trophozooid has become differentiated into a discoid crown, the
anthocyathus (_Cy_) and a pedicle, the anthocaulus (_Ca_). (After G. C.
Bourne.)]
DISTRIBUTION OF REEF CORALS.—The principal reef-forming corals reach their
greatest size and grow with greatest rapidity in the warm, shallow waters
of the world, but they are not confined to this habitat. A species of
_Madrepora_ has been found in the very cold waters of Archangel, and
_Manicina areolata_ occurs in Simon's Bay, Cape of Good Hope, many degrees
south of the region of the East African coral reefs. As regards the
distribution of these corals in depth, very little is known at present. The
face of the growing coral reef that is turned towards the open sea is so
steep that it has been found impossible to determine to what depth the
living reef corals actually extend.
The survey of the Macclesfield bank proved that a considerable number of
reef corals are to be found alive at depths {390}ranging from 30 to 50
fathoms.[408] To give one example:—In the dredging No. 50, depth 32 to 35
fathoms, living examples of the following genera of corals were obtained:
_Madrepora_, _Montipora_, _Psammocora_, _Pavonia_, and _Astraeopora_.
CORAL REEFS AND ATOLLS.—In many regions of the tropical seas, banks and
islands are found which are built up of blocks of coral, coral detritus,
and altered or modified limestone. These are the famous coral reefs of
which so much has been said and written during the last half-century. There
can be little doubt that the superficial strata of these formations are
entirely due to the action of coral-forming animals and plants living in
warm, shallow sea-water.
[Illustration: FIG. 172.—Plan of Minikoi Atoll in Laccadive Archipelago.
_A_, the land elevated above the level of high-water mark; _Ch_, the boat
channel; _5 fm_, the five fathom line; _2 fm_, the two fathom line; _L_,
the lagoon with a maximum depth of 7 fathoms; _R_, the reef continuing the
circle on the east side of the atoll, awash at high tides. (After Stanley
Gardiner.)]
Three classes of coral reefs are usually recognised: the "fringing reefs"
which follow the contour of the coast at a distance of a few hundred yards,
and are separated from the beach at low tide by sand flats or a shallow
lagoon; the "barrier reefs," following the contour of the coast less
regularly than the fringing reefs, but at a much greater distance, and
separated from the beach by a lagoon of sufficient depth to serve as a
harbour for ships of great size; and, finally, the "atolls," which are
ring-shaped, or broken circlets of low islands enclosing a lagoon which is,
in some cases, of considerable depth.
It was observed by the early surveyors that in many cases the sea-bottom
slopes downwards steeply or almost precipitously from the outer edge of the
barrier reefs and atolls to very great {391}depths—to depths, in fact, at
which reef-forming corals do not live.
It seems obvious, therefore, that the atolls and barrier-reefs are resting
upon some stratum which could not possibly have been formed by
reef-building organisms at the same relative position it has now, and the
questions arose, What is the substratum and how was it formed?
If this stratum is a coral rock, it is clear that it must have been formed
at a time when it was nearer to the surface of the sea than it is now, and
that it must have subsided subsequently to greater depths. If, on the other
hand, it is a primitive rock, we must assume that in such regions as the
Indian Ocean and the South Pacific, where the archipelagoes of atolls
extend for hundreds of miles, there are chains of mountain ranges with
peaks reaching to a uniform level beneath the surface of the sea. "But we
cannot believe that a broad mountain summit lies buried at the depth of a
few fathoms beneath every atoll, and nevertheless that throughout the
immense areas above named not one point of rock projects above the level of
the sea. For we may judge of mountains beneath the sea by those on land,
and where can we find a single chain, much less several such chains many
hundred miles in length, and of considerable breadth, with broad summits
attaining the same height from within 120 to 180 feet?"[409]
To account for the observed facts of the atolls and barrier-reefs, Darwin
conceived and expounded the subsidence theory. According to this theory,
the regions where atolls now occur were at one time dry land, or an
archipelago of volcanic islands surrounded by fringing reefs of the
ordinary type. A gradual subsidence of the land took place, and the area of
the land diminished; but the area enclosed by the coral reefs did not
diminish in a corresponding degree, and the young corals growing on the
débris of the older ones as they sank continued the growth of the reef in a
direction nearly vertical to the sea-bottom. The fringing reefs thus became
barrier reefs, and they were separated from the land by a lagoon of
considerable depth. Finally, when the mountain peaks disappeared beneath
the waves, a ring-shaped reef or atoll was all that was left to mark the
position of the former land.
The fundamental assumption in the subsidence-theory is that {392}the
substratum of the coral reefs and islands is coral-formed limestone. To
test the truth of this assumption an expedition was sent out to obtain, by
boring, evidence of the character of the substratum of a typical atoll. The
island of Funafuti in the Ellice group of the Pacific Ocean was selected,
and after several attempts a successful boring was made to a depth of 1114
feet. The material from the boring was found to consist of rocks or sands
entirely derived from the calcareous skeletons of marine Invertebrate
animals and calcareous Algae.[410] Moreover, in the cores from various
depths down to the lowermost the fossilised skeletons of the common genera
of recent corals, and very few or no representatives of genera of corals
now extinct were discovered.
[Illustration: FIG. 173.—Section of the outer edge of one of the Maldive
Atolls. A, foundation of primitive rock cut down by the currents; B,
upgrowth of the rim by the deep-sea-, intermediate depth- and (B')
reef-organisms; C, extension outwards by means of the talus slope; D,
lagoon. Scale in fathoms. (After Stanley Gardiner.)]
These facts, therefore, prove the justice of Darwin's assumption as to the
nature of the substratum—and give support to the subsidence-theory as
applied to this particular island. A strong opinion has, however, been
expressed by several authors of recent years that the subsidence-theory
cannot account for the formation of all the atolls and barrier reefs that
have now been investigated, and alternate hypotheses have been put forward
to account for particular cases. The main chain of the Maldive Archipelago
in the Indian Ocean, for example, presents special difficulties to the
acceptance of the subsidence-theory as one of general application.[411] The
main chain of these islands is more than 300 miles long, and lies at right
angles to the monsoon currents of the {393}Indian Ocean. Here the action of
the currents appears to have cut down a great tract of land to form a
plateau more than 100 fathoms in depth. The outer rim of this plateau may
have grown in height by the deposit of the skeletons of surface-swimming
animals, and the skeletons of deep-sea corals, until it reached a level
where reef-forming corals can thrive. A certain number of channels would be
retained and even deepened as the rim grew up, and thus the coral would
eventually reach the surface not as a single large atoll, but as a series
of coral islands. When the coral reef has thus reached the surface and
cannot grow farther in height, it spreads radially like a fairy ring on the
talus formed by broken corals that have fallen down the slopes. The central
parts, no longer protected by living organisms, are continually subject to
the solvent action of the sea water penetrating the porous substratum, and
sink to form the lagoon.
It is not only in the reefs of the Indian Ocean, however, but in many of
the archipelagoes of the Pacific Ocean, where there is evidence of very
extensive elevation of the land areas in the neighbourhood of atolls and
barrier reefs, that the subsidence-theory does not satisfactorily account
for all the observed facts. It appears probable, therefore, that although a
gradual subsidence of the land may have been the primary cause of coral
reef formation in some areas, similar reefs may have been formed in other
areas by other natural methods.
FOSSIL CORALS.—A great number of the genera of corals found in the newer
Tertiary deposits, and a smaller number of those occurring in the older
Tertiary and Cretaceous strata clearly belong to families now represented
by recent corals. In the earlier strata, however, fossils are found which
cannot be placed in our system with any degree of certainty. Attempts have
been made from time to time to arrange these corals in their proper
positions by the careful study and comparison of their skeletal features,
but the reasons given are not convincing. The genus _Syringopora_, and the
families Favositidae, Heliolitidae, and Coccoseridae have been noticed in
the chapter on Alcyonaria (pp. 343-346). The family Zaphrentidae will be
noticed when dealing with the order Zoanthidea.
Among the families of fossil corals of uncertain position which may still
be included in the order Madreporaria, the more important are:—
{394}CYATHOPHYLLIDAE, a family of solitary and colonial corals with
numerous radially arranged septa, extending from the Silurian to the
Carboniferous limestone. It includes the genera _Cyathophyllum_, which was
very abundant in Devonian times, and _Lithostrotion_, which, in the times
of the formation of the Carboniferous limestone, occurred in continuous
masses extending over great areas of the sea-bottom. The Cyathophyllidae
may possibly be ancestral to the representatives of both Astraeidae and
Fungiidae, which appeared in the Triassic strata.
The CYATHAXONIIDAE form a family of solitary turbinate or horn-shaped
corals, with septa showing a regular, radial arrangement, and may have been
the ancestors of the modern family Turbinoliidae. They have the same
geological range as the Cyathophyllidae.
