric cavities of
the zooids. There is no evidence that fission of a colony to form secondary
colonies ever occurs. Gemmation leads to the increase in the number of
zooids forming a colony, but not to an increase in the number of colonies.
Fission of the zooids is of extremely rare occurrence; a single case,
however, has been recorded by Studer in the genus _Gersemia_. Sexual
reproduction usually occurs once in a year; it is doubtful whether it ever
occurs continuously. The colonies appear to be nearly always dioecious,
only one case of hermaphroditism having yet been recorded.[370] The ova and
sperm sacs are usually formed and matured on the six ventral mesenteries,
rarely on the dorsal pair of mesenteries (Fig. 148, B) as well. The
spawning season varies with the locality. _Alcyonium digitatum_ spawns at
Plymouth at the end of December, and somewhat later at Port Erin. The
Pennatulid _Renilla_ and the Gorgonid _Leptogorgia_ spawn in the summer
months on the coast of North America. In the Mediterranean _Alcyonium
palmatum_ spawns in September and October (Lo Bianco), _Gorgonia cavolinii_
in May and June.
{341}It is not known for certain when the fertilisation of the ova is
effected, but in _Alcyonium digitatum_, and in the majority of the
Alcyonarians, it probably takes place after the discharge of the ova from
the zooids. A few forms are, however, certainly viviparous, the larvae of
_Gorgonia capensis_ being retained within the coelenteric cavity of the
parent zooid until they have grown to a considerable size. The other
viviparous Alcyonarians are _Corallium nobile_ (de Lacaze Duthiers), the
"Clavulaires petricoles," and _Sympodium coralloides_ (Marion and
Kowalevsky), and three species of _Nephthya_ found at depths of 269 to 761
fathoms (Koren and Danielssen). The general features of the development are
very similar in all Alcyonarians that have been investigated. The egg
contains a considerable amount of yolk, and undergoes a modified form of
segmentation. The free-swimming larva is called a "sterrula." It consists
of an outer layer of clear ciliated ectoderm cells, surrounding a solid
endodermic plasmodium containing the yolk. As the yolk is consumed a cavity
appears in the endoderm, and the larva is then called a "planula" (Fig.
150). The mouth is subsequently formed by an invagination of the ectoderm
at the anterior pole. The development of the mesenteries has not yet been
fully described.
[Illustration: FIG. 150.—Ciliated "planula" larva of _Alcyonium digitatum_.
_Ec_, Ectoderm; _End_, endoderm.]
CLASSIFICATION.—The sub-class Alcyonaria may conveniently be classified as
follows:—
Grade A. PROTALCYONACEA.
Grade B. SYNALCYONACEA.
Order 1. STOLONIFERA.
Order 2. COENOTHECALIA.
Order 3. ALCYONACEA.
Order 4. GORGONACEA.
Order 5. PENNATULACEA.
{342}GRADE A. PROTOALCYONACEA.
This Grade includes those genera which, like many sea-anemones, do not
reproduce by continuous gemmation to form colonies.
Several genera have been described, and they have been placed together in
one family called the HAIMEIDAE.
_Haimea funebris_, M. Edwards, was found off the coast of Algeria; _H.
hyalina_, Koren and Danielssen, in Norway; _Hartea elegans_, Wright, from
the Irish coast; _Monoxenia darwinii_, Haeckel, from the Red Sea, and a
large new species found by the "Siboga" Expedition in deep water off Ceram.
All these species, however, are very rare, and there is no satisfactory
evidence at present that they remain solitary throughout life.
GRADE B. SYNALCYONACEA.
The sub-division of the Synalcyonacea into orders presents many
difficulties, and several different classifications have been proposed.
Only two orders of the five that are here recognised are clearly defined,
namely, the Coenothecalia, containing the single living genus _Heliopora_,
and the Pennatulacea or Sea-pens; the others are connected by so many
genera of intermediate characters that the determination of their limits is
a matter of no little difficulty.
ORDER I. STOLONIFERA.
These are colonial Alcyonaria springing from a membranous or ribbon-like
stolon fixed to a stone or some other foreign object. The body-walls of the
individual zooids may be free or connected by a series of horizontal bars
or platforms (autothecalous); never continuously fused as they are in other
orders (coenothecalous).
In the simplest form of this order, _Sarcodictyon catenatum_ Forbes, the
ribbon-like strands of the stolon meander over the surface of stones,
forming a red or yellow network, from the upper surface of which the clear
transparent anthocodiae of the zooids protrude. When retracted the
anthocodiae are drawn down below the surface of the general coenenchym, and
their position is indicated by small cushion-like pads on the stolon.
{343}_Sarcodictyon_ is found in depths of 10 to 22 fathoms in the Irish
Sea, off the west coast of Scotland, the Shetlands, and off the Eddystone
Lighthouse, South Devon.
Another very important genus is _Tubipora_, in which the tubular body-wall
of each zooid is very much longer in proportion to its diameter than it is
in _Sarcodictyon_, and the anthocodia is retracted not into the stolon, but
into the basal part of the body-wall. The zooids are connected together by
horizontal platforms on which new zooids are formed by gemmation. Both
horizontal platforms and the body-walls of the zooids are provided with a
skeleton of fused spicules of a red colour.
This genus is the well-known Organ-pipe coral, and is found sometimes in
immense quantities on the coral reefs of both the old and new world.
It may be seen in pools on the edge of the reefs at low tides in colonies
frequently a foot or more in diameter. The tentacles are often of a bright
emerald green colour, and as the anthocodiae stand expanded in the clear
water they contribute a brilliant patch of colour to the many beauties of
their surroundings. When the coral is disturbed, or the water shallows and
the anthocodiae are retracted, the dull red colour of the skeleton
gradually takes the place of the bright green of the tentacles.
[Illustration: FIG. 151.—_Tubipora musica_, a young colony growing on a
dead Madrepore branch (_M_). _Hp_, The connecting horizontal platforms;
_p_, _p_, the skeletal tubes of the zooids; _St_, the basal stolon.]
It is probable that this order of Alcyonaria was better represented on the
reefs of some of the earlier periods of the world's history than it is at
present. The fossil _Syringopora_, which is found abundantly in the
carboniferous limestone and other strata, was probably an Alcyonarian
belonging to this order. It resembles _Tubipora_ in its mode of growth, but
in place of the horizontal platforms connecting the zooids there are rods
or bars from which new zooids spring (Fig. 152). Similar connecting bars
are found in the recent _Clavularia_ (_Hicksonia_, Delage) {344}_viridis_
of the East Indian reefs (Fig. 153). Other fossil forms belonging to the
order are _Favosites_, a very abundant coral of the Upper Silurian rocks,
and possibly _Columnaria_.
[Illustration: FIG. 152.—_Syringopora_, a fossil, showing autothecalous
tubes (_th_), funnel-shaped tabulae (_tab_), and tubular cross-bars (_t_).]
[Illustration: FIG. 153.—_Clavularia_ (_Hicksonia_) _viridis_, with
creeping stolon and transverse connecting tubes.]
The principal families of the Stolonifera are:—
Fam. 1. CORNULARIIDAE.—Without spicules; _Cornularia_, Lamarck,
Mediterranean; _Stereosoma_, Hickson, Celebes.
Fam. 2. CLAVULARIIDAE.—_Clavularia_, Quoy and Gaimard; _Sarcodictyon_,
Forbes, British; _Sympodium_, Ehrb.; _Syringopora_, Goldfuss, fossil.
Fam. 3. TUBIPORIDAE.—_Tubipora_, Linnaeus, tropical shallow water.
Fam. 4. FAVOSITIDAE.—_Favosites_, Lamarck; _Syringolites_, Hinde;
_Stenopora_, King.
ORDER II. COENOTHECALIA.
This order contains the single genus and species _Heliopora coerulea_ among
recent corals, but was probably represented by a large number of genera and
species in earlier periods.
{345}It is found at the present day in many localities in the warm shallow
waters of the tropical Pacific and Indian Oceans. It usually flourishes on
the inside of the reef, and may form masses of stone five or six feet in
diameter. The coral may easily be recognised, as it is the only one that
exhibits a blue colour. This colour usually penetrates the whole skeleton,
but in some forms is absent from the superficial layers.
The skeleton consists of a number of parallel tubes with imperforate walls,
which are fused together in honey-comb fashion. On making a vertical
section through a branch of the coral it is found that the tubes are
divided into a series of chambers by transverse partitions or "tabulae."
The soft living tissues of the coral, the zooids and coenosarc, are
confined to the terminal chambers, all the lower parts being simply dead
calcareous skeleton supporting the living superficial layer. Among the
parallel tubes there may be found a number of larger chambers that seem to
have been formed by the destruction of the adjacent walls of groups of
about nineteen tubes. These chambers are provided with a variable number of
pseudo-septa, and have a remarkable resemblance to the thecae of some
Zoantharian corals. That _Heliopora_ is not a Zoantharian coral was first
definitely proved by Moseley, who showed that each of these larger chambers
contains an Alcyonarian zooid with eight pinnate tentacles and eight
mesenteries. The zooids arise from a sheet of coenosarc that covers the
whole of the living branches of the coral mass, and this sheet of coenosarc
bears a plexus of canals communicating on the one hand with the zooids, and
on the other with a series of blind sacs, each of which occupies the cavity
of one of the skeletal tubes as far down as the first tabula. The zooids of
_Heliopora_ are very rarely expanded during the day-time, and it has been
found very difficult to get them to expand in an aquarium. The coral,
however, is frequently infested with a tubicolous worm allied to the genus
_Leucodora_, which freely expands and projects from the surface. So
constant and so numerous are these worms in some localities that it has
actually been suggested that _Heliopora_ should be regarded as a Polychaete
worm and not as an Alcyonarian. According to Mr. Stanley Gardiner, however,
these worms do not occur in association with the _Heliopora_ found on the
reefs of the Maldive Archipelago.
{346}There is very strong reason to believe that certain fossil corals were
closely related to _Heliopora_; that _Heliopora_ is in fact the solitary
survivor of a group of Alcyonarian corals that in past times was well
represented on the reefs, both in numbers and in species. The evidence is
not so convincing that other fossil corals are closely related to
_Heliopora_, and their true zoological position may remain a matter for
surmise. The order may be classified as follows:—
FAM. 1. HELIOLITIDAE.[371]—Coenothecalia with regular, well-developed
septa, generally twelve in number, in each calicle.
_Heliolites_, Dana, Silurian and Devonian. _Cosmiolithus_, Lindström, Upper
Silurian. _Proheliolites_, Klaer, Lower Silurian. _Plasmopora_, Edwards and
Haime, Upper Silurian. _Propora_, E. and H., Upper Silurian.
_Camptolithus_, Lindström, Upper Silurian. _Diploëpora_, Quenst, Upper
Silurian. _Pycnolithus_, Lindström, Upper Silurian.
FAM. 2. HELIOPORIDAE.[372]—Coenothecalia with small irregularly arranged
coenosarcal caeca, and a variable number of septa or septal ridges.
_Heliopora_, de Blainville, recent, Eocene and Upper Cretaceous.
_Polytremacis_, d'Orbigny, Eocene and Upper Cretaceous. _Octotremacis_,
Gregory, Miocene.
The family COCCOSERIDAE is regarded by Lindström as a sub-family of the
Heliolitidae, and the families THECIDAE and CHAETETIDAE are probably
closely related to the Helioporidae.
ORDER III. ALCYONACEA.
This order contains a large number of genera of great variety of form. The
only characters which unite the different genera are that the body-walls of
some groups of zooids, or of all the zooids, are fused together to form a
common coenenchym penetrated by the coenosarcal canals, and that the
spicules do not fuse to form a solid calcareous, or horny and calcareous,
axial skeletal support.
The affinities with the order Stolonifera are clearly seen in the genera
_Xenia_ and _Telesto_. Some species of _Xenia_ form flattened or domed
colonies attached to stones or corals, with non-retractile anthocodiae and
body-walls united for only a {347}short distance at the base. Young _Xenia_
colonies are in fact Stolonifera in all essential characters. In _Telesto
prolifera_ we find a network of stolons encrusting coral branches and other
objects after the manner of the stolons of many species of _Clavularia_,
although the zooids do not arise from these stolons singly, but in groups,
with their body-walls fused together for a certain distance. In _Telesto
rubra_ the spicules of the body-walls are fused together to form a series
of perforated tubes very similar in some respects to the tubes of
_Tubipora_.
A remarkable genus is _Coelogorgia_. Here we find a branching colony
arising from a basal stolon, and the axis of the main stem and of each
branch consists of a single very much elongated zooid bearing on its
thickened walls the branches of the next series and other zooids. It is
true that in this genus there is very little fusion of neighbouring zooids,
and the amount of true coenenchym is so small that it can hardly be said to
exist at all. Bourne[373] has united this genus with _Telesto_ into a
family Asiphonacea, which he joins with the Pennatulida in the order
Stelechotokea; but their affinities seem to be closer with the Alcyonacea
than with the Pennatulacea, from which they differ in many important
characters.
[Illustration: FIG. 154.—_Alcyonium digitatum_, a single-lobed specimen,
with some of the zooids expanded.]
The genus _Alcyonium_ not only contains the commonest British Alcyonarian
(_A. digitatum_), but it is one of the most widely distributed genera of
all Alcyonaria that occur in shallow water.
The genera _Sarcophytum_ and _Lobophytum_ occur in shallow water in the
tropics of the old world. The former frequently consists of huge toad-stool
shaped masses, soft and spongy in {348}consistency, of a green, brown, or
yellow colour. On some reefs the colonies of _Sarcophytum_ form a very
conspicuous feature, and from their very slimy, slippery surface, add to
the minor dangers of wading in these regions. Both genera are dimorphic.
Some species of the genus _Sclerophytum_,[374] which occur in the Indian
Ocean, are so hard and brittle that they might readily be mistaken for a
Zoantharian coral. This character is due to the enormous number of tightly
packed spicules borne by the coenenchym. Some of these spicules in _S.
querciforme_ are 7 mm. × 1.7 mm.; the largest, though not the longest
(_vide_ p. 335) of any spicules occurring in the order.
Another very important genus occurring on coral reefs, and of very wide
distribution, is _Spongodes_. This genus forms bushy and rather brittle
colonies of an endless variety of beautiful shapes and colours. Arising
from the neck of each anthocodia there are one or two long, sharp,
projecting spicules, which give the surface a very spiny or prickly
character.
The genera _Siphonogorgia_ and _Chironephthya_ form large brittle,
branching colonies which might readily be mistaken for Gorgonians. The
strength of the branches, however, is mainly due to the large, densely
packed, spindle-shaped spicules at the surface of the coenenchym, the long
coelenteric cavities of the zooids penetrating the axis of both stem and
branches. _Siphonogorgia_ is usually uniformly red or yellow in colour.
_Chironephthya_, on the other hand, exhibits a great variety of colour in
specimens from the same reef, and indeed in different branches of the same
colony.
FAM. 1. XENIIDAE.—Alcyonacea with non-retractile zooids. Spicules very
small discs, usually containing a relatively small proportion of lime.
_Xenia_, Savigny; Indian Ocean and Torres Straits. _Heteroxenia_, Kölliker;
Red Sea, Cape of Good Hope, and Torres Straits.
FAM. 2. TELESTIDAE.—Colonies arising from an encrusting membranous or
branching stolon. The erect stem and branches are formed by the body-walls
of two or three zooids only, from which secondary zooids and branches of
the next order arise.
_Telesto_, Lamouroux, widely distributed in warm waters of the Atlantic,
Pacific, and Indian Oceans. The genus _Fascicularia_, Viguier, from the
coast of Algiers, seems to be related to _Telesto_, {349}but the groups of
zooids are short, and do not give rise to branches.
FAM. 3. COELOGORGIIDAE.—The colony arborescent, attached by stolon-like
processes. The stem formed by an axial zooid with thickened body-walls.
Branches formed by axial zooids of the second order, and branchlets by
axial zooids of the third order, borne either on two sides or in spirals by
the main stem. Genus _Coelogorgia_, Zanzibar.
FAM. 4. ALCYONIIDAE.—The colonies of this family are usually soft and
fleshy, and the spicules, evenly distributed throughout the coenenchym, do
not usually fuse or interlock to form a continuous solid skeleton. They may
be unbranched or lobed, never dendritic in form. The principal genera
are:—_Alcyonium_, Linnaeus, cosmopolitan, but principally distributed in
temperate and cold waters. _Alcyonium digitatum_ is the commonest British
Alcyonarian. It is found in shallow water, from the pools left at low
spring tides to depths of 40 or 50 fathoms, at most places on the British
shores. It is stated by Koehler to descend into depths of over 300 fathoms
in the Bay of Biscay. There are two principal varieties; one is white or
pale pink in the living condition, and the other yellow. In some localities
the two varieties may be found in the same pools. Another species,
_Alcyonium glomeratum_, placed in a distinct genus (_Rhodophyton_) by Gray,
and distinguished from the common species by its red colour and long
digitate lobes, is found only off the coast of Cornwall. _Paralcyonium_,
Milne Edwards; Mediterranean. _Sclerophytum_, Pratt; sometimes dimorphic,
Indian Ocean. _Sarcophytum_, Lesson; dimorphic, principally tropical.
_Lolophytum_, Marenzeller; dimorphic, tropical. _Anthomastus_, Verrill;
dimorphic, Atlantic Ocean, deep water. _Acrophytum_, Hickson; dimorphic,
Cape of Good Hope.
FAM. 5. NEPHTHYIDAE.—Colonies dendritic. Usually soft and flexible in
consistency. _Nephthya_, Savigny; Indian and Pacific Oceans. _Spongodes_,
Lesson; widely distributed in the Indian and Pacific Oceans.
FAM. 6. SIPHONOGORGIIDAE.—Colonies often of considerable size. Dendritic.
Spicules usually large and abundant, giving a stiff, brittle consistency to
the stem and branches. _Siphonogorgia_, Kölliker; Red Sea, Indian, and
Pacific tropics. _Chironephthya_, Wright and Studer; Indian and Pacific
Oceans. _Lemnalia_, {350}Gray; Zanzibar. _Agaricoides_, Simpson;[375]
Indian Ocean, 400 fathoms.
ORDER IV. GORGONACEA.
This order contains a very large number of dendritic and usually flexible
corals occurring in nearly all seas and extending from shallow waters to
the very great depths of the ocean. A large proportion of them are brightly
coloured, and as the principal pigments are fixed in the spicules, and are
therefore preserved when the corals are dead and dried, they afford some of
the most attractive and graceful objects of a natural history museum.
The only character that separates them from the Alcyonacea is that they
possess a skeletal axis that is not perforated by the coelenteric cavities
of the zooids. The coelenteric cavities are usually short. The order may
conveniently be divided into two sub-orders.
SUB-ORDER 1. PSEUDAXONIA.
The axis in this sub-order consists of numerous spicules tightly packed
together, or cemented together by a substance which is probably allied to
horn in its chemical composition. This substance may be considerable in
amount, in which case it remains after decalcification as a spongy, porous
residue; or it may be so small in amount, as in _Corallium_, that the axis
appears to be composed of solid carbonate of lime. The statement is usually
made that the axis is penetrated by nutritive canals in certain genera, but
the evidence upon which this is based is unsatisfactory and in some cases
unfounded. There can be no doubt, however, that in some genera the axis is
porous and in others it is not, and this forms a useful character for the
separation of genera.
FAM. 1. BRIAREIDAE.—The medullary substance consists of closely packed but
separate spicules embedded in a soft horny matrix, which is uniform in
character throughout its course. Nearly all the genera form dendritic
colonies of considerable size.
The principal genera are:—_Solenocaulon_, Gray; Indian Ocean and North
Australia. Many of the specimens of this genus have fistulose stems and
branches. The tubular character of the stem and branches is probably caused
by the activity of a Crustacean, {351}_Alpheus_, and may be regarded as of
the nature of a gall-formation.[376] _Paragorgia_, M. Edwards; Norwegian
fjords, in deep water. This genus forms very large tree-like colonies of a
ruby-red or white colour. It is perhaps the largest of the dendritic
Alcyonarians. It is dimorphic. _Spongioderma_, Kölliker; Cape of Good Hope.
The surface of this form is always covered by an encrusting sponge.
_Iciligorgia_, Ridley; Torres Straits. The stem and branches are compressed
and irregular in section.
FAM. 2. SCLEROGORGIIDAE.—The medullary mass forms a distinct axis
consisting of closely packed elongate spicules with dense horny sheaths.
_Suberogorgia_, Gray, has a wide distribution in the Pacific Ocean, Indian
Ocean, and the West Indies. _Keroeides_, W. and S., comes from Japan.
FAM. 3. MELITODIDAE.—The axis in this family exhibits a series of nodes and
internodes (Fig. 155), the former consisting of pads formed of a horny
substance with embedded spicules, the latter of a calcareous substance with
only traces of a horny matrix. The internodes are quite rigid, the nodes
however give a certain degree of flexibility to the colony as a whole.
Neither the nodes nor the internodes are penetrated by nutritive canals,
but when dried the nodes are porous.
[Illustration: FIG. 155.—_Melitodes dichotoma_, showing the swollen nodes
and the internodes.]
The principal genera are:—_Melitodes_, Verrill; widely distributed in the
Indian and Pacific Oceans, Cape of Good Hope, etc. This genus is in some
localities extremely abundant and exhibits great brilliancy and variety of
colour. The branching is usually dichotomous at the nodes. _Wrightella_,
Gray. This is a delicate dwarf form from Mauritius and the coast of South
Africa. _Parisis_, Verrill; Pacific Ocean from Formosa to Australia but not
very common. One species from Mauritius. The branches arise from the
internodes.
{352}FAM. 4. CORALLIIDAE.—The axis is formed by the fusion of spicules into
a dense, solid, inflexible, calcareous core.
_Corallium_, Lamarck. _Corallium nobile_, Pallas, the "precious coral,"
occurs in the Mediterranean, chiefly off the coast of North Africa, but
also on the coasts of Italy, Corsica, Sardinia, and it extends to the Cape
Verde Islands in the Atlantic Ocean. _C. japonicum_, Kishinouye, called
Akasango by the fishermen, occurs off the coast of Japan, and _C. reginae_,
Hickson, has recently been described from deep water off the coast of
Timor.[377] The genus _Pleurocorallium_, Gray, is regarded by some authors
as distinct, but the characters that are supposed to distinguish it,
namely, the presence of peculiar "opera-glass-shaped spicules," and the
occurrence of the verrucae on one side of the branches only, are not very
satisfactory. The following species are therefore placed by Kishinouye[378]
in the genus _Corallium_:—_C. elatius_, Ridley (Momoirosango); _C.
konojoi_, Kishinouye (Shirosango); _C. boshuensis_, K.; _C. sulcatum_, K.;
_C. inutile_, K.; and _C. pusillum_, K.,—all from the coast of Japan. Of
the coral obtained from these species, the best kinds of Momoirosango vary
in price from £30 per pound downwards according to the quality. The
Shirosango is the least valuable of the kinds that are brought into the
market, and is rarely exported.[379] Three species of _Corallium_
(_Pleurocorallium_) have been described from Madeira,[380] and one of
these, _C. johnsoni_, has recently been found in 388 fathoms off the coast
of Ireland.[381] Other species are _C. stylasteroides_, from Mauritius; _C.
confusum_, Moroff,[382] from Sagami Bay in Japan; and an undescribed
species obtained by the "Siboga," off Djilolo. These corals range from
shallow water to depths of 300-500 fathoms. _Pleurocoralloides_, Moroff,
differs from the others in having very prominent verrucae and in the
character of the large spindle-shaped and scale-like spicules. It was found
in Sagami Bay, Japan. Specimens attributed to the genus _Pleurocorallium_
have been found fossil in the white chalk of France, but Corallium has been
found only in the tertiaries.[383]
{353}SUB-ORDER 2. AXIFERA.