The CYSTIPHYLLIDAE.—This family consists of solitary corals with very thin
septa; the interseptal spaces are filled with an abundant vesicular
substance called the "stereoplasm." The systematic position of this family
is very doubtful, as the structure is evidently much destroyed, but by some
authors it is supposed to be ancestral to the family Eupsammiidae.
These three families, together with the Zaphrentidae (p. 406), were
formerly grouped together as the Tetracoralla or Rugosa.
SUB-ORDER 1. ENTOCNEMARIA.
Madreporaria forming perforate coralla, with calices that do not project
above, or project only slightly above the surface of the coenosarc. The
zooids of each colony are usually small and crowded. The mesenteries arise
in bilateral pairs, and the increase in their number takes place in the
chamber between the ventral or the dorsal pairs of directives. The corals
included in this order are among the most important of the reef-builders.
On many of the recent coral reefs they occur in enormous numbers and of
great individual size. But although so prevalent upon recent reefs, they
appear to have played a far less important part in the formation of the
reefs of the early Tertiary times, and in the reefs of times antecedent to
the Tertiary they were rare or absent.
Judging from the structure of the skeleton and the palaeontological history
alone it might be thought that the Entocnemaria {395}represent the most
recent types of Madreporarian structure, but the anatomy of the zooids
points to a contrary conclusion. The zooids are of very simple structure;
the mesenteries are found only in bilateral pairs, and all the new
mesenteries formed after the protocnemes originate in one of the directive
chambers. These are characters indicating a very ancient history,
suggesting affinities with the Edwardsiidea on the one hand, and some
ancient type of Cerianthidea on the other. There can be little doubt that
it was owing to the evolution of a porous skeleton of rapid growth that
these corals have caught up and passed the Astraeidae and other more
specialised forms in the struggle for predominance on the coral reefs.
FAM. 1. MADREPORIDAE.—The calices of the corallum are small and contain a
few perfectly distinct septa. The coenosteum is porous and contains a
plexus of the coenosarcal canals, which connects the cavities of
neighbouring zooids. This family is divided into a number of sub-families,
but it is only necessary here to mention the peculiarities of a few of the
well-known genera.
_Madrepora._—This genus is represented by an immense number of forms on the
coral reefs of both the old and new world. Attempts have been made at
various times to divide these forms into specific groups, and a large
number of species have been defined and named. The differences between
these species, however, are such as may be due to varying conditions of
life upon the reefs and not to characters transmitted from generation to
generation by heredity. There can be no doubt that when our knowledge of
the soft tissues of these corals is extended the number of species will be
greatly reduced. There are, however, three principal forms of growth or
_facies_ in the genus.
1. The flabellate or palmate colonies with large flat or concave fronds,
radiating from an encrusting base: _Forma palmata_.
2. Much branched colonies, several branches radiating obliquely from a
common centre: _Forma prolifera_.
3. Large and more erect colonies, less branched except towards the
periphery: _Forma cervicornis_.
On some reefs one of these forms of growth predominates, and for miles the
reef seems to be built up mainly of corals of this shape. On other reefs
two or sometimes all three of these forms may be found within a stone's
throw of one another. {396}Notwithstanding the difficulty of distinguishing
the species, the genus itself is quite well defined. The calices project
slightly from the surface of the branches and contain six septa, of which
the pair that is parallel with the axis of the branch is the strongest.
This strong pair of septa can usually be well seen when a slender branch of
a Madrepore is examined by a lens by transmitted light. At the apex of each
branch there is a terminal zooid and in the skeleton an apical calyx. The
terminal zooid is (in some species at least) different from the lateral or
radial zooids. The former is radially symmetrical and has six long equal
digitiform tentacles, the latter have usually twelve tentacles, of which
six are larger than the others. These tentacles alternate, but they are so
arranged on the disc as to give a distinctly bilateral appearance to the
zooids.
The colour of the West Indian Madrepores appears to be entirely due to
Zooxanthellae (pp. 86, 125). They are lighter or darker shades of brown,
sometimes becoming green, yellow, or orange. On the Australian barrier reef
and other reefs of the eastern seas the growing points of the branches are
variable and often brilliantly coloured, emerald green, violet, or red;
giving some of the most wonderful colour effects for which the reef pools
are famous. The cause of these brilliant apical colours has not yet been
ascertained.
The genus is found in shallow water of all seas of the tropical belt except
on the western side of the continent of America.
_Montipora._—In this genus the calices are small and situated in
depressions in the coenosteum, and there are six, sometimes twelve, septa
of approximately equal size. There is no terminal calyx at the apex of the
branches. This is a genus of very variable form and wide distribution in
all tropical seas except on the shores of the Atlantic Ocean.
_Turbinaria._—This genus is usually cup-shaped or foliaceous and twisted in
form. The septa may be six to thirty in number. Some of the species of this
genus attain to a very great size in favourable localities. There is a
specimen in the British Museum that is 16 feet in circumference and
weighed, when dried, 1500 lbs.
FAM. 2. PORITIDAE.—The corallum is usually encrusting, foliaceous, lobed or
tufted, rarely dendritic. The whole skeleton is built up of a system of
trabeculae and stout cross bars, and in {397}section the limits of the
calices are not well defined. The septa are represented by twelve
trabeculae. The zooids are small and are usually provided with twelve
tentacles. The most important genus is _Porites_, which is so abundant on
many reefs that it may be said to rival _Madrepora_ itself in the
luxuriance of its growth. On the Australian barrier reef a species of
_Porites_ builds up coralla over twenty feet in length and as many in
height. According to Saville Kent they are usually found on the outer side
of the reef and form a basis of support for the high-level Madreporas and
other corals.[412]
The colours of _Porites_ are very variable and often beautiful. In
Jamaica[413] the prevailing colours are bright blue, pale yellow, and
yellowish green. In Australia the colours are less brilliant perhaps, but
among the prevailing tints are light or bright lilac, a delicate pink, dark
yellow, and brown. The genus _Porites_ occurs in Eocene and Miocene
deposits, and is now found on all the more important coral reefs of the
world.
The genus _Alveopora_ is usually placed with the Poritidae. According to
Bernard,[414] however, its affinities with this family are remote, and it
is more closely related to the Favositidae (see p. 344). The walls of the
calices are contiguous and the septa are reduced to rows of spines, as in
the Favositidae. It is found in shallow water in the Pacific, the Indian
Ocean, and the Red Sea.
SUB-ORDER 2. CYCLOCNEMARIA.
Madreporaria forming perforate or imperforate coralla. Solitary or
colonial. The zooids have usually a large number of mesenteries arranged in
two or more cycles. The mesenteries beyond the protocnemic pairs arise in
unilateral pairs in chambers other than those between the directives.
SECT. 1. APOROSA.—Cyclocnemaria in which the theca and septa are not
perforated. The zooids of the colonial forms may communicate by means of
superficial canals of the coenosarc, or they may be in contact with one
another only at their edges.
Several families are included in this section, of which the more important
are:—
{398}FAM. 1. TURBINOLIIDAE.—The corals included in this family are mostly
solitary forms attached to foreign objects, or living partly embedded in
sand. In some cases a small colony is formed by gemmation.
The genus _Flabellum_ is a solitary coral of a compressed top shape. It has
a large number of septa arranged radially on the cup-wall. This cup-wall is
not a true theca but an epitheca. In some forms root-like tubes grow out
from the sides of the cup near its base and may serve to support the coral
on solid objects. In some remarkably fine specimens recently obtained from
the Persian Gulf these tubes served to attach the coral to a telegraph
cable. _Flabellum_ seems to be cosmopolitan in its distribution. It is
usually found in deep or moderately deep water, but some specimens have
been dredged in water of 2 to 9 fathoms.
[Illustration: FIG. 174.—Side view of _Trochocyathus hastatus_, with exsert
septa, well-marked costae (_c_), and with three spinous projections (_Sp_)
at the base formed by outgrowths from primary costae. (After G. C.
Bourne.)]
_Caryophyllia_ is a conical coral fixed by a slightly expanded base. The
cup-wall is a true theca covered below by an epitheca. There is a spongy
columella surrounded by a single circle of pali. There is one British
species, _C. smithii_. It is found attached to shells at a depth of about
thirty fathoms near the Eddystone Lighthouse and in other localities in the
English Channel. It also occurs between tide marks in the Scilly Islands,
and is found off the Shetlands, on the west coast of Scotland, and the
south-west of Ireland. The genus is widely distributed and extends from
shallow water to depths of 1500 fathoms. _Caryophyllia_ sometimes occurs in
clusters which have the appearance of an incipient colony. This may be due
to the embryos fixing themselves upon the epitheca of existing individuals
and developing there. It is doubtful whether the species ever reproduce
asexually either by gemmation or by fission. When the zooid is fully
expanded it projects some distance above the corallum and shows a very
transparent body-wall with a crown of some fifty tentacles. Each tentacle
terminates in a globose head (Fig. 169) charged with nematocysts. The
general colour is pale pink, and there is a broad brown circle {399}round
the mouth. Large specimens may be three-quarters of an inch in diameter.
_Turbinolia_ is a common Eocene fossil genus found in England and France,
and is stated to occur in the Caribbean Sea. The columella stands up like a
stylet and the septa are "exsert," _i.e._ project above the rim of the
theca.
_Trochocyathus_ is a genus with well-marked "costae" occurring in tropical
shallow water (Fig. 174).
FAM. 2. OCULINIDAE.—Colonial forms, dendritic or encrusting, with
relatively large and rather prominent calices separated by considerable
stretches of compact coenosteum. The zooids bear a crown of ten to
forty-eight or more capitate tentacles.
_Neohelia_ has a fistulose stem lined internally by a horny membrane. There
seems to be some reason for supposing that this membrane is formed by the
zooids themselves. A similar membrane is found in the fistulose stems of
_Amphihelia_ and perhaps other Oculinidae. If this membrane is really
formed by the activity of the corals it forms an exception to the general
rule that the skeleton of the Madreporaria is entirely calcareous. Others
maintain, however, that this membrane is formed by the Chaetopod worms
which are found in the tubes, and that the fistulose stem of the coral is
formed by folding round and encrusting the horny tubes of the worm.
_Neohelia_ is found in the Pacific Ocean.[415]
_Lophohelia_ is a genus forming dendritic colonies of considerable size.
The calices have thick walls and are very deep. _Lophohelia prolifera_ has
been found in deep water off the island of Skye and in other localities off
the west coast of Scotland. It is also not uncommon in some of the
Norwegian fjords and in other parts of the world.
_Oculina_ is another widely distributed genus found in the shallow tropical
waters of the West Indies, the Indian and Pacific Oceans. It forms
dendritic colonies of considerable size. The calices are usually arranged
in a spiral manner on the branches. The colour of the West Indian species
is stated to be light or dark brown when alive. The tentacles are arranged
in three cycles, and are usually twenty-four in number. Asexual
reproduction takes place by budding at the apex of the branches.
FAM. 3. ASTRAEIDAE.—This is a very large family, and {400}authorities are
not agreed as to its limits or classification. Excluding the simple forms
for the present, the family may be said to be distinguished by having the
calices so closely crowded that there is little or no coenosteum between
them. The corallum is compact and massive, unless bored and perforated by
algae, worms, and other coral-destroying organisms.
The genera of Astraeidae that form colonies may be divided into two groups:
the GEMMANTES and the FISSIPARANTES. In the group GEMMANTES asexual
reproduction is effected by gemmation, and each zooid of a colony is a
distinct individual with two pairs of directive mesenteries. Among the best
known of recent corals included in this group may be mentioned _Galaxea_.
In this genus there is a good deal more coenosteum between the calices than
there is in most of the Astraeidae. The calices are long and project some
distance above the coenosteum. The septa are exsert. In _Galaxea esperi_
examined by Fowler[416] there are twelve septa, twelve pairs of
mesenteries, and twenty-four tentacles, of which twelve are very small and
twelve rather larger. The colour is green or brown. The genus is found in
shallow water in the tropics of the old world.
In _Astrangia solitaria_ the zooids are either isolated or more generally
united by thin strands of perithecal tissue to form encrusting colonies.
The septa are not exsert as in _Galaxea_. Six are prominent and belong to
the first cycle, six smaller ones form a second cycle, and an incomplete
third and fourth cycle may be seen. Corresponding with each septum there is
a tentacle. The tentacles of the innermost cycle are the longest (3 mm. in
length). All the tentacles terminate in a knobbed apex. The living zooids
are colourless throughout, or display only very delicate tints within
restricted areas.[417] This genus occurs principally on the coasts of the
American continent, extending as far south as the Straits of Magellan.
Other well-known genera of Astraeidae Gemmantes are _Orbicella_,
_Cladocora_, _Phyllangia_.
In the group FISSIPARANTES asexual production takes place by fission
without the production of morphologically complete zooids. The tentacles,
mesenteries, and septa, when fission is established, are not arranged in
regular hexameral cycles, and no {401}new directive mesenteries arise. In
some cases very large corals are formed, and, if our conception is correct,
these must be regarded, not as a colony of zooids, but as a single
individual zooid divided into a considerable number of incompletely
separated parts. Among the well-known genera belonging to this group are
_Euphyllia_, _Mussa_, _Meandrina_, _Coeloria_, _Favia_, and _Goniastraea_.
In such genera as _Euphyllia_ the parts of the colony become separated by
deep grooves, and have the superficial appearance of being distinct
individuals; but in the Brain-coral _Coeloria_ and others the surface of
the coral presents a series of more or less bent or curved grooves, each
with a row of slit-shaped mouths and bordered by rows of tentacles.
A number of genera of solitary corals united in the subfamily
Trochosmiliacea are generally included in the family Astraeidae. The study
of their skeletal characters has suggested[418] that they are more closely
allied to the Turbinoliidae. The principal genera thus transferred would be
_Trochosmilia_, _Placosmilia_, _Parasmilia_, and _Asterosmilia_. As these
genera and their allies are nearly all extinct, and nothing is known of the
structure of the living zooids, their removal from the Astraeidae may be
regarded as not fully justified.
FAM. 4. POCILLOPORIDAE.—The general anatomy of the zooids of this family of
corals has some resemblance to that of the Entocnemaria, and it is possible
that they will eventually find a place in our classification near to, if
not actually within that group. The fact, however, that the skeleton is
imperforate is sufficient for the present to justify the inclusion of the
family in the section Aporosa. There are but two genera at present known,
and in both of them the zooids have twelve tentacles, twelve mesenteries,
and only two mesenterial filaments. The zooids are connected together by an
elaborate system of canals running in the superficial coenosarc. The
calices are bilaterally symmetrical, and in _Seriatopora_ the septa which
are parallel with the axis of the branch are united in the centre of the
calyx, and are very much larger than the others, as in _Madrepora_. In all
these characters the family shows affinities with the Entocnemaria. In the
characters of the skeleton, which is imperforate and tabulate, the
affinities are rather with the {402}Cyclocnemaria. The two genera are
widely distributed on the coral reefs of the old world, and in some
localities are very abundant. Neither genus is found in the West Indies.
They are both of recent origin, but _Pocillopora_ occurs in the Miocene. It
is a remarkable feature of the family that both genera may be attacked by
the gall-forming crab _Hapalocarcinus_. From some reefs nearly all the
Pocilloporidae show crab-galls on a large number of their branches, whereas
other Madreporaria are free from them.
[Illustration: FIG. 175.—A portion of a colony of _Pocillopora_ from the
Maldive Archipelago.]
[Illustration: FIG. 176.—A single calyx of _Pocillopora septata_, showing
_Co_, the columella; _S_, _S_, the septa; _Th_, the theca wall. (After
Gardiner.)]
_Pocillopora_ is a coral that forms encrusting masses, rising into lobes or
branches of considerable size, terminating in blunt apices. _Seriatopora_
is much more slender and ramified, the branches terminating in sharp
points.
SECT. 2. FUNGACEA.—This section of Cyclocnemaria contains a number of
solitary and colonial corals of very varied form united in the possession
of a number of cross-bars called "synapticula" connecting the septa, and
thereby giving strength to the calyx apart from any increase in the
thickness of the calyx-wall. The family Fungiidae shows many peculiarities
which separate it very distinctly from both the Cyclocnemaria and the
Aporosa. The Eupsammiidae, however, approach the {403}Cyclocnemaria in many
respects, and the Plesiofungiidae form a connecting link with the
Astraeidae. It is very probable that this section had a dual origin, and
therefore does not represent a single line of descent.
FAM. 5. PLESIOFUNGIIDAE.—This family is related to the Aporosa in the
possession of septa that are generally solid and imperforate, and to the
Astraeidae in particular in the possession of dissepiments. They differ
from them, however, in the presence of synapticula and in certain
peculiarities of the tentacles.
The genus _Siderastraea_ has recently been studied by Duerden.[419] The
colony is usually massive and encrusting in habit. The zooids when expanded
do not rise much above the level of the corallum. The tentacles are short
and are arranged in irregular cycles on the disc. They terminate in knobbed
extremities, and those of the inner cycles are bifurcated. The colour of
_S. sideraea_ is reddish-brown when alive. _Siderastraea_ is found in
shallow water on the coral reefs, and is widely distributed.