The axis in this sub-order may be horny, or horny with a core of calcium
carbonate, or composed of horn impregnated with calcium carbonate, or of
nodes of horn alternating with internodes of calcium carbonate. It may be
distinguished from the axis of the Pseudaxonia by the fact that in no case
have definite spicules been observed to take part in its formation. It has
been suggested that as the Axifera represent a line of descent distinct
from that of the Pseudaxonia they should be placed in a separate order.
Apart from the character of the axis, however, the two sub-orders show so
many affinities in their general anatomy that it is better to regard the
two lines of descent as united within the Gorgonacean limit. It is very
improbable that the two groups sprang independently from a stoloniferous
ancestor.
FAM. 1. ISIDAE.—This family includes all those Axifera in which the axis is
composed of alternate nodes of horn and internodes of calcareous substance.
There can be little doubt of the close affinities of many of the genera of
this family with the Melitodidae among the Pseudaxonia. In both the
coenenchym is thin and the coelenteric cavities short. No important
differences have been observed between the structure of the zooids of the
two families, and now that we know that the "nutritive canals" of
_Melitodes_ do not perforate the nodes there is no important difference
left between the coenosarcal canal systems. The structure and method of
calcification of the internodes of the two families are very similar. The
main difference between them is that the nodes of the Isidae are purely
horny, whereas in the Melitodidae the horny substance of the nodes contains
calcareous spicules.
The principal genera are:—_Isis_, Linnaeus; Pacific Ocean. This genus forms
substantial fan-shaped colonies with, relatively, a thick coenenchym, short
stout internodes and black horny nodes. _Mopsea_, Lamouroux; Coast of
Australia. The verrucae are club-shaped and are arranged in spiral rows
round the stem. _Acanella_, Gray; principally found in deep water in the
Atlantic Ocean but also in the Pacific. The internodes are long and the
branches arise from the nodes. Most of the species occur in deep water,
some in very deep water (_A. simplex_, 1600 to 1700 fathoms). In this and
the following genera the coenenchym is {354}thin and the zooids imperfectly
or not retractile. _Ceratoisis_, Wright; Atlantic Ocean, extending from
shallow to deep water. The branches arise from the nodes. _Chelidonisis_,
Studer; deep water off the Azores. _Isidella_, Gray; Mediterranean Sea.
_Bathygorgia_, Wright; off Yokohama, 2300 fathoms. This genus is
unbranched, with very long internodes and short nodes. The zooids are
arranged on one side only of the stem.
FAM. 2. PRIMNOIDAE.—This is a well-marked family. The axis of the colonies
is horny and calcareous. The coenenchym and the non-retractile zooids are
protected by scale-like spicules, which usually overlap and form a complete
armour for the protection of the soft parts. On the aboral side of the base
of each tentacle there is a specialised scale, and these fit together, when
the tentacles are folded over the peristome, to form an operculum.
The principal genera are:—_Primnoa_, Lamouroux; Atlantic Ocean, occurring
also in the Norwegian fjords. This genus is usually found in moderately
deep water, 100 to 500 fathoms. _Primnoella_, Gray. This genus seems to be
confined to the temperate seas of the southern hemisphere. It is
unbranched. The zooids are arranged in whorls round the long whip-like
stem. _Plumarella_, Gray; southern hemisphere, in moderately deep water.
This is branched pinnately in one plane. The zooids are small and arise at
considerable intervals alternately on the sides of the branches.
_Stenella_, Gray; widely distributed in deep water. The zooids are large
and are arranged in whorls of three situated at considerable distances
apart. _Stachyodes_, W. and S.; Fiji, Kermadecs, Azores, in deep water.
Colony feebly branched. Zooids in regular whorls of five. Other genera
belonging to this group of Primnoidae are _Thouarella_, Gray, and
_Amphilaphis_, Antarctic seas.
The following genera are placed in separate sub-families:—_Callozostron_,
Wright; Antarctic Sea, 1670 fathoms. The axis is procumbent and the zooids
are thickly set in rows on its upper surface. The zooids are protected by
large imbricate scales, of which those of the last row are continued into
long spine-like processes. _Calyptrophora_, Gray; Pacific Ocean, in deep
water. The base of the zooids is protected by two remarkably large scales.
_Primnoides_, W. and S.; Southern Ocean. The opercular scales are not
distinctly differentiated and the calyx is therefore imperfectly protected.
{355}FAM. 3. CHRYSOGORGIIDAE.[384]—The axis in this family is composed of a
horny fibrous substance with interstratified calcareous particles, and it
springs from a calcareous plate, which sometimes gives off root-like
processes. It may be unbranched or branched in such a way that the branches
of the second, third, and subsequent orders assume in turn the direction of
the base of the main axis. The axis is frequently of a metallic iridescent
appearance. The zooids usually arise in a single straight or spiral row on
the branches, and are not retractile. The coenenchym is thin. The spicules
vary considerably, but in a very large proportion of the species they are
thin, oval, or hour-glass plates (Fig. 149, 10, p. 336).
By some authors this family is considered to be the simplest and most
primitive of the Axifera; but the delicate character of the axis of the
main stem and branches, the thinness of the coenenchym, the position of the
zooids on one side of the branches only, and the tenuity of the calcareous
spicules may be all accounted not as primitive characters, but as special
adaptations to the life in the slow uniform currents of deep water.
The principal genera are:—_Lepidogorgia_, Verrill; Atlantic and Pacific
Oceans, 300 to 1600 fathoms. Axis unbranched. Zooids large and arranged in
a single row. _Trichogorgia_, Hickson; Cape of Good Hope, 56 fathoms.
Colony branching in one plane. Zooids numerous and on all sides of the
branches. _Chrysogorgia_, D. and M.; deep water. Axis branched. Spicules on
the zooids always large. _Metallogorgia_, Versluys; Atlantic Ocean, 400 to
900 fathoms. Basal part of the stem unbranched (monopodial). _Iridogorgia_,
Verrill. Spiral stem and branches. _Pleurogorgia_, Versluys. Axis branched
in one plane. Coenenchym thick. _Riisea_, D. and M. Monopodial stem and
thick coenenchym.
FAM. 4. MURICEIDAE.—This is a large family, exhibiting very great variety
of habit. The spicules are often very spiny, and project beyond the surface
of the ectoderm, giving the colony a rough appearance. A great number of
genera have been described, but none of them are very well known. The
family requires careful revision.
The more important genera are:—_Acanthogorgia_, Gray; principally in deep
water in the Atlantic Ocean. The calices are {356}large, cylindrical, and
spiny. _Villogorgia_, D. and M.; widely distributed. Delicate, graceful
forms, with thin coenenchym. _Echinomuricea_, Verrill; _Muricea_,
Lamouroux; _Paramuricea_, Köll; _Acamptogorgia_, W. and S.; _Bebryce_,
Philippi.
FAM. 5. PLEXAURIDAE.—In this family we find some of the largest and most
substantial Gorgonids. The axis is usually black, but its horny substance
may be impregnated with lime, particularly at the base. The coenenchym is
thick, and the zooids are usually completely retractile, and the surface
smooth. The species of the family are principally found in shallow water in
warm or tropical regions.
The principal genera are:—_Eunicea_, Lamouroux. The calices are prominent,
and not retractile. _Plexaura_, Lamouroux; _Euplexaura_, Verrill.
_Eunicella_, Verrill. With an outer layer of peculiar torch-shaped
spicules. The only British species of this order is _Eunicella cavolini_
(formerly called _Gorgonia verrucosa_). It is found in depths of 10 to 20
fathoms off the coast of the English Channel and west of Scotland.
Occasionally specimens are found in which a gall-like malformation with a
circular aperture is seen, containing a Barnacle. Such gall formations,
common enough in some species of Madreporaria, are rarely found in
Alcyonaria.
[Illustration: FIG. 156.—_Eunicella cavolini._ Some branches of a large
dried specimen, showing a gall formed by a Cirripede.]
FAM. 6. GORGONIIDAE.—This family contains some of the commonest and
best-known genera of the order. They usually form large flexible branched
colonies with delicate horny axes and thin coenenchym. The zooids are
usually completely retractile.
The principal genera are:—_Gorgonia_, Linn. This genus {357}includes
_Gorgonia_ (_Rhipidogorgia_) _flabellum_, the well-known fan Gorgonia with
intimately anastomosing branches, from the warm waters of the Atlantic
Ocean. The genera _Eugorgia_, Verrill, and _Leptogorgia_, Milne Edwards,
differ from _Gorgonia_ in the character of the spicules. In _Xiphigorgia_,
Milne Edwards, from the West Indies, the branches are much compressed,
forming at the edges wing-like ridges, which bear the zoopores in rows.
_Malacogorgia_, Hickson, has no spicules. Cape of Good Hope.
FAM. 7. GORGONELLIDAE.—In this family the horny axis is impregnated with
lime. The surface of the coenenchym is usually smooth, and the spicules
small. The colonies are sometimes unbranched (_Juncella_). In the branching
forms the axis of the terminal branches is often very fine and thread-like
in dimensions.
[Illustration: FIG. 157.—_Verrucella guadaloupensis_, with an epizoic
Brittle star (_Oph._) of similar colour.]
The principal genera are:—_Gorgonella_, with a ramified flabelliform axis;
_Ctenocella_, with a peculiar double-comb manner of branching; and
_Juncella_, which forms very long unbranched or slightly branched colonies,
with club-shaped spicules. All these genera are found in shallow water in
the tropical or semi-tropical regions of the world. _Verrucella_ is a genus
with delicate anastomosing branches found principally in the shallow
tropical waters of the Atlantic shores. Like many of the Gorgonacea, with
branches disposed in one plane (flabelliform) _Verrucella_ frequently
carries a considerable number of epizoic Brittle stars, which wind their
flexible arms round the branches, and thus obtain a firm attachment to
their host. There is no reason to suppose that these Brittle stars are in
any sense parasitic, as a specimen that bears many such forms shows no sign
of injury or degeneration, and it is possible they may even be of service
to {358}the _Verrucella_ by preying upon other organisms that might be
injurious. An interesting feature of the association is that the Brittle
stars are of the same colour as the host, and the knob-like plates on their
aboral surface have a close resemblance to the verrucae (Fig. 157).
ORDER V. PENNATULACEA.
The Sea-pens form a very distinct order of the Alcyonaria. They are the
only Alcyonarians that are not strictly sedentary in habit, that are
capable of independent movement as a whole, and exhibit a bilateral
symmetry of the colony. No genera have yet been discovered that can be
regarded as connecting links between the Pennatulacea and the other orders
of the Alcyonaria. Their position, therefore, is an isolated one, and their
relationships obscure.
The peculiarities of the order are due to the great growth and modification
in structure of the first formed zooid of the colony. This zooid (Oozooid,
Hauptpolyp, or Axial zooid) increases greatly in length, develops very
thick fleshy walls, usually loses its tentacles, digestive organs, and
frequently its mouth, exhibits profound modification of its system of
mesenteries, and in other ways becomes adapted to its function of
supporting the whole colony.
[Illustration: FIG. 158.—Diagram of a Sea-pen. _L_, leaves composed of a
row of autozooids; _R_, rachis; _St_, stalk; _T_, anthocodia of the axial
zooid, usually suppressed. (After Jungersen.)]
The axial zooid shows from an early stage of development a division into
two regions: a distal region which produces by gemmation on the body-wall
numerous secondary zooids, and becomes the rachis of the colony; and a
proximal region which becomes the stalk or peduncle, and does not produce
buds (Fig. 158). The secondary zooids are of two kinds: {359}the autozooids
and the siphonozooids. The former have the ordinary characters of an
Alcyonarian zooid, and produce sexual cells; the latter have no tentacles,
a reduced mesenteric system, and a stomodaeum provided with a very wide
siphonoglyph.
The arrangement of the autozooids and siphonozooids upon the axial zooid is
subject to great modifications, and affords the principal character for the
classification of the order. In the Pennatuleae the autozooids are arranged
in two bilaterally disposed rows on the rachis, forming the leaves or
pinnae of the colony (Fig. 158). The number in each leaf increases during
the growth of the colony by the addition of new zooids in regular
succession from the dorsal to the ventral side of the rachis[385] (Fig.
159). In other Pennatulacea the autozooids are arranged in rows which do
not unite to form leaves (_Funiculina_), in a tuft at the extremity of a
long peduncle (_Umbellula_), scattered on the dorsal side of the rachis
(_Renilla_, Fig. 160), or scattered on all sides of the rachis
(_Cavernularia_, Fig. 161). In those forms in which the autozooids are
scattered the bilateral symmetry of the colony as a whole becomes obscured.
The siphonozooids may be found on the leaves (_Pteroeides_), but more
frequently between the leaves or rows of autozooids, or scattered
irregularly among the autozooids. Usually the siphonozooids are of one kind
only, but in _Pennatula murrayi_ there is one specially modified
siphonozooid at the base of each leaf,[386] which appears to have some
special but unknown function.
[Illustration: FIG. 159.—Diagram of a portion of a rachis of a Sea-pen,
_aut_, The rows of autozooids; 1-6, the order of age of the autozooids
composing a leaf; _D_, the dorsal side of the rachis; _Si_, the
siphonozooids; _V_, the ventral side of the rachis. (After Jungersen.)]
In _Umbellula gracilis_ each siphonozooid bears a single pinnate tentacle,
and in some other species of the same genus there is a tentacle which is
not pinnate.[387]
{360}The zooids and coenenchym are usually protected by a crust of coloured
or colourless, long, smooth, needle-like, calcareous spicules, situated
principally in the superficial layer, so as to leave the subjacent tissues
soft and spongy in texture. In some cases the spicules are smooth double
clubs, rods, discs, or irregular granules, and in _Sarcophyllum_,
_Chunella_, some species of _Umbellula_ and others, there is no calcareous
skeleton. The tuberculated spindles, so common in other Alcyonaria, are not
found in any species. In most genera a horny, or calcified horny rod is
embedded in the central part of the axial polyp, serving as a backbone or
support for its muscles. It is absent, however, in _Renilla_, and reduced
or absent in _Cavernularia_.
The sexual organs are borne by the mesenteries of the autozooids only, and
each colony is either male or female. There is no record of hermaphroditism
in the order. The eggs contain a considerable amount of yolk, and
fertilisation is effected in the sea-water after their discharge. The
segmentation is irregular, and the free-swimming ciliated larva (of
_Renilla_) shows the rudiments of the first buds from the axial polyp
before it settles down in the mud.
The Sea-pens are usually found on muddy or sandy sea-bottoms, from a depth
of a few fathoms to the greatest depths of the ocean. It is generally
assumed that their normal position is one with the peduncle embedded in the
mud and the rachis erect. Positive evidence of this was given by Rumphius,
writing in 1741, in the case of _Virgularia rumphii_ and _V. juncea_ at
Amboina,[388] and by Darwin in the case of _Stylatula darwinii_ at Bahia
Blanca.[389]
"At low water," writes Darwin, "hundreds of these zoophytes might be seen
projecting like stubble, with the truncate end upwards, a few inches above
the surface of the muddy sand. When touched or pulled they suddenly drew
themselves in with force so as nearly or quite to disappear."
It is not known whether the Pennatulids have the power of moving from place
to place when the local conditions become unfavourable. It is quite
probable that they have this power, but the accounts given of the Sea-pens
lying flat on the sand do not appear to be founded on direct observation.
The fable of {361}_Pennatula_ swimming freely "with all its delicate
transparent polypi expanded, and emitting their usual brilliant
phosphorescent light, sailing through the still and dark abyss by the
regular and synchronous pulsations of the minute fringed arms of the whole
polypi," appears to be based on a statement made by Bohadsch in 1761, and
picturesque though it be, is undoubtedly erroneous.
The brilliant phosphorescence of many species of Pennatulacea has been
observed by many naturalists, and it is very probable that they all exhibit
this property to some degree. The phosphorescence appears to be emitted by
the mesenteric filaments of the autozooids, but it is not yet determined
whether the phenomenon is confined to these organs or is more generally
distributed.
The Pennatulacea are usually devoid of epizoites, but occasionally the
parasitic or semi-parasitic Entomostracan _Lamippe_ is found in the zooids.
A small crab is also frequently found between the large leaves of species
of _Pteroeides_. The most remarkable case of symbiosis, however, has
recently been observed in the form of an encrusting Gymnoblastic
Hydroid[390] living on the free edge of the leaves of a species of
_Ptilosarcus_.
The order Pennatulacea is divided into four sections.
SECT. 1. PENNATULEAE.—In this section the colony is distinctly bilaterally
symmetrical, and the autozooids are arranged in rows with their body-walls
fused to form leaves.
The genus _Pteroeides_, the representative genus of the family
PTEROEIDIDAE, is a fleshy Sea-pen found in shallow sea water in the warm
waters of the Pacific Ocean and in the Mediterranean. It has large leaves
with long spiny, projecting spicules, and the siphonozooids are borne by
the leaves. _Pennatula_, the representative genus of the family
PENNATULIDAE, has a wider distribution in area and in depth. _Pennatula
phosphorea_ is a common British species, found in depths of 10 to 20
fathoms in many localities off our coasts. It is about 5 inches in length.
There are several varieties of this species distributed in Atlantic waters.
_Pennatula grandis_ is a magnificent species found in Norwegian fjords, in
the Faeroe Channel, and off the northern coasts of N. America, in depths of
from 50 to 1255 fathoms. Specimens have been {362}obtained no less than 2½
feet in length. _P. murrayi_ and _P. naresi_ are species of the genus found
at depths of a few hundred fathoms in tropical seas.
The genus _Virgularia_, belonging to the family VIRGULARIIDAE, is
represented in the British seas by _V. mirabilis_, a long slender Sea-pen
found in many localities off the Scottish coasts.
SECT. 2. SPICATAE.—This section includes those Sea-pens in which the
autozooids are arranged bilaterally on the axial zooid in rows or more
irregularly, but do not unite to form leaves. It is a large section and
contains many widely divergent genera.
The family FUNICULINIDAE is represented on our coasts by _Funiculina
quadrangularis_, a long and slender Sea-pen 2 to 3 feet in length. The
autozooids are arranged in oblique rows, and the siphonozooids are on the
ventral side of the rachis. There is one point of special interest in this
genus. The siphonozooids appear to change as the colony grows and to become
autozooids. If this is the case it may be more correct to describe the
genus as devoid of true siphonozooids.
The family ANTHOPTILIDAE contains the species _Anthoptilum grandiflorum_,
which has a wide distribution in depths of 130 to 500 fathoms in the N. and
S. Atlantic Ocean. It is perhaps the largest of all the Pennatulacea,
specimens having been obtained from the Cape of Good Hope over 4 feet long
with expanded autozooids, each more than half an inch in length.
The family KOPHOBELEMNONIDAE contains a number of forms with remarkably
large autozooids arranged in irregular rows on the two sides of the rachis.
The siphonozooids are numerous and scattered, and their position is
indicated by small papilliform calices on the coenenchym. The surface of
these pens is usually rough, owing to the presence of numerous coarse
projecting spicules. _Kophobelemnon_ occurs in the Mediterranean in deep
water, off the coasts of Ireland and Scotland, and in other regions.
The family UMBELLULIDAE contains some of the most remarkable and
interesting examples of the deep-sea fauna. The peduncle is very long and
the rachis stunted and expanded. The autozooids are of great size,
non-retractile, and arranged in a cluster or rosette on the terminal
rachis. There is a wide structural range between the species. Some species
have numerous large spicules, others have none. In some species the
siphonozooids have a single pinnate or digitate tentacle, in others the
siphonozooids {363}are of the usual type. _Umbellula_ appears to be a
somewhat rare but cosmopolitan genus in deep water, extending from the
Arctic to the Antarctic region in water ranging from 200 to 2500 fathoms.
The interesting genus _Chunella_ was discovered by the German "Valdivia"
Expedition at a depth of about 420 fathoms off the coast of E. Africa, and
subsequently by the Dutch "Siboga" Expedition at a depth of about 500
fathoms in the Malay Archipelago. According to Kükenthal,[391] this genus
with another closely allied genus _Amphianthus_ should form a new section
of Pennatulacea, the VERTICILLADEAE. _Chunella_ has a long and very
delicate rachis and peduncle, and the former terminates in a single
autozooid and has five or six whorls of three autozooids, situated at
considerable distances from one another. Spicules are absent. The full
description of this genus has not yet been published, but it is clear that
it occupies a very isolated position in the order.
[Illustration: FIG. 160.—_Renilla reniformis_, a small specimen (34 mm.),
showing the dorsal side of the expanded rachis. _A_, autozooid; _H_, the
mouth of the axial zooid; _s_, siphonozooid; _St_, the short stalk. (After
Kölliker.)]
SECT. 3. RENILLEAE.—This section contains a single family RENILLIDAE and a
single genus _Renilla_ (Fig. 160). The rachis is expanded into a flattened
cordate form set at an angle to the peduncle, and the zooids are confined
to the dorsal surface, which is uppermost in the natural position of the
colony. The peduncle is short and does not contain an axial skeleton. The
colour of {364}this Sea-pen is usually violet when dried or preserved.
Specimens of _Renilla_ are very abundant in shallow water in some
localities on the Atlantic and Pacific coasts of N. America, but the genus
has also been obtained from the Red Sea and the coast of Australia. A
popular name for this genus is "Sea pansy."
SECT. 4. VERETILLEAE.—This section contains a number of genera in which the
bilateral arrangement of the zooids is obscured by their gradual
encroachment on the dorsal side of the axial polyp. The rachis and peduncle
are thick and fleshy, and the autozooids and siphonozooids are irregularly
distributed all round the rachis. The genus _Cavernularia_ is not
uncommonly found in moderate depths of water in the Indian and Pacific
Ocean, and is distinguished from the other genera by the reduction of the
skeletal axis. Other genera are _Veretillum_, Mediterranean and Atlantic
Ocean, and _Lituaria_, Indian Ocean.