In _Agaricia_ the colony is more foliaceous. The tentacles are rudimentary
or small. The colour of the living zooids is very similar to that of
_Siderastraea_. _Epistrelophyllum_ is a solitary coral, from the Jurassic
series, belonging to the family.
FAM. 6. FUNGIIDAE.—_Fungia_ is an unattached solitary coral of a flat
disc-like shape with very numerous exsert imperforate septa. It is
frequently of considerable size (six to twelve inches in diameter). On many
of the coral reefs of the old world it is extremely abundant, and
consequently it is one of the commonest corals of our collections. When
alive the corallum is almost hidden by the disc, which is studded all over
with very numerous long tentacles.[420] The colour varies in different
species, but is usually brown. One species on the Australian barrier reef,
_F. crassitentaculata_, is of a dark olive green colour, the tentacles
terminating in white knobs.
The free adult Fungias are derived from a fixed stock called the
trophozooid, from which the young Fungias are detached by transverse
fission (see p. 388). The thecal wall of the young _Fungia_ when detached
from the trophozooid is perforated, but {404}the pores become largely
filled up during the later growth of the coral.
There are several genera of colonial Fungiidae of less frequent occurrence,
such as _Halomitra_, _Herpetolitha_, and _Cryptabacia_.
FAM. 7. CYCLOSERIDAE.—These are solitary or colonial Fungacea with an
imperforate theca. _Bathyactis_ occurs at great depths. _Diaseris_, shallow
water on coral reefs.
FAM. 8. PLESIOPORITIDAE.—The septa in this family are trabeculate and
perforate, resembling in this respect the septa of Poritidae.
_Leptophyllia_, _Microsolena_, extinct.
FAM. 9. EUPSAMMIIDAE.—This family of perforate corals is usually placed
with the Madreporidae and Poritidae in the old group Perforata. The
researches of Fowler and Gardiner have shown that the arrangement of the
mesenteries is that of the Cyclocnemaria, and the presence of synapticula
connecting the septa suggests affinities with the Fungacea. The synapticula
of the Eupsammiidae, however, are peculiar in being arranged, not in a
vertical series, but alternately with one another or quite irregularly in
position. The members of this family are solitary or colonial in habit.
_Stephanophyllia_ is a flattened disc-shaped coral, with perforate and
dentate septa, found in the Pacific Ocean and as a fossil in various strata
since Cretaceous times.
In _Leptopenus_, from depths of about 1500 fathoms, the perforations are
much larger than in the last-named genus, and the skeleton is reduced to a
system of slender trabeculae.
_Rhodopsammia_ has a conical shape, and gives rise by gemmation to a number
of young zooids, which remain attached for some time to the parent form
before becoming free.
Among the colonial genera are _Dendrophyllia_, _Coenopsammia_, and the
well-known Mediterranean genus _Astroides_.
ORDER IV. ZOANTHIDEA.
This order of Zoantharia consists of a number of solitary or colonial
Anemones that do not form a skeleton of horn or carbonate of lime, and are
distinguished from the Actiniaria by the peculiar arrangement of their
mesenteries.
FAM. 1. ZOANTHIDAE.—_Sphenopus_ is a solitary coral and terminates aborally
in a small sucker-like base, by which it may {405}be attached to foreign
bodies. The genera _Gemmaria_ and _Isaurus_ include solitary forms.
In the majority of the species of Zoanthids, however, a basal encrusting
stolon is formed, which may be thick and fleshy or membranous, or may
consist of a plexus of bands from which several zooids rise and on which
the new buds are formed.
The tentacles are numerous, simple, usually short, and arranged in one or
two circles on the margin of the disc. Most Zoanthidae are encrusted with
sand, shell fragments, or sponge spicules, but _Zoanthus_ and _Isaurus_ are
naked. The foreign particles that form the incrustation are firmly attached
to the ectoderm, and as a rule many of them sink down into the mesogloea to
give additional support to the body-wall. It is the presence of so much
incorporated sand that frequently gives these Zoantharia such a very
brittle character. The stomodaeum usually exhibits a well-marked ventral
siphonoglyph. The mesenteries consist of a pair of complete ventral
directives, a pair of incomplete dorsal directives, while of the remaining
protocnemes the lateral mesenteries which are first and second in the order
of appearance are complete, the sixth is incomplete, whereas the fifth is
complete in the Macrocneminae and incomplete in the Brachycneminae.
Duerden[421] has found in specimens of three species that the arrangement
of the mesenteries is "brachycnemic" (the sixth protocneme imperfect) on
one side and "macrocnemic" (the sixth protocneme perfect) on the other. The
metacnemes appear in the spaces between the sixth protocnemes and the
ventral directives in unilateral pairs, of which one becomes complete and
the other always remains incomplete (Fig. 163, 4, p. 368).
The Zoanthidae are usually dioecious, but hermaphroditism undoubtedly
occurs in the genera _Zoanthus_ and _Isaurus_. Little is known of their
development, but a larval form discovered by Semper off the Cape of Good
Hope, of cylindrical shape, with an opening at each end and distinguished
by a longitudinal band of cilia running from one end to the other, is
probably the larva of a Zoanthid. It is commonly known as Semper's larva.
Other larvae provided with a ring of cilia have also been attributed to
this group.
A great many Zoanthidae are epizoic in habit. Thus several {406}species of
_Epizoanthus_ form colonies on the shells of Gasteropods inhabited by
hermit crabs. _Parazoanthus tunicans_ is found on the stem of a
_Plumularia_; _Parazoanthus separatus_, from Jamaica, is associated with a
sponge. The base of the bundle of long spicules of the Sponge _Hyalonema_
(p. 204) is almost invariably sheathed by a colony of _Epizoanthus
stellaris_.
The only genera occurring within the British area are _Epizoanthus_ (with
six species), _Parazoanthus_ (with four species), and _Zoanthus sulcatus_.
Of the species of _Epizoanthus_, _E. incrustatus_ is fairly common, in
depths of twenty to eighty fathoms on all our coasts, and is frequently
commensal with different species of hermit crabs, while _E. paguriphilus_
is found in much deeper water off the west coast of Ireland and is always
commensal with hermit crabs. _Parazoanthus anguicomus_ is found at depths
of a hundred fathoms off the Shetlands and west of Ireland, and is usually
associated with various species of Sponges.
_Gerardia savalia_ is the largest "black coral" of the Mediterranean. The
colony begins by encrusting the stem of one of the Gorgoniidae, but soon
surpassing its support in growth, it forms a basal horny skeleton of its
own and builds up very large branching colonies. A specimen in the British
Museum,[422] from twenty fathoms off the island Negropont, is two metres
high and two metres wide. The genus appears to be related anatomically to
_Parazoanthus_.
[Illustration: FIG. 177.—_Zoanthus macgillivrayi_, a small colony. The
tentacles are shown somewhat contracted by the preservative. Each zooid is
about 25 mm. in length. (After Haddon.)]
FAM. 2. ZAPHRENTIDAE.—This family of Palaeozoic corals is usually placed
with the Turbinoliidae or in the separate group Tetracoralla. Recently
Duerden[423] has given reasons, based on the method of increase of the
septa in _Lophophyllum_, for believing that their affinities lie rather
with the Zoanthidae than {407}with the Madreporaria. They are solitary
turbinate corals, with numerous septa exhibiting a distinct bilateral
symmetry in arrangement. _Zaphrentis_, _Lophophyllum_.
ORDER V. ANTIPATHIDEA = ANTIPATHARIA.
The members of this order can readily be distinguished from all other
Zoantharia by the presence of a horny axial skeleton (sclerobase) and the
absence of any spicules of calcium carbonate. The skeleton is covered by a
thin bark which consists of a number of simple, naked zooids united at
their edges. The zooids bear six tentacles, or if there are more than six,
six large prominent tentacles. In most genera there are but ten
mesenteries, in others twelve. In _Cladopathes_ only six mesenteries are
found. The skeleton of the Antipathidea is simple in _Stichopathes_ and
_Cirripathes_, but in all other genera it is ramified. The ramification is
usually profuse and irregular. The horny substance of which it is composed
is free from any deposit or infiltration of lime. The surface of the
younger branches is beset with numerous short spines, the number and
arrangement of which are characters largely used in the determination of
species. The basal parts of the main axis and the thicker branches are
frequently bare, the zooids having died and become disintegrated. In these
cases the spines wear away and the skeleton appears to be smooth. The
presence of spines on some of the branches is, however, generally
sufficient to enable the naturalist to distinguish a dried Antipathid from
the axis of a Gorgonid, with which alone it might be confounded.