[Illustration: FIG. 161.—_Cavernularia obesa._ _Au_, autozooid; _Si_,
siphonozooid; _St_, stalk. (After Kölliker.)]
{365}CHAPTER XIV
ANTHOZOA (_CONTINUED_): ZOANTHARIA
SUB-CLASS II. ZOANTHARIA.
The Zoantharia exhibit a great deal more diversity of form and structure
than the Alcyonaria. The sub-class is consequently difficult to define in a
few words, and it may be taken to include all the Anthozoa which do not
possess the typical Alcyonarian characters.
All the orders, with the exception of the Antipathidea and Zoanthidea,
contain genera of solitary zooids, and the orders Edwardsiidea and
Cerianthidea contain no genera that form colonies. In the Madreporaria,
Zoanthidea, and Antipathidea, on the other hand, colonies are formed
composed of a very large number of individuals which frequently attain to a
very great size. The term "Sea-anemone" is commonly used in writing about
the solitary Zoantharia which do not form any skeletal structures, and the
term "Coral" is applied to all those Zoantharia which do form a skeleton.
In a scientific treatise, however, these popular terms can no longer be
satisfactorily employed. The "Sea-anemones" exhibit so many important
differences in anatomical structure that they must be placed in at least
three distinct orders that are not closely related, and the organisms to
which the term Coral has been applied belong to so many organisms—such as
Alcyonaria, Hydrozoa, Polyzoa, and even Algae—that its use has become
indeterminate.
Whilst these terms must disappear from the systematic part of Zoology, they
may still be employed, however, in the description of a local fauna or
coral reef to signify the soft solitary zooids on {366}the one hand, and
the organisms, animals or plants, which form large, massive skeletons of
carbonate of lime, on the other.
The form of the solitary zooids and of the colony of zooids in the
Zoantharia, then, may be very divergent. In the Actiniaria we find single
soft gelatinous zooids of considerable size adherent to rocks or
half-buried in the sand. Among the Madreporaria we find great branching
colonies of thousands of zooids supported by the copious skeleton of
carbonate of lime that they have secreted. Among the Antipathidea, again,
we find a dendritic skeleton of a dark horny substance, formed by a colony
of small zooids that cover it like a thin bark. The majority of the
Zoantharia are, like other zoophytes, permanently fixed to the floor of the
ocean. Where the embryo settles, there must the adult or colony of adults
remain until death. Some of the common Sea-anemones can, however, glide
slowly over the surface on which they rest, and thus change their position
according to the conditions of their surroundings. Others (the Minyadidae)
float upside down in the sea, and are carried hither and thither by the
currents. Others, again (_Cerianthus_, _Edwardsia_, _Peachia_), burrow in
the sand or mud at the sea-bottom.
The structure of the zooid varies considerably, but in the following
characters differs from the zooid of the Alcyonaria. The TENTACLES are
usually simple finger-like processes, and when they bear secondary pinnae
these can readily be distinguished from the rows of secondary pinnules of
the Alcyonarian tentacle. The number of tentacles is very rarely eight
(young _Halcampa_), and in these cases they are not pinnate. The number of
tentacles may be six (many Antipathidea and some zooids of _Madrepora_),
twelve (_Madrepora_), some multiple of six, or an indefinite number. In the
Thalassianthidae and some other families of Actiniaria the tentacles are
plumose, but do not exhibit the regular pinnate form of the tentacles of
Alcyonaria.
[Illustration: FIG. 162.—Large (A) and small (B) plumose tentacles of
_Actinodendron plumosum_. Large (C) and small (D) plumose tentacles of _A.
glomeratum_. (After Haddon.)]
As regards the number of MESENTERIES, the Zoantharia exhibit {367}very
great variety. It has been shown that there is frequently a stage in their
development during which there are only eight mesenteries. This stage is
usually called the _Edwardsia_ stage. These eight mesenteries are arranged
in bilateral pairs as follows:—One pair is attached to the body-wall and
reaches to the dorsal side of the stomodaeum, and is called the pair of
dorsal directives; a corresponding pair attached to the ventral side of the
stomodaeum is called the pair of ventral directives. The other two pairs
are the lateral mesenteries. To these four pairs are added, at the close of
the _Edwardsia_ stage, two additional pairs, making in all twelve
mesenteries (cf. Fig. 163).
These six primary pairs of mesenteries, conveniently called the
"protocnemes" by Duerden, may be traced in the development and recognised
in the adult of the majority of Zoantharia. But the number of the
mesenteries is usually increased in the later stages by the addition of
other mesenteries called the "metacnemes." The metacnemes differ from the
protocnemes in that they usually appear in unilateral pairs, that is to
say, in pairs of which both members arise on the same side of the
stomodaeum, and the number is very variable throughout the group. The space
enclosed by a pair of mesenteries is called an "entocoele," and the space
between two pairs of mesenteries is called an "ectocoele."
The twelve protocnemes are usually complete mesenteries, that is to say,
they extend the whole distance from the body-wall to the stomodaeum, while
the metacnemes may be complete or incomplete; in the latter case extending
only a part of the distance from the body-wall towards the stomodaeum.
We find, therefore, in making a general survey of the anatomy of the
Zoantharia that there is no general statement to be made, concerning the
number or arrangement of the mesenteries, which holds good for the whole or
even for a considerable portion of the genera.
The bands of retractor muscles are, as in the Alcyonaria, situated on one
face only of the mesenteries (except in the Antipathidea and Cerianthidea),
but an important character of the Zoantharia is that the muscle bands on
the ventral pair of directives are situated on the dorsal faces of these
mesenteries, and not on the ventral faces as they are in Alcyonaria.
In the Edwardsiidea there are only eight complete mesenteries, {368}but a
variable number of other rudimentary and incomplete mesenteries have
recently been discovered by Faurot.[392] In the Zoanthidea the mesenteries
are numerous, but the order is remarkable for the fact that the dorsal
directives are incomplete, and that, of the pairs of metacnemes that are
added, one mesentery becomes complete and the other remains incomplete. In
most of the genera of the Antipathidea there are only ten mesenteries, but
in _Leiopathes_ there are twelve, and as they bear no bands of retractor
muscles it is difficult to determine accurately their true relation to the
mesenteries of other Zoantharia.
[Illustration: FIG. 163.—Diagrams of transverse sections of 1, Alcyonarian;
2, _Edwardsia_; 3, _Cerianthus_; 4, _Zoanthus_; 5, _Favia_; 6, _Madrepora_.
_DD_, the dorsal directive mesenteries; _VD_, the ventral directives;
_I-VI_, the protocnemes in order of sequence.]
In the Cerianthidea the mesenteries are very numerous, and increase in
numbers by the addition of single mesenteries alternately right and left in
the ventral inter-mesenteric chamber throughout the life of the individual.
These mesenteries do not bear retractor muscles.
In the Actiniaria and Madreporaria, with the exception of the genera
_Madrepora_, _Porites_, and a few others, there are also very many
mesenteries. The two pairs of directives are usually present, but they may
not occur in those zooids that are produced {369}asexually by fission (see
p. 388). The metacnemes are frequently formed in regular cycles, and in
many genera appear to be constantly some multiple of six (Fig. 163, 5).
In _Madrepora_ and _Porites_[393] the two pairs of directives and two pairs
of lateral protocnemes are complete; the other two pairs of protocnemes
are, however, incomplete; and metacnemes are not developed (Fig. 163, 6).
The stomodaeum is usually a flattened tube extending some distance into the
coelenteric cavity and giving support to the inner edges of the complete
mesenteries; in many of the Madreporaria, however, it is oval or circular
in outline. In most of the Actiniaria there are deep grooves on the dorsal
and ventral sides of the stomodaeum, but in Zoanthidea the groove occurs on
the ventral side only and in the Cerianthidea on the dorsal side only. In
the Madreporaria these grooves do not occur or are relatively
inconspicuous.[394] In the Alcyonaria the siphonoglyph exhibits a very
marked differentiation of the epithelium (see Fig. 148, p. 334), and the
cilia it bears are very long and powerful. It has not been shown that the
grooves in the Zoantharia show similar modifications of structure, and they
are called by the writers on Zoantharia the sulci. There is no difference
in structure, and rarely any difference in size, between the dorsal sulcus
and the ventral sulcus in the Actiniaria, and the use of the
word—sulculus—for the former is not to be commended.
The mesenteries bear upon their free edges the mesenteric filaments. These
organs are usually more complicated in structure than the corresponding
organs of the Alcyonaria, and the dorsal pair of filaments is not
specialised for respiratory purposes as it is in that group.
In many genera the mesenteric filaments bear long, thread-like
processes—the "acontia"—armed with gland cells and nematocysts which can be
protruded from the mouth or pushed through special holes (the "cinclides")
in the body-wall.
The gonads in the Zoantharia are borne upon the sides of the mesenteries
and are usually in the form of long lobed ridges instead of being spherical
in form, and situated at the edges of the mesenteries as they are in the
Alcyonaria.
{370}Nearly all the zooids and even the colonies of the Zoantharia are
unisexual, but some species, such as _Manicina areolata_ (Wilson),
_Meandrina labyrinthica_ (Duerden), _Cerianthus membranaceus_, and others,
are hermaphrodite. Mr. J. S. Gardiner has recently given reasons for
believing that the genus _Flabellum_ is protandrous.
SKELETON.—The soft tissues of the Zoantharian zooids may be supported or
protected by hard skeletal structures of various kinds. In the Zoanthidea
and the Actiniaria there are many species that have no skeletal support at
all, and are quite naked. These seem to be sufficiently well protected from
the attacks of carnivorous animals by the numerous nematocysts of the
ectoderm, and perhaps in addition by a disagreeable flavour in their
tissues. Anemones do not seem to be eaten habitually by any fish, but cases
have been described of _Peachia hastata_ being found in the stomach of the
Cod, and of _Edwardsia_ in the stomach of the Flounder.[395] On the
Scottish coasts Anemones are occasionally used with success as a bait for
cod.[396] The body-wall of _Edwardsia_, however, is protected to a certain
extent by the secretion of a mucous coat in which grains of sand and mud
are embedded. Some Anemones, such as _Urticina_, _Peachia_, and others, lie
half-buried in the sand, and others form a cuticle, like that of
_Edwardsia_, to which foreign bodies are attached.
_Cerianthus_ is remarkable for constructing a long tube composed of a
felt-work of discharged nematocysts mixed with mud and mucus, into which it
retires for protection. In the Zoanthidea the body-wall is frequently
strengthened by numerous and relatively large grains of sand, which are
passed through the ectoderm to lie in the thick mesogloea.
In the Madreporaria a very elaborate skeleton of carbonate of lime is
formed. In the solitary forms it consists of a cup-shaped outer covering
for the base and column of the zooid called the "theca," of a series of
radial vertical walls or "septa" projecting into the intermesenteric
chambers carrying the endodermal lining of the coelenteric cavity with
them, and in some cases a pillar, the "columella," or a series of smaller
pillars, the "pali" projecting upwards from the centre of the base of the
{371}theca towards the stomodaeum. In the colonial forms the theca of the
individual zooids is continuous with a common colonial skeleton called the
"coenosteum." This is solid in the Imperforate corals, and it supports at
the surface only a thin lamina of canals and superficial ectoderm. In the
Perforate corals, however, the coenosteum envelopes and surrounds the
canals during its formation, and thereby remains perforated by a network of
fine channels. In the colonial Madreporaria the skeletal cups which support
and protect the zooids are called the "calices."
The skeleton of the Antipathidea is of a different nature. It is composed
of a horny substance allied to keratin. When it is old and thick, it
usually has a polished black appearance, and is commonly termed "black
coral." The surface of this kind of coral is ornamented with thorny or
spiny projections, but it is never perforated by calices or canal systems.
It forms a solid axis for the branches of the corals, and all the soft
parts of the zooids and coenosarc are superficial to it.
It was formerly considered that this type of coral, which shows no trace of
the shape and form of the living organisms that produce it, is of a
different character to the calcareous skeleton which exhibits calices,
septa, pores, and other evidence of the living organism, and it was called
a "sclerobase" to distinguish it from the "scleroderm" of the Madreporaria.
It is now known that both the sclerobasic skeleton and the sclerodermic
skeleton are products of the ectoderm, and consequently these expressions
are no longer in general use.
ASEXUAL REPRODUCTION in the Zoantharia may be effected by continuous or
discontinuous fission or gemmation.
In the Edwardsiidea, Actiniaria, and Cerianthidea, that is to say in the
animals popularly known as Sea-anemones, asexual reproduction does not
commonly occur, but nevertheless a good many instances of it are now known
in individual genera. In _Actinoloba_ (_Metridium_), for example, Parker
has described a case of complete longitudinal fission, and Duerden states
that it occurs in the West Indian Anemones _Actinotryx_ and _Ricordea_. A
still more remarkable form of asexual reproduction known as transverse
fission has been described in the genus _Gonactinia_.[397] In this case,
the body of the Anemone becomes constricted in {372}the middle, a circlet
of tentacles is formed below the constriction, and division takes place.
The upper half floats away with the original tentacles and stomodaeum and
becomes attached by the base in another place; the lower half remains
behind and develops a new stomodaeum, mesenteric filaments, and sexual
organs. In some of the Actiniaria another form of asexual reproduction
occurs, known as "Pedal laceration." In the common British _Actinoloba_,
for example, so often kept in aquaria, the pedal disc sometimes spreads on
the glass or rock upon which the animal rests, in the form of a thin
membrane or film of an irregular circular shape, nearly twice the diameter
of the column. As the Anemone glides along, the film remains behind and
breaks up into a number of hemispherical droplets, which in a few days
develop tentacles, a mouth, mesenteries, and the other organs of a complete
and independent Anemone. A similar method of reproduction has been observed
in several species of _Sagartia_. A true process of discontinuous gemmation
has also been observed in _Gonactinia_, in _Corynactis_, and in
_Actinoloba_.
[Illustration: FIG. 164.—Longitudinal fission of _Actinoloba_. (After
Agassiz and Parker.)]
In the Madreporaria, Zoanthidea and Antipathidea, the usual method of
reproduction to form the colonies is continuous gemmation. The new zooids
that are added to the colony as it grows arise as buds, either from the
superficial canals of the coenenchym, or from the base or body-wall of the
older zooids. In these cases the young zooids acquire the same number of
mesenteries, and the same characters of the stomodaeum as the original
parent. Some further particulars of asexual reproduction in the
Madreporaria are given on p. 387.
The SEXUAL REPRODUCTION of a great many species of Zoantharia has now been
observed. The eggs are, as a general rule, ripened in batches, and
fertilisation is effected before their discharge from the body. In some
cases the sexual condition is seasonal. In temperate climates the
generative organs ripen in the spring and {373}summer months, and remain
small and relatively inconspicuous in the colder weather; but British
Sea-anemones, when kept in an aquarium and regularly fed, will breed nearly
all the year round. The corals of the tropics living in warmer water of a
more regular temperature show considerable variety in their breeding
habits. Thus Duerden found that colonies of _Favia_, _Manicina_,
_Siderastraea_ and _Porites_ are fertile at nearly all times, whereas
colonies of _Madrepora_, _Orbicella_ and _Cladocora_ were rarely so. In
nearly all cases the fertilisation is effected, and segmentation of the
ovum occurs within the body of the parent, the young Zoantharian beginning
its independent life as an oval or pear-shaped ciliated larva.
There are a great many cases among the Actiniaria in which the embryos are
retained within the coelenteron, or in special brood pouches of the parent
(p. 379), until a stage is reached with twelve or more tentacles.
The oval or pear-shaped larva swims about for a few days or hours, and then
settles down on its aboral end. In swimming, the aboral end is always
turned forwards. In the larva of _Lebrunia coralligens_ and _Rhodactis
sancti-thomae_, a distinct sense organ has been observed upon the aboral
extremity, and a similar but less distinct organ on the larva of _Actinia
equina_. These organs are of considerable interest, as they are probably
the only specialised sense organs known to occur in the Zoantharia.
The larvae of Zoantharia present, as a rule, very little variation from the
type described, and live but a short time if they fail to find a suitable
place for fixation. The colour is usually white and opaque, but in some
species the endoderm may be coloured yellow by Zooxanthellae (cf. pp. 86,
125).
The larvae of the Cerianthidea, however, are remarkable and exceptional.
After the larva of these animals has passed through the gastrula stage, a
certain number of mesenteries and tentacles are formed, and it rises in the
water to live a pelagic life of some duration. This larva is known as
_Arachnactis_, and is not unfrequently found in the plankton.
The character of the FOOD of the Zoantharia varies with the size of the
zooids, the occurrence of Zooxanthellae in the endoderm, and local
circumstances; but in general it may be said to consist mainly of small
living animals.
{374}Sea-anemones kept in an aquarium will readily seize and devour pieces
of raw beef or fragments of mussel that are offered to them; but they may
also be observed to kill and swallow the small Crustacea that occur in the
water. When a living animal of a relatively small size comes within range
of the tentacles, it appears to be suddenly paralysed by the action of the
nematocysts and held fast. The tentacles in contact with it, and others in
the neighbourhood but to a lesser extent, then bend inwards, carrying the
prey to the mouth. The passage of the food through the stomodaeum is
effected partly by ciliary, and partly by muscular action, and the food is
then brought to the region of the mesenteric filaments where it is rapidly
disintegrated by the digestive fluids they secrete. Any unsavoury or
undigested portions of the food are ejected by the mouth.
Very little is known concerning the food of the Madreporarian Corals. Many
investigators have noticed that the zooids of preserved specimens very
rarely contain any fragments of animal or plant bodies that could possibly
be regarded as evidence of food. It is possible that many Corals derive a
part, perhaps in some cases a considerable part, of their nourishment from
the symbiotic Zooxanthellae (pp. 86, 125) which flourish in the endoderm;
but it is improbable that in any case this forms the only source of food
supply. The absence of food material in the cavities of the zooids may
perhaps be accounted for by the fact that nearly all the Corals are fully
expanded, and therefore capable of catching their food only at night.
Corals are usually collected during the daytime, and therefore during the
period of rest of the digestive organs.
It is true that nearly all Corals do exhibit Zooxanthellae in their
endoderm, but there are some species from which they are nearly or wholly
absent, such as _Astrangia solitaria_ and _Phyllangia americana_ on the
West Indian reefs,[398] and the Pocilloporidae. The absence of any signs of
degeneration in the tentacles or digestive organs of those corals with
Zooxanthellae as compared with those without them suggests, at any rate,
that the Zooxanthellae do not supply such a large proportion of the food
necessary for the support of the colonies as to warrant any relaxation of
the efforts to obtain food by other means. Mr. Duerden found that when
living Annelids are placed upon the {375}tentacles of a living
_Siderastraea_—a genus with Zooxanthellae, the tentacles at once close upon
them and prevent their escape. The general conclusion seems to be,
therefore, that the Madreporarian Corals feed upon small animals in much
the same way as the Sea-anemones, whether they have Zooxanthellae or not,
but that in general they feed only at night.
AGE.—It is known that Sea-anemones kept in an aquarium and regularly fed
will live for a considerable number of years without showing signs of
weakness or failing health. Dalyell kept in an aquarium a specimen of
_Actinia mesembryanthemum_, which lived for sixty-six years and then died a
natural death; and specimens of _Sagartia_, still living, are known to be
about fifty years old.[399] The unnatural conditions of life in an aquarium
may have favoured the longevity of these specimens, and it would not be
reasonable to conclude from these records that the average life of a
full-grown Anemone on the rocks is more than thirty or thirty-five years,
and perhaps it is a good deal less.
As regards the Madreporarian Corals, we know but little concerning their
duration of life. An examination of any living coral reef is sufficient to
convince an observer that the power of asexual reproduction of the colonial
forms is not unlimited; that colonies, like individuals, have a definite
span of life, and that they grow old, senile, and then die a natural death
if spared in their youth from accident and disease. Mr. Gardiner has
calculated that the duration of life in solitary Corals like _Flabellum_ is
about twenty-four years, in colonial forms such as _Goniastraea_,
_Prionastraea_, _Orbicella_, and _Pocillopora_, from twenty-two to
twenty-eight years.
ORDER I. EDWARDSIIDEA.
This order contains only a few genera and species of small size living in
shallow water in various parts of the world. In external features they
closely resemble several genera of the Actiniaria, particularly those
belonging to the family Halcampidae. The distinguishing character of the
order is to be found in the system of mesenteries. In all the species only
eight mesenteries are complete, namely, the first two pairs of protocnemes,
and the two pairs of directives (Fig. 163, 2), {376}and these usually
support such large and powerful muscle-bands that they appear to be the
only mesenteries present. A careful examination of transverse sections,
however, reveals the fact that other mesenteries are present. The fifth and
sixth pairs of protocnemes seem to be invariably represented, and two or
three pairs of metacnemes can also be traced in some species.
The tentacles are variable in number. In _Edwardsia beautempsii_, for
example, they may be 14-16 in number, arranged in a single row round the
oral disc. In _E. timida_ they vary from 20 to 24. The normal number
appears to be eight tentacles of the first cycle, corresponding to the
eight primary inter-mesenteric chambers, _plus_ 6 or 12 tentacles,
corresponding with the chambers limited by the more rudimentary
mesenteries,—making a total of 14 or 20 tentacles; but by the suppression
of the two primary dorso-lateral tentacles, or by the addition of tentacles
of another cycle, the actual number is found to vary considerably. The
Edwardsiidea are not fixed to the bottom, but are usually found deeply
embedded in sand, the aboral extremity being pointed and used for burrowing
purposes. The general colour of the body is yellow or yellowish brown, but
it is partly hidden by a short jacket of mud or sand and mucous secretion.
The oral crown frequently shows beautiful colours. De Quatrefages relates
that in _Edwardsia beautempsii_ the oral cone is golden yellow, and the
tentacles, transparent for the greater part of their extent, terminate in
opaque points of a beautiful yellowish red colour.
[Illustration: FIG. 165.—_Edwardsia beautempsii._ Nat. size. (After de
Quatrefages.)]
{377}FAM. 1. EDWARDSIIDAE.—Several species of this family have been found
in the British area. They are very local in their distribution, but
sometimes occur in great numbers.
_Edwardsia beautempsii_ occurs in shallow water near the shores of the
English Channel and has been found in Bantry Bay; and _E. carnea_ and _E.
timida_ have also been found in the Channel. _E. tecta_ is a recently
described species from the S. Irish coast, and _E. allmani_ and _E.
goodsiri_ are found in Scottish waters.
FAM. 2. PROTANTHEIDAE.—This family, constituted for the reception of three
remarkable genera, is now usually included in the order Edwardsiidea on the
ground that not more than eight mesenteries are complete.