There are six complete mesenteries in each zooid, but as they bear no
retractor muscles it is not certain that they represent the first six
protocnemes of other Zoantharia. In a great many species the zooids are
oval in shape, the longer diameter being parallel with the axis of the
branch. The mouth and stomodaeum are compressed and at right angles to this
diameter. It is usually assumed that the mesenteries attached to the angles
of the stomodaeum are the directives, and that the remaining pair, which is
axial in direction, corresponds with the first pair of protocnemes. The
axial pair of mesenteries is frequently very well developed and alone bears
the gonads. When other mesenteries are formed they always arise in
bilateral pairs between the axial mesenteries {408}and the directives. The
tentacles correspond with the intermesenteric chambers. In some genera
there is a constriction of the zooid between the pairs of the tentacles on
each side of the axial mesenteries and the directive tentacles. This gives
them the appearance of a division into three zooids with two tentacles
apiece, one with a mouth and two without a mouth; and as the mouthless
parts alone bear the gonads on the single axial mesentery, they have been
called the "gastrozooids" and "gonozooids" respectively. This must not be
regarded, however, as a case of true dimorphism, as the cavities of the
so-called gastrozooid and gonozooids are continuous.
The Antipatharia are widely distributed in nearly all the great seas of the
world. Some species are found in shallow water in the tropics, but most of
them occur in depths of fifty to five hundred fathoms. The genus
_Bathypathes_ is only found at enormous depths ranging from 1070 to 2900
fathoms. Specimens of _Cirripathes spiralis_, _Antipathella gracilis_, and
another species have recently been obtained in deep water off the west
coast of Ireland,[424] but these are the only Antipatharia known to occur
within the British area.
The very simple structure of the Antipatharia is usually attributed to
degeneration. On this view the Antipathidae with only six complete
mesenteries are the most modified, whereas the Leiopathidae with twelve
mesenteries are more closely related to the ancestral forms, and _Gephyra
dohrnii_ (see p. 382) is a link connecting the order with the Actiniaria.
There is no reason, however, for supposing that _Gephyra_ is specially
related to this order, and, as pointed out recently by Roule,[425] the
simple structure of the zooids of the Antipathidea is more easily explained
if they are regarded as primitive forms.
_Gerardia_ (p. 406), from the Mediterranean, forms a horny axial skeleton
like that of the Antipathidea, but this genus is probably a Zoanthid.
FAM. 1. ANTIPATHIDAE.—In this family the zooids have six tentacles and six
or ten mesenteries. It includes nearly all the familiar genera, such as
_Stichopathes_, _Cirripathes_, _Antipathes_, _Antipathella_, _Cladopathes_,
and _Bathypathes_. _Schizopathes_ and {409}its allies occurring in deep
water are the forms regarded by Brook as dimorphic.
[Illustration: FIG. 178.—A portion of a branch of _Antipathes ternatensis_,
showing three zooids and the horny axis beset with thorn-like projections.
(After Schultze.)]
FAM. 2. LEIOPATHIDAE.—This family includes the single genus _Leiopathes_ of
the Mediterranean Sea. It is distinguished from the others by the presence
of twelve mesenteries.
FAM. 3. DENDROBRACHIIDAE.—This family also consists of a single genus,
_Dendrobrachia_, from 400 fathoms in the South Atlantic. It is
distinguished by having pinnate retractile tentacles.
ORDER VI. CERIANTHIDEA.
This order contains the remarkable Sea-anemone called _Cerianthus_. Two of
the species have been placed in separate genera, but they do not appear to
be of more than sub-generic rank. _Cerianthus_ has a long cylindrical body
with a double crown of numerous long tentacles at the oral extremity and
tapering to a blunt point or rounded at the aboral extremity.
There are numerous mesenteries, which increase in number by the addition of
bilateral pairs, arising only in the ventral inter-mesenteric space
throughout the greater part, if not the whole, of the life of the zooid.
The right mesentery of each young pair is always more advanced than the
left, so that the mesenteries have the appearance of arising alternately
right and left. None of the mesenteries bear conspicuous bands of retractor
muscles. The movements of the body are effected by a thick band of
longitudinal fibres lying between the ectoderm and the mesogloea in the
body-wall.
The absence or very slight development of muscles on the mesenteries
renders it difficult to recognise the homologues of the protocnemes of
other Zoantharia in the adult. From the evidence of embryology, however, it
seems certain that the six dorsal pairs of mesenteries represent the
protocnemes (Fig. 163, 3, p. 368) and the others are metacnemes.
{410}The stomodaeum exhibits a single long deep siphonoglyph, which is
probably dorsal in position.
There are two tentacles to each inter-mesenteric space, one being marginal
and the other circumoral. The gonads are borne upon alternate mesenteries,
and both ova and spermatozoa are produced by the same individual.
[Illustration: FIG. 179.—_Cerianthus membranaceus._ Colour pink, with
tentacles annulated pink and brown. About 35 cm. in length. (After
Andres.)]
The ectoderm of _Cerianthus_ is remarkable for the immense number of
nematocysts and gland cells. The latter secrete a quantity of mucus which
binds the threads of the discharged nematocysts into a sticky feltwork and
this secures particles of sand and mud, the whole forming a long tube in
which the animal freely moves. This tube is often of considerable
thickness. It is tough and resistant, smooth inside but ragged and muddy
outside. It is often many times the length of the animal's body.
The embryo of _Cerianthus_ is set free before the completion of
segmentation, and it gives rise to a floating pelagic larva known as
_Arachnactis_. It has a variable number of tentacles and mesenteries
according to its age, but when it reaches a size of {411}about 15 mm. in
length it has developed characters which are sufficient to determine its
position as a Cerianthid.
The genus _Cerianthus_ appears to be widely distributed. _C. membranaceus_
is the common species in the Mediterranean Sea, but a smaller species has
been described from Naples under the name _C. oligopodus_ by Cerfontaine.
_C. americanus_ occurs on the eastern coasts of North America. The British
and North European species is _C. lloydii_, but another species, _C.
vogti_, has been found at a depth of 498 fathoms in the North Sea. _C.
nobilis_ is a gigantic species supposed to be about 1 foot in length when
complete, from Torres Straits.
_C. bathymetricus_ of Moseley, placed by Andres in the genus _Bathyanthus_,
is a species of small size (25 mm.), obtained by the "Challenger" from a
depth of 2750 fathoms in the North Atlantic. It exhibits a remarkable
prolongation of the stomodaeum into the coelenteron in the form of a sack
which contained food. Moseley described a species of _Cerianthus_, 6 inches
long, living on the coral reef at Zebu in the Philippines fully expanded in
the tropical sunshine.
Several species of _Arachnactis_ larvae have been described. Of these
_Arachnactis lloydii_ appears to be undoubtedly the larva of _C. lloydii_.
The adult forms of _Arachnactis albida_ from various stations in the
Atlantic Ocean and of _Arachnactis americana_ are not known. The larva of
_Cerianthus membranaceus_ has been called _Dianthea nobilis_, and is
characterised by the great length of the column, by the general opacity of
all parts of the body, and by the precocious appearance of the median
marginal tentacle. A considerable number of remarkable pelagic larvae have
been described by van Beneden[426] from the Atlantic Ocean, and
provisionally assigned by him to five different genera. The adult forms of
these larvae are not known, but they are probably members of this order.
{412}CHAPTER XV
CTENOPHORA
The Ctenophora are spherical, lobed, thimble-shaped, or band-like animals,
usually very transparent and gelatinous in structure. They are exclusively
marine, and are found floating at or near the surface of the sea.
Although they are generally classified with the Coelenterata, they are
regarded by some authors as having closer affinities with the Polyclad
Turbellaria (cf. Vol. II. p. 7). They agree, however, with neither of these
divisions in their essential characters, and the only way to indicate and
emphasise their unique position is to place them in a separate Phylum.
They differ from all the Coelenterata in the absence of nematocysts, and in
the presence in development of a definite mesoblast. The character from
which they derive their name, Ctenophora, is the presence on the surface of
bands of swimming plates. The plates are called the "combs" (κτείς, gen.
κτενός = a comb) or "ctenophoral plates." They occur in all genera included
in the Phylum except in _Coeloplana_ (Fig. 183, p. 422).
Another peculiarity of all Ctenophora (except the Beroidae) is the
presence, at some stage in the life-history, of two long and extremely
contractile tentacles. There is also a well-developed sense-organ
(statocyst) in the centre of the aboral area of the body.
The Ctenophora differ from the Turbellaria in the presence of the combs and
of the two long tentacles, in the position and relative importance of the
statocyst, and, with the exception of _Coeloplana_, in the general
characters of the alimentary canal.