The genus _Gonactinia_ exhibits the very exceptional character of having a
thick layer of muscles in the body-wall (cf. Cerianthidea, p. 409), and it
is also remarkable for the frequency with which it reproduces itself
asexually by longitudinal and, more rarely, by transverse fission. It has
been found in Norway, the Mediterranean, and on the reefs of New Caledonia.
The other genera of the family are _Oractis_ from California, and
_Protanthea_ from the coast of Sweden.
ORDER II. ACTINIARIA.
This order contains nearly all the animals popularly known as Sea-anemones.
They are usually found in shallow water, attached by a broad basal disc to
shells, stones, or sea-weeds. In the Halcampidae, however, the aboral
extremity ends in a blunt point as in the Cerianthidea and Edwardsiidea,
and the animals live half-buried in sand or mud. The Minyadidae of the
southern oceans are pelagic in habit, floating near the surface of the sea
with the mouth turned downwards. They are supported in the water by a
bladder, formed by an involution of the pedal disc, and filled with gas.
Many of the Sea-anemones are found in symbiotic association with other
animals. The common _Adamsia_ of the British coasts is found on whelk
shells containing hermit crabs. The crab is probably protected from the
attacks of some of its enemies by the presence of the Anemone, which in its
turn has the advantage of securing some fragments of the food captured and
torn to {378}pieces by the crab. The association, therefore, seems to be
one of mutual advantage to the messmates. It is a noteworthy fact that in
these associations the species of Sea-anemone associated with a particular
hermit crab is nearly always constant. Thus in the English Channel,
_Adamsia palliata_ is almost invariably found associated with _Eupagurus
prideauxii_, and _Adamsia rondeletii_ with _Eupagurus bernhardus_. But,
perhaps, the most remarkable association of this kind is to be seen in the
case of the little shore crab of the Indian Ocean, _Melia tesselata_, which
invariably holds in each of its large claws a small Sea-anemone. Möbius,
who originally described this case, relates that when the crab is robbed of
its Anemone it appears to be greatly agitated, and hunts about on the sand
in the endeavour to find it again, and will even collect the pieces, if the
Anemone is cut up, and arrange them in its claw.[400]
Another very interesting association is that of certain fish and Crustacea
with the large Sea-anemones of the tropical Australian coast.[401] Thus
_Stoichactis kenti_ almost invariably contains two or more specimens of the
Percoid fish _Amphiprion percula_. This fish is remarkable for its
brilliant colour, three pearly white cross-bands interrupt a ground plan of
bright orange-vermilion, and the ends of the cross-bands as well as the
fins are bordered with black. In another species a prawn of similar
striking colours is found. These companions of the giant Anemones swim
about among the tentacles unharmed, and when disturbed seek refuge in the
mouth. It has been suggested that these bright and attractive animals serve
as a lure or bait for other animals, which are enticed into striking
distance of the stinging threads of the Anemone, but how the commensals
escape the fate of the animals they attract has yet to be explained.
In a considerable number of Sea-anemones, such as _Actinoloba marginata_
and _A. dianthus_, some species of _Sagartia_, _Actinia cari_, _Anemonia
sulcata_, and _Calliactis parasitica_, the fertilisation of the eggs and
their subsequent development take place in the sea water.[402] In a great
many others, such as _Bunodes_ (several species), _Cereactis aurantiaca_,
_Sagartia troglodytes_, _Bunodactis {379}gemmacea_, etc., the embryos are
discharged into the water from the body-cavity of the parent, at a stage
with six or twelve tentacles. In the Arctic species of the genera
_Urticina_ and _Actinostola_, however, the embryos are retained within the
body of the parent until several cycles of tentacles are developed, and in
_Urticina crassicornis_ the young have been found with the full number of
tentacles already formed. In _Epiactis prolifera_ from Puget Sound, the
young Anemones attach themselves to the body-wall of the parent after their
discharge, and in _Epiactis marsupialis_, _Pseudophellia arctica_,
_Epigonactis fecunda_, and other species from cold waters, the young are
found in numerous brood sacs opening in rows on the body-wall. It is not
known for certain how these embryos enter the brood sacs, but it is
possible that each sac is formed independently for a young embryo that has
settled down from the outside upon the body-wall of the parent. The most
specialised example of this kind of parental care in the Sea-anemones is
seen in _Marsupifer valdiviae_ from Kerguelen, in which there are only six
brood sacs, but each one contains a great many (50-100) embryos.
The wonderful colours of our British Sea-anemones are familiar to most
persons who have visited the sea-side. The common _Actinia
mesembryanthemum_ of rock pools, for example, is of a purple red colour.
The base is usually green with an azure line. Around the margin of the disc
there are some twenty-five turquoise blue tubercles. On each side of the
mouth there is a small purple spot, and the numerous tentacles forming a
circlet round the mouth are of a pale roseate colour. Nothing could be more
beautiful than the snowy-white _Actinoloba dianthus_ or the variegated
_Urticina crassicornis_.
Similar wonderful variety and beauty of colour are seen in the Sea-anemones
of other parts of the world. Thus Saville Kent[403] in describing a species
of the gigantic _Stoichactis_ of the Australian Barrier Reef says, "the
spheroidal bead-like tentacles occur in irregularly mixed patches of grey,
white, lilac, and emerald green, the disc being shaded with tints of grey,
while the oral orifice is bordered with bright yellow."
The order Actiniaria contains a large number of families, presenting a
great variety of external form and of detail in general anatomy. The
definitions of the families and their {380}arrangement in larger groups
have presented many difficulties, and have led to considerable differences
of opinion; and even now, although our anatomical knowledge has been
greatly extended, the classification cannot be regarded as resting on a
very firm basis. The families may be grouped into two sub-orders:—
SUB-ORDER 1. ACTINIINA.—The tentacles are simple and similar, and there is
one tentacle corresponding to each intermesenteric chamber (endocoel).
SUB-ORDER 2. STICHODACTYLINA.—The tentacles are simple and similar, or
provided with teat-like or ramified pinnules. One or more tentacles may
correspond with an endocoel, and there may be two kinds of tentacles
(marginal and accessory) in the same genus.
SUB-ORDER 1. ACTINIINA.
FAM. 1. HALCAMPIDAE.—This family is clearly most closely related to the
Edwardsiidea. There are, however, twelve complete mesenteries of the first
cycle, and a second cycle of more or less incomplete mesenteries. The
tentacles are usually twelve in number, but may be twenty or twenty-four.
There is no pedal disc, but the base is swollen and rounded or pointed at
the end.
The genus _Halcampa_ includes a considerable number of small species
occurring in the shallow waters of the temperate northern hemisphere, and
of the Kerguelen Islands in the south. Three British species have been
described, of which _Halcampa chrysanthellum_ alone is common. The larva
with eight tentacles and eight mesenteries has been found living on the
Medusa _Thaumantias_.
_Peachia_ is a genus containing Anemones of much larger size (10-25 cm.).
It is remarkable for the very large siphonoglyph on the ventral side of the
stomodaeum, prolonged into a papillate lip projecting from the mouth called
the "conchula." The genera _Scytophorus_ from 150 fathoms off Kerguelen and
_Gyractis_ from Ceylon, although showing some remarkable peculiarities of
their mesenteric system, appear to be closely related to this family.
_Ilyanthus mitchellii_ is a large Anemone with a vesicular base,
forty-eight tentacles and mesenteries, occurring in the English Channel,
but it is not very common. It is usually {381}placed in a separate family,
but is in many respects intermediate in character between the Halcampidae
and the Actiniidae.
FAM. 2. ACTINIIDAE.—This family contains some of the commonest British
Sea-anemones. There is a large flat pedal disc by which the body is
attached to stones and rocks. The body-wall is usually smooth, and not
perforated by cinclides. The edge of the disc is usually provided with
coloured marginal tubercles. There are no acontia.
_Actinia._—This genus contains the widely distributed and very variable
species _Actinia mesembryanthemum_, one of the commonest of the
Sea-anemones found in rock pools on the British coast. The colours of this
species are often very beautiful (see p. 379) but variable.
_Anemonia_ is a genus with remarkably long tentacles which are not
completely retractile. _A. sulcata_ (sometimes called _Anthea cereus_) is
very common in the rock pools of our southern coasts.
_Bolocera tuediae_ is, next to _Actinoloba dianthus_, the largest of the
British Anemones. It has very much the same colour as the common varieties
of _Actinia mesembryanthemum_, but the body-wall is studded with minute,
rounded warts. It is found between tide marks in the Clyde sea-area, but
usually occurs in deeper water.
FAM. 3. SAGARTIIDAE.—This family includes several genera with a contractile
pedal disc, with the body-wall usually perforated by cinclides, and
provided with acontia.
The genera may be arranged in several sub-families distinguished by
well-marked characters. Among the well-known Sea-anemones included in the
family may be mentioned:—
_Sagartia troglodytes_, a very common British species found in hollows in
rocks. It is usually of an olive green or olive brown colour, and the upper
third or two-thirds of the body-wall is beset with numerous pale suckers.
_Adamsia palliata_ has a white body-wall spotted with bright red patches,
and is associated with the hermit crab _Eupagurus prideauxii_.
_Actinoloba_ (frequently called _Metridium_) _dianthus_ is considered the
handsomest of all the British Sea-anemones. It has a lobed disc frilled
with numerous small tentacles, and is uniformly coloured, creamy-white,
yellow, pale pink, or olive brown. It lives well in captivity, and
sometimes reaches a length of 6 inches with a diameter of 3 inches (Fig.
164).
{382}_Aiptasia couchii_ is a trumpet-shaped Anemone, found under stones at
low-water mark in Cornwall and the Channel Islands, with relatively slight
power of retraction.
_Gephyra dohrnii_ is an interesting species with twelve tentacles, which
was supposed at one time to form a connecting link between the Actiniaria
and the Antipathidea. It is found attached to the stems and branches of
various Hydrozoa and Alcyonaria, sometimes in such numbers and so closely
set that it gives the impression of having formed the substance of its
support. Haddon[404] has described specimens found on the stems of
_Tubularia_ from deep water off the south and south-west coasts of Ireland.
It also occurs in the Mediterranean and the Bay of Biscay.
FAM. 4. ALICIIDAE.—The members of this family have a large flat contractile
base and simple tentacles. The body-wall is provided with numerous simple
or compound outgrowths or vesicles, usually arranged in vertical rows.
_Alicia mirabilis_ is a rare Anemone from Madeira with a very broad base,
capable of changing its position with considerable activity, and of
becoming free and floating upside down at the surface of the sea. Other
genera of the family are _Bunodeopsis_ and _Cystiactis_. The genus
_Thaumactis_, described by Fowler,[405] from the Papeete reefs, has many
peculiarities, but is probably capable of crawling rapidly and of floating
at the surface like other members of the family. The remarkable Anemone
_Lebrunia_ from the West Indies may be included in this family.
FAM. 5. PHYLLACTIDAE.—These are distinguished by the presence of a broad
collar of foliaceous or digitate processes outside the circle of tentacles.
The processes have some resemblance to the foliaceous tentacles of the
Stichodactylinae. They are found in the Mediterranean, Red Sea, and on the
shores of the Atlantic Ocean, but have not yet been found in the British
area.
FAM. 6. BUNODIDAE.—This family is characterised by prominent verrucae and
tubercles of the body-wall. It contains several British species, of which
_Bunodes gemmacea_ found between tide marks on our southern shores is
fairly common. The very common British species _Urticina_ (_Tealia_)
_crassicornis_ is usually placed in this family, but exhibits some
peculiarities which seem {383}to warrant its removal to another division of
the Actiniaria. It is found in tide pools attached to rocks, but is usually
partially hidden by adherent sand or small stones.