SHAPE.—Several of the Ctenophora are conical or spherical in shape, but
exhibit at the pole where the mouth is situated {413}(Fig. 180, _M_) a
slight conical projection, and at the opposite pole where the sense-organ
is placed a slight depression (_Ab_). In others, the sides of the body are
drawn out into a pair of wing-like lobes (Lobata), and the body is
considerably flattened or compressed (Fig. 181). The Cestoidea have a long
flattened ribbon- or band-shape (Fig. 182), and the Platyctenea (Fig. 183)
are flattened in the oro-apical axis and exhibit a well-marked distinction
between the dorsal and ventral surfaces. The shape of _Beroe_ is that of a
hollow cone or thimble.
[Illustration: FIG. 180.—_Hormiphora plumosa._ _Ab_, position of the aboral
sense-organ; _Ct_, rib of ctenophoral plates; _M_, mouth; _t_, tentacle,
with two kinds of pinnae. (After Chun.)]
CTENOPHORAL PLATES.—In many Ctenophora eight lines can be traced, like the
lines of longitude on a globe, from the area of the sense-organ to the base
of the mouth-cone or hypostome. In the course of these lines are situated
the ctenophoral plates. In some species they extend along the greater part
of these lines of longitude, but in others they are more restricted. That
part of the line that bears the plates is called the "rib" or "costa."
These plates or combs form the principal organs of locomotion of the
Ctenophores. They consist of a row of cilia fused at the base (cf. p. 141)
to form the plate, but free at the extremity where they form the comb-like
edge. They are alternately raised, by a rapid contractile action, and then
slowly flattened down again. The plates are raised in succession from the
aboral to the oral end of each rib, and the appearance given to the bands
in the living animal is that of a series of waves travelling down the lines
of longitude from the sensory area towards the mouth. The effect of these
rhythmic movements of the combs is to {414}drive the animal slowly through
the water with the oral cone forwards. In some Ctenophores the costæ are
phosphorescent.[427]
TENTACLES.—In all the Ctenophora, except the Beroidae and the adult stages
of Lobata and Cestoidea, there is a single pair of tentacles. They are
attached to the base of a blind funnel-shaped pit which opens to the
exterior near the equator of the animal's body. The pits are on opposite
sides of the body, and the plane which passes through them both vertically
divides the body into approximately equal parts. It is called the
"tentacular" or "transverse" plane (Fig. 180). The plane at right angles to
this, which also passes through the mouth and statocyst, is called the
"sagittal" plane.
The tentacles are solid, and in the Cydippidae, of considerable length.
During life they are usually extended, and trail behind the animal as it
progresses through the water. But they are extremely contractile, and when
the animal is alarmed are suddenly withdrawn into the shelter of the
tentacular pits. Each tentacle usually bears a row of short pinnae. The
surfaces of the tentacles and of their pinnae are crowded with remarkable
cells which carry little globules of an adhesive secretion, and are called
the glue-cells or "colloblasts." These cells stick to any foreign body they
touch, and may be drawn out some distance from the tentacle, but they
remain attached to it by a long spiral thread which unwinds as the cell is
pulled out. Although the colloblasts have the function of catching prey
similar to that of the nematocysts of Coelenterata, they are true animal
cells and are not therefore homologous with nematocysts, which are the cell
products of the cnidoblasts.[428]
The Lobata and Cestoidea pass through a stage in development called the
Cydippiform or _Mertensia_ stage, when they possess a single pair of long
tentacles similar to those described above. In the adult condition,
however, these tentacles are absent, and their functions are performed by
numerous small accessory tentacles or tentilla arranged in rows on definite
lines along the body-wall.
SENSE-ORGAN.—At the aboral pole of the Ctenophore there is a hard
granulated calcareous body, the "statolith." This is {415}supported by four
tufts of fused cilia, and is usually covered by a dome of delicate
protoplasmic texture, which is believed to be formed by a fusion of cilia.
The dome enclosing the statolith is called the "statocyst."
Supporting the statocyst there is a circular or oval area of ciliated
epithelium which is usually supposed, but on insufficient evidence, to be
specially sensory in function. Extending from this area in the sagittal
plane there are two strips of ciliated epithelium called the "polar
fields."
The aboral sense-organ of the Ctenophora is one of the most characteristic
organs of the Phylum. The aboral pole of the Medusae of Coelenterata is
usually devoid of any special modification of the ectoderm of the bell, and
in the Tiarid genus _Stomatoca_ the little tassel at the aboral pole of the
Medusa cannot in any sense be regarded as a homologue of the sense-organ of
the Ctenophore. If the aboral sense-organ of the Ctenophora can be compared
with that of any other group of animals, it would be with the statocyst of
many of the Turbellaria, such as that of _Convoluta_, but it is far more
satisfactory to regard it as an organ peculiar to the Ctenophora and as
having no true relationship with any sense-organ found in other animals.
ALIMENTARY CANAL.—The mouth of the Cydippiform Ctenophores opens into a
sac-like chamber called the "stomodaeum," flattened in the sagittal plane
and stretching from the oral pole as far as the centre of the body. The
stomodaeum passes into a chamber flattened in the transverse plane called
the "infundibulum." From the infundibulum a narrow tube passes in the
direction of the aboral pole called the "intestine," and from the extremity
of this four short tubes pass to the sides of the polar fields at the place
where these fields join the sensory area. Two, or, in some cases, all four
of these tubes open to the exterior; but they do not appear to serve the
purpose of ejecting the undigested portions of the food, which usually pass
to the exterior by the mouth as in Coelenterata and Turbellaria.
From the lateral extremities of the infundibulum four pairs of tubes pass
to the equatorial region of the body, where each one joins a longitudinal
vessel which runs immediately beneath the epithelium supporting the ribs.
These are called the longitudinal or "sub-costal" canals. From the
infundibulum there also {416}passes a single pair of blind canals, the
"paragastric canals," one on each side of the stomodaeum, to end in the
oral cone.
In the Lobata the paragastric canals communicate with the longitudinal
canals under the transverse costae,[429] and send long blind processes into
the lobes. In the Cestoidea the arrangement of the canals is considerably
modified in adaptation to the needs of the ribbon-like body. In the
Beroidae the paragastric and longitudinal canals are in communication by a
peripheral network of canals, and in the Platyctenea there is also a
network of canals but without any definite longitudinal vessels.
SEXUAL ORGANS.—Most of the Ctenophora are undoubtedly hermaphrodite, but
Willey was unable to find ova in some of his specimens of _Ctenoplana_ that
were producing spermatozoa. In the Cydippidea the ova are produced on one
side of the longitudinal canal and the spermatozoa on the other. Each
longitudinal canal therefore performs the functions of a hermaphrodite
gland. When the sexual cells are ripe they escape into the infundibulum and
are discharged by the mouth. In _Ctenoplana_ there are definite and direct
male genital ducts.
The ova are very small when discharged and undergo complete segmentation in
the sea water. The development of the Cydippidea is really direct, but
there is a stage passed through in which the tentacles are relatively very
prominent and situated close to the aboral pole, and this stage is very
different in appearance from the adult. In the Lobata and Cestoidea there
is, however, a definite larval stage, of the general appearance of a
_Mertensia_, and during this stage fertile eggs and spermatozoa are formed
and set free.
DISTRIBUTION.—Ctenophora are found at the surface of nearly all seas, and
many of the genera have a cosmopolitan distribution. Some of the Lobata,
the Cestoidea, and the Platyctenea are more commonly found in the warmer
regions of the world. _Pleurobrachia pileus_, _Bolina infundibulum_, _Beroe
ovata_, and _B. cucumis_ occur off the British coast.
Most of the Ctenophora are from 5 to 20 mm. in diameter, but _Beroe_
reaches the length of 90 mm., _Eucharis multicornis_ {417}a height of 250
mm., and _Cestus veneris_ has been found no less than 1½ metres from one
extremity to the other.
Ctenophores usually go about in shoals, and in the case of _Beroe cucumis_
and _Eucharis multicornis_ the shoals may be of very great extent.
_Pleurobrachia pileus_ of the British coasts is often found at the end of
the season (July) as a series of isolated individuals; but in June they
occur in small shoals, swimming so close together that they will choke a
tow-net in a very short space of time.
CLASS I. TENTACULATA
Ctenophora provided with a pair of tentacles in the larval stages only or
in both larval and adult stages.
ORDER I. CYDIPPIDEA.
This order includes a number of spherical or oval Ctenophores, with a pair
of tentacles retractile into deep tentacular pits in the adult stage.
FAM. 1. MERTENSIIDAE.—The body is compressed in the transverse plane, and
the ribs on the transverse areas are longer than those on the sagittal
areas. The family includes the genus _Euchlora_, which occurs in the
Mediterranean and in the northern part of the Atlantic Ocean. In
_Charistephane_ there are only two enormous ctenophoral plates in each of
the longitudinal tracts. These plates are so broad that they almost meet
laterally to form two continuous circlets round the body of the animal.
This genus is found in the Mediterranean, but a few specimens have also
been obtained in the Atlantic.
In _Tinerfe_ the body is almost cylindrical, and there is a pair of
kidney-shaped swellings at the sides of the aboral pole. It has a pale blue
colour, and is found in the Guinea and south equatorial currents of the
Atlantic Ocean.
The name _Mertensia_ has been given to several forms that are undoubtedly
the young stages of genera belonging to the Lobata, but Chun retains the
name _M. ovum_ for a species which is very abundant in the Arctic currents
of the North Atlantic.
FAM. 2. CALLIANIRIDAE.—Two or four wing-like processes, into {418}which the
longitudinal canals extend, are found at the aboral pole. _Callianira_ has
two of these processes arranged in the transverse plane, and _Lophoctenia_
has four. _Callianira_ is found in the Mediterranean and in the Atlantic
from the Arctic to the Antarctic waters.
FAM. 3. PLEUROBRACHIIDAE.—The body is almost spherical in form, and the
eight ribs are equal in length.
This family includes the genus _Pleurobrachia_, in which the ribs extend
for a considerable distance along the lines of longitude of the spherical
body, but do not reach either the oral or the aboral areas. _P. pileus_ is
the commonest British Ctenophore, and may be found in shoals in May, June,
and July at the surface of the sea or cast up on the sand as the tide ebbs.
It is widely distributed in the North Atlantic waters. _P. rhodopis_ of the
Mediterranean has rather shorter ribs than _P. pileus_. Two new species
have recently been described from the Malay Archipelago.[430] _Hormiphora_
(Fig. 180, p. 413) differs from _Pleurobrachia_ in having much shorter
ribs, and in possessing two kinds of pinnae on the tentacles, those of the
ordinary kind and others much larger and sometimes palmate in character.
This genus has a world-wide distribution.
In _Lampetia_ and _Euplokamis_ the body is more cylindrical in shape than
it is in the other genera, but the ribs and subjacent longitudinal canals
extend up to the margin of the aboral field. Both these genera occur in the
Mediterranean, but _Lampetia_ is also found in the Malay Archipelago.
ORDER II. LOBATA.
The body is considerably flattened in the transverse plane, and the
sagittal areas are extended into the form of two wide peristomial lobes.
The oral ends of the areas between the transverse and sagittal ribs are
extended to form four flaps, called the "auricles." There are no tentacles
nor tentacle-sheaths of the ordinary kind in the adult form; but numerous
tentilla, similar in some respects to the pinnae of the tentacles of other
Ctenophora, form a fringe round the margin of the auricles and the
peristome. A single pair of long, filamentous, non-retractile tentacles
arise from the sides of the peristomium in _Eucharis {419}multicornis_.
These tentacles have no sheaths, and do not bear pinnae. They are probably
not homologous with those of other Ctenophora.
The characters that separate the families of Lobata are chiefly those of
varying size, shape, and position of the peristomial lobes and auricles. In
the Lesueuriidae the peristomial lobes are rudimentary; in the other
families they are moderately or very large. In the Bolinidae the auricles
are short, but in most of the other families they are long and ribbon-like.
In _Eucharis_ they can be spirally twisted in repose.
The modifications of the external form seen in the Lobata are accompanied
by some modifications of the internal structure. Among these, perhaps the
most interesting is a communication between the transverse longitudinal and
the paragastric canals, and the long convoluted tubes given off to the
peristomial lobes by the sagittal longitudinal canals. Very little is known
about the life-history and development of most of the Lobata, but Chun has
shown that in _Eucharis_ and _Bolina_ there is a Cydippiform larval stage
which produces ripe ova and spermatozoa. This is followed by a period of
sterility, but when the adult characters are developed they become again
sexually mature. To this series of sexual phenomena the name "Dissogony" is
given.
[Illustration: FIG. 181.—_Ocyroe crystallina._ _Ab_, aboral sense-organ;
_au_, auricle; _Can_, diverticulum from the paragastric canal passing into
peristomial lobe; _Ct_, costae; _M_, mouth; _Par_, paragastric canal
passing outwards to join one of the transverse subcostal canals; _P.L_,
peristomial lobe; _w_, wart-like tubercles on the lobe. (After Mayer.)]
The order contains only fifteen genera, but they are usually arranged in
the following eight families:—
1. LESUEURIIDAE. _Lesueuria._
2. BOLINIDAE. _Bolina, Bolinopsis._
3. DEIOPEIDAE. _Deiopea._
4. EURHAMPHAEIDAE. _Eurhamphaea._
5. EUCHARIDAE. _Eucharis._ {420}
6. MNEMIIDAE. _Mnemia, Mnemiopsis._
7. CALYMMIDAE. _Calymma._
8. OCYROIDAE. _Ocyroe._
Most of these Ctenophores occur in the warm and tropical seas; but _Bolina_
is found occasionally at Plymouth in the month of May, on the west coast of
Ireland, and at other stations on the British coasts. _Eucharis_ is
regarded as one of the most beautiful of the Phylum. A swarm, some miles in
length, of large specimens of _E. multicornis_ was met by the Plankton
Expedition in the south equatorial current of the Atlantic during the month
of September.
ORDER III. CESTOIDEA.
In this order the body is so much compressed in the transverse plane and
elongated in the sagittal plane that it assumes the shape of a long narrow
band or ribbon. The tentacular sheaths are present but the tentacles are
degenerate in the adult. The tentacular functions are performed by numerous
tentilla situated in long grooves extending along the whole length of the
oral side of the band-like body. The transverse ribs are reduced; the
sagittal ribs extend along the whole of the aboral side.
FAM. CESTIDAE.—This is the only family of the order. _Cestus veneris_, the
Venus's girdle of the Mediterranean Sea, is also found in the Atlantic
Ocean, and specimens belonging to the same genus, but probably to a
different species, occur as far north as the White Sea. Some of the larger
specimens are considerably over 1 metre in length.
[Illustration: FIG. 182.—_Cestus pectenalis._ _Ab_, aboral sense-organ;
_Ct_, the sagittal ribs; _M_, mouth. (After Bigelow.)]
_C. pectenalis_ was found in abundance off one of the Maldive Islands [431]
and differs from _C. veneris_ in having a large and {421}prominent orange
patch at each end of the body. It is said to be extremely graceful in the
water, moving with slow, ribbon-like undulations, and shining in the
sunlight with a violet iridescence. _Vexillum_, from the Mediterranean Sea
and Canary Islands, is rather more pointed at the extremities than
_Cestus_, and differs from it in some important anatomical characters.
ORDER IV. PLATYCTENEA.
This order has been constituted for two remarkable genera, in which the
oro-apical axis is so much reduced that distinct dorsal and ventral
surfaces can be distinguished.
There is a single pair of long milky-white tentacles capable of complete
retraction into tentacular sheaths.
FAM. 1. CTENOPLANIDAE.—_Ctenoplana_ was discovered by Korotneff in 1886
floating with the Plankton off the coast of Sumatra. In 1896 Willey [432]
discovered four specimens on a cuttle-bone floating off the coast of New
Guinea. To these authors we are indebted for the only accounts of this
animal that have been published.
When the _Ctenoplana_ is creeping on the bottom of a dish or with its
dorsal side downwards on the surface film of the water, it has the form of
a flattened disc with a notch on each side. On the upper or dorsal surface
eight short rows of ctenophoral plates may be seen, and in a position
corresponding with the two notches in the margin of the body are situated
the two sheaths from which the long pinnate tentacles protrude. In the
exact centre of the dorsal surface is situated the statolith, supported by
stiff processes from adjacent cells; and forming a circlet round the
statolith there is a row of short ciliated tentacles. These tentacles,
however, when examined carefully in the living animal, are found to be
arranged in two sets of about nine in each, separated by narrow gaps on
each side, the gaps corresponding in position with the axis through the
tentacles.
When the animal is swimming it assumes a helmet-shape by depressing the
sides of the body like a pair of flaps on the tentacular axis, and then the
ctenophoral plates come into play and produce the progressive movements of
the animals. The pinnate tentacles are opaque white in colour, and have
peculiar serpentine {422}movements. Very little is known at present
concerning many details of the internal anatomy, but there is one point of
considerable theoretical interest—namely, the presence of definite male
genital ducts.
Three of Dr. Willey's specimens were mottled with a green pigment, whereas
his fourth specimen and Korotneff's only specimen were mottled with a red
pigment. It has yet to be determined whether the differences which have
been observed in the individual specimens are of specific value.