FAM. 7. MINYADIDAE.—This family contains a number of floating Anemones. The
basal disc is folded over to form a gas bladder lined by a cuticular
secretion. The species are principally found in the seas of the southern
hemisphere.
SUB-ORDER 2. STICHODACTYLINA.
FAM. 1. CORALLIMORPHIDAE.—In this family the marginal cycle of tentacles
and accessory tentacles are all of the same kind. The accessory tentacles
are arranged in radial rows. All the tentacles are knobbed at the
extremity. The musculature is weak. _Capnea sanguinea_, _Corynactis
viridis_, and _Aureliania heterocera_ belong to the British fauna. They are
all small Anemones of exquisite colours, but are not very common. The genus
_Corallimorphus_ is principally found in the southern hemisphere.
FAM. 2. DISCOSOMATIDAE.—The tentacles are all of one kind and are very
numerous. The mesenteries are also very numerous. The sphincter muscle is
strong.
This family includes a rather heterogeneous assembly of forms, and will
probably require some rearrangement as our knowledge increases. Nearly all
the species are found in the shallow waters of the tropics, and among them
are to be found some of the largest Anemones of the world. _Stoichactis
kenti_, from the Barrier Reef, is from one to four feet in diameter across
the disc. In the West Indies these Anemones do not attain to such a great
size, but _Homostichanthus anemone_ from Jamaica is sometimes 8 inches in
diameter.
FAM. 3. RHODACTIDAE.—In this family the body-wall is smooth and the oral
disc greatly expanded. The tentacles are of two kinds. On the margin there
is a single cycle of minute tentacles, while on the disc there are numerous
tuberculate or lobed tentacles. Many of the species of this family are
quite small, but _Actinotryx mussoides_ from Thursday Island has an oral
disc 8 inches in diameter. The genera and species are widely distributed in
the warm, shallow waters of the world.
FAM. 4. THALASSIANTHIDAE.—The tentacles are simple or {384}ramified (Fig.
166), and in some cases very long (_Actinodendron arboreum_). Many of the
specimens of _A. plumosum_ and _Megalactis griffithsi_ are of very large
size, 8 to 12 inches in diameter. Of the former of these two species
Saville Kent remarks: "The colours are lacking in brilliancy, being chiefly
represented by varying shades of light brown and white, which are probably
conducive to its advantage by assimilating it to the tint of its sandy bed.
When fully extended the compound tentacles are elevated to a height of 8 or
10 inches, and bear a remarkable resemblance to certain of the delicately
branching, light brown sea-weeds that abound in its vicinity." The same
author calls attention to their stinging, which is "nearly as powerful as
the ordinary stinging nettle."
[Illustration: FIG. 166.—_Actinodendron plumosum._ _D_, disc of attachment;
_Si_, siphonoglyph; _t_, _t_, lobes of the marginal disc bearing the
tentacles; _W_, body-wall. Height of the column 200 mm. (After Haddon.)]
ORDER III. MADREPORARIA.
The Madreporaria form a heterogeneous group of Zoantharia characterised by
a single common feature, the formation of an extensive skeletal support of
carbonate of lime. In a great many cases the skeleton exhibits cups or
"calices" into which the zooids may be completely or partially retracted,
and these calices usually exhibit a series of radially disposed vertical
laminae, the "septa," corresponding with the inter-mesenteric spaces of the
zooids. Calices and structures simulating septa also occur in _Heliopora_,
which is an Alcyonarian, and in certain fossil corals which are probably
not Zoantharians. The anatomy of the zooids of a great many Madreporaria is
now known, and, {385}although a great deal of work yet remains to be done,
it may be said that the Madreporaria exhibit close affinities in structure
with the Actiniaria. The chief points in the anatomy of the zooids are
described under the different sub-divisions, but a few words are necessary
in this section to explain the principal features exhibited by the
skeleton.
There is no more difficult task than the attempt to explain upon any one
simple plan the various peculiarities of the Madreporarian skeleton.[406]
The authorities upon the group are not agreed upon the use of the terms
employed, nor are the current theories of the evolution of the skeleton
consistent. It is necessary, however, to explain the sense in which certain
terms are employed in the systematic part that follows, and in doing so to
indicate a possible line of evolution of the more complicated compound
skeletons from the simple ones.
[Illustration: FIG. 167.—Series of diagrams to illustrate the structure of
the Madreporarian skeleton. A, young stage of a solitary coral with simple
protheca (_p.t_). B, solitary coral, with theca (_th_), epitheca (_e.t_),
and prototheca (_p.t_). C, young stage of colonial coral, showing
coenosteum (_coe_) and theca (_th_), and the formation of the theca of a
bud (_b_). D, two zooids of a more advanced stage of a colonial coral.
_coe_, Coenosteum; _th_, theca. The black horizontal partitions are the
tabulae. E, transverse section of a calyx. _c_, Costa; _col_, columella;
_d_, dissepiment; _g_, septum; _p_, pali.]
There can be no doubt whatever that the whole of the skeleton of these
animals is formed by the ectoderm, and is external to their bodies. If we
could get rid of the influence of tradition upon our use of popular
expressions we should call this skeleton a shell. There can be little
doubt, moreover, that this skeleton is formed by a single layer of
specialised ectoderm cells called the "calicoblasts."
{386}The calicoblasts form, in the first instance, a skeletal plate at the
aboral end of the coral embryo, which becomes turned up at the edges to
form a shallow saucer or cup. This cup is called the "prototheca."[407] At
this stage the body-wall of the living zooid may or may not overflow the
edge of the prototheca. In the former case the growth of the rim of the
prototheca is brought about by the calicoblasts of an inner and outer layer
of epiblast, and the cup is then called the "theca." In the latter case,
the growth of the rim of the prototheca is continued by the calicoblasts of
one layer of epiblast only, and it is called the "epitheca" (_Flabellum_).
With the continued growth of the theca the tissues that have overflowed—the
"episarc"—retreat from the base, and in doing so the ectoderm of the edge
and, to some extent, the outer side of the episarc secrete a layer of
epitheca which becomes more or less adherent to the theca. Thus the cup may
have a double wall, the theca and the epitheca (_Caryophyllia_).
[Illustration: FIG. 168.—Diagram of a vertical section of a young
_Caryophyllia_, showing the septa (_S_) covered with endoderm projecting
into the coelenteric cavity. _M_, mouth; _St_, stomodaeum. (After G. von
Koch.)]
[Illustration: FIG. 169.—A young _Caryophyllia_, viewed from above, showing
the tentacles (_t_) and the stomodaeum (_St_). The letter _m_ points to a
space between a pair of mesenteries, and the darker shading in this place
shows a septum projecting radially from the wall of the theca. (After G.
von Koch.)]
With the growth of the theca and epitheca a certain number of radially
disposed laminae of lime rise from the walls and grow centripetally. These
are the "septa." Additional ridges on {387}the inner wall of the cup
between the septa are called the "dissepiments." Corresponding with the
septa there may be a circle of columns or bands rising from the basal parts
of the prototheca—the "pali"; and from the actual centre a single column
called the "columella." The longitudinal ridges on the outside of the
theca, corresponding in position with the septa inside, are called the
"costae" (Fig. 167, E, _c_).
We may imagine that in the primitive forms that gave rise to colonies, the
episarc of the primary zooid overflowed on to the substance to which it was
attached, and gave rise to successive layers of epithecal skeleton, which
may be called the "coenosteum." The ectoderm at the base of the original
prototheca is in some corals periodically dragged away from the skeleton,
and forms another cup or platform of lime at a little distance from it—the
"tabula." New zooids are developed at some distance from the primary one by
a process of gemmation in the episarc, and independent thecae, septa, etc.,
are formed in it; the skeleton of the new zooid thus originated being
connected with that of the primary zooid by the coenosteum.
There are many modifications of this simple description of skeleton
formation to be considered before a thorough knowledge of coral structure
can be understood, but sufficient has been said to explain the use of the
terms that it is necessary to employ in the description of the families.
When it is necessary to speak of the cup in which the zooid is situated
without expressing an opinion as to the homology of its wall, it is called
the calyx.
There are many forms of asexual reproduction observed in the Madreporaria.
Of these the most frequent is gemmation. The buds are formed either on the
episarc or on the canals running between zooids at the surface of the
coenenchym. When the young zooids that have been formed by gemmation reach
maturity they have the same characters as their parents. Fission occurs in
the production of a great many colonies of Madreporaria. It occurs
occasionally in such genera as _Madrepora_ and _Porites_, where
reproduction by gemmation prevails, but it is said that gemmation never
occurs in those forms such as the Astraeidae Fissiparantes where fission is
the rule. In fission a division of the zooid takes place in a vertical
plane passing through the stomodaeum and dividing the zooid into two equal
parts. In some cases these two parts become separated during the further
{388}growth of the coral. In other cases, however, further divisions of the
stomodaeum occur before the separation of the zooids, and then elongated,
serpentine polyps are produced (as in _Meandrina_, etc.), which consist of
a number of imperfectly separated zooids, each with a distinct mouth and
stomodaeum but with continuous coelenteric cavities. Two kinds of fission
must be distinguished from each other. In _Madrepora_ and _Porites_ the
plane of fission passes dorso-ventrally through the zooids, that is,
between the dorsal and ventral pairs of directive mesenteries. In these
cases the zooids produced by fission are similar to the parent form. In
most Madreporaria, however, the plane of fission appears to be more or less
at right angles to this, and the resulting zooids are unlike the original
parent form in having either no directive mesenteries at all or only one
pair of them.
[Illustration: FIG. 170.—Diagrammatic transverse sections of _Porites_ to
illustrate the process of fission. A, before division; B, fission nearly
completed. In A four bilateral pairs (_a_, _b_, _c_, _d_) of mesenteries
have appeared in the entocoele of the ventral directives (_VD_). These are
increased to six pairs and then fission commences as seen in B, the plane
of fission passing through the entocoeles of the last pair of secondary
mesenteries (_f_) and of the dorsal directives (_DD_). _I_, _II_, _V_,
_VI_, the protocnemes in the order of their development. (After Duerden.)]
The section Fungacea presents us with some exceptional and remarkable forms
of asexual reproduction. The embryo _Fungia_ gives rise to a conical fixed
coral called a "trophozooid." The upper part of the calyx of this
trophozooid expands and becomes disc-shaped. This is called the
"anthocyathus," and after it has reached a certain size it breaks away from
the rest of the trophozooid as an adult _Fungia_. Several anthocyathi may
be formed in succession from one trophozooid. This may be described as a
process of successive transverse fission. In _Diaseris_ the disc divides
into four quadrants, and each quadrant appears to be capable of acquiring
the shape and size of the undivided parent.
{389}Without doubt a process of sexual reproduction occurs in all
Madreporaria. In some genera sexual reproduction appears to be almost
continuous throughout the year; in others the sexual organs are formed only
at periods separated by considerable intervals of sterility. According to
the researches of Duerden the Madreporaria appear to be usually viviparous,
the early stages of development are passed through within the body of the
parent, and the young coral is discharged into the water as a free-swimming
ciliated larva. The larvae are spheroidal, oval, or pear-shaped, but change
their shape a good deal, and sometimes become elongated, straight, or
spirally twisted rods. The larvae are at first dense and opaque, but
subsequently they become distended by the absorption of water, and more
nearly transparent. They swim about for one or two days, and then settle
down by the aboral pole and become fixed. The tentacles are not formed, in
any species that has yet been observed, during the free-swimming stage of
existence.