FAM. 2. COELOPLANIDAE.—_Coeloplana_ was originally discovered by Kowalevsky
in the Red Sea, but has recently been found by Abbott [433] on the coast of
Japan.
[Illustration: FIG. 183.—_Coeloplana mitsukurii_, floating at the surface
of the sea with the dorsal side downwards. _T_, _T_, the tentacles
expanded. (After Abbott.)]
The Japanese species are found principally on encrusting Algae, _Zostera_,
_Melobesia_, etc., which they resemble very closely in colour. The Red Sea
species is, according to Kowalevsky, ciliated all over, but the Japanese
species are ciliated only on the ventral surface. As in _Ctenoplana_, the
body of _Coeloplana_ is a flattened disc with a notch at each end of the
tentacular axis, when creeping; but _Coeloplana_ does not swim, nor at any
time does it assume a helmet-shape. The tentacles are very long and of a
chalky-white colour. They can be retracted into tentacle-sheaths. When the
animal is excited it throws out the whole tentacle in a cloud of white
filaments, "and to watch it at such a time, shooting out and retracting the
tentacles, moving along the side of the aquarium like a battleship in
action is truly a remarkable spectacle."[434] On the dorsal side of the
body there is a series of processes which are called the dorsal tentacles.
The statolith is very small, and is not surrounded by sensory processes as
it is in _Ctenoplana_. There are no ctenophoral plates. The colours of the
Japanese {423}species are scarlet or carmine red and dirty brown or
brownish yellow. They are from 1 to 2 centimetres in diameter.
CLASS II. NUDA
Ctenophora without tentacles.
FAM. BEROIDAE.—_Beroe_, the only genus of this family and class, differs
from other Ctenophora in several important particulars. There are no
tentacles, and the stomodaeum is so large that the body-form assumes that
of a thimble with moderately thick walls. The infundibulum is small. The
paragastric and longitudinal canals give rise to numerous ramifications
which form a network distributed throughout the surface of the body. The
statolith is unprotected by a dome, and the polar fields are bordered by a
number of small branching papillae. The eight ribs extend for nearly the
whole length of the body. _Beroe_ is almost cosmopolitan, and is frequently
found at the surface of the sea in great numbers. _B. ovata_ is found off
the Shetlands, Hebrides, and west coast of Ireland, but is rare on the east
coast of the British Islands and in the English Channel. At Valencia it is
common in August and September, and sometimes reaches the great size of 90
mm. in length by 50 mm. in breadth. It is usually of a pale pink colour.
APPENDIX TO CTENOPHORA
_Hydroctena salenskii_ has recently been discovered by Dawydoff[435]
floating with the Plankton off the island Saparua in the Malay Archipelago.
It is claimed to be a connecting link between the Ctenophora and the
Medusae of the Hydrozoa.
In external features it is like one of the Narcomedusae, having a
transparent jelly-like bell with a wide bell-mouth guarded by a velum (Fig.
184, _V_). There are only two simple but solid tentacles (_t_), provided
with tentacle-sheaths, but inserted on opposite sides of the bell—not on
the margin, but, as in the Ctenophore, at a level not far removed from the
aboral pole. At the aboral pole there is a minute pore surrounded by a high
ciliated epithelium bearing an orange pigment. This leads into {424}a short
blind canal, which terminates in an ampulla bearing two statoliths
supported by elastic processes from the ampullar epithelium.
The sub-umbrellar cavity extends for a distance of about one-half the
height of the bell. The mouth (_M_), which opens into this cavity, leads
into a wide cavity that gives off a short blind canal to the side of each
tentacular sheath, and a straight tube that leads straight to the
statocyst, where it also ends blindly. There are no radial canals and no
ring canal at the margin of the umbrella. There are also no ctenophoral
plates. In the absence of any information concerning the position of the
genital glands, the character of the epithelium of the tentacles and the
development, we are not justified in regarding _Hydroctena_ either as a
Ctenophore or as a connecting link between the Ctenophora and the
Hydromedusae. It may be regarded simply as a Craspedote Medusa, probably
related to the Narcomedusae, with a remarkable aberrant aboral sense-organ.
[Illustration: FIG. 184.—_Hydroctena salenskii._ _ab_, Aboral organ; _M_,
manubrium; _t_, tentacle; _V_, velum. (After Dawydoff.)] ECHINODERMATA
BY
E. W. MACBRIDE, M.A., FRS.
Formerly Fellow of St. John's College
Professor of Zoology in McGill University, Montreal.
{427}CHAPTER XVI
ECHINODERMATA—INTRODUCTION—CLASSIFICATION—ANATOMY OF A STARFISH—SYSTEMATIC
ACCOUNT OF ASTEROIDEA
The name Echinodermata[436] means literally "spiny-skinned," and thus
brings into prominence one very conspicuous feature of most of the animals
belonging to this phylum. All, it is true, do not possess spines; but with
one or two doubtful exceptions, all have calcareous plates embedded in the
skin, and these plates, in many cases, push out projections which raise the
skin into corresponding elevations, which are called the spines. The spines
are, like the other plates, inside the skin, and to speak of an Echinoderm
living in its shell, as we speak of a Snail, is a serious error. The shell
of a Mollusc is fundamentally a secretion poured forth from the skin, and
is thus entirely external to the real living parts; but the plates and
spines of an Echinoderm may be compared to our own bones, which are
embedded deeply in the flesh. Hence the name _ossicle_ (little bone) is
used to designate these organs.
Besides the possession of these spines, Echinoderms are characterised by
having their organisation pervaded by a fundamental radial symmetry. The
principal organs of the body are repeated and are arranged like the spokes
of a wheel round a central axis instead of being, as, for example, in
Chaetopoda, arranged behind one another in longitudinal series.
In addition to these striking peculiarities, Echinoderms possess a most
interesting internal organisation, being in this respect almost exactly
intermediate between the Coelenterata {428}and the higher Invertebrata.
Like so many of the latter, the Echinodermata have an anus, that is, a
second opening to the alimentary canal through which indigestible material
is rejected; like them also, they have a body-cavity or coelom surrounding
the alimentary canal—from the lining of which the genital cells are
developed. On the other hand, there is no definite circulatory system, nor
any specialised excretory organ, and the nervous system exhibits no
concentration which could be called a brain, and is, moreover, in close
connexion with the skin. In all these points the Echinodermata resemble the
Coelenterata.
One of the most characteristic features of the internal anatomy of
Echinodermata is the presence of a peculiar series of organs, known
collectively as the water-vascular system or HYDROCOEL. This is really a
special division of the coelom or body-cavity which takes on the form of a
ring-shaped canal embracing the mouth, from which are given off long radial
canals, usually five in number, running to the more peripheral parts of the
body.[437] Each radial canal carries a double series of lateral branches,
which push out the skin so as to appear as appendages of the body. These
appendages are known as tentacles or tube-feet; they are both sensory and
respiratory in function, and often in addition, as the name TUBE-FOOT
indicates, assist in locomotion. As a general term for these appendages, to
be applied in all cases without reference to their function, the name
PODIUM has been suggested and will be employed here. A system of canals, in
many ways resembling the water-vascular system, is found in Brachiopoda,
Gephyrea and Polyzoa, but the peculiarity of Echinodermata is the way in
which it is kept filled with fluid. From the ring-canal in the interval (or
INTERRADIUS) between two radial canals, a vertical canal, termed the
STONE-CANAL, is given off, which communicates with the exterior by means of
a sieve-like plate, the MADREPORITE, pierced by fine canals. These canals
and the stone-canal itself are lined with powerful cilia, which produce a
strong inward current, and keep the water-vascular system tensely filled
with sea water.
The phylum includes the familiar Starfish and Sea-urchins, which in
sheltered spots are found between tide-marks; the {429}Brittle Stars and
Sea-cucumbers, which can be dredged up from below low-water mark, and
lastly the beautiful Feather-stars, of which there are comparatively few
species still living, although huge beds of limestone are composed of the
remains of fossil Feather-stars.
One species of Sea-cucumber (_Synapta similis_)[438] is said to enter
brackish water in the mangrove swamps of the tropics; but, with this
exception, the whole phylum is marine. A few species can endure partial
exposure to the air when left bare by the receding tide, but the
overwhelming majority are only found beneath low-water mark, and a
considerable number live in the deepest recesses of the ocean.
Their distribution is, no doubt, partly determined by food, a number of
species being strictly confined to the neighbourhood of the shore. On the
other hand, since a very large number of species live on the layer of mud
impregnated with animal remains which forms the superficial layer of the
deposit covering the sea-floor, it is not surprising to learn that many
have an exceedingly wide range, since this deposit is very widely
distributed. Another equally important factor in determining distribution
is wave-disturbance, and it is surprising to learn to what a depth this
extends. Off the west coast of Ireland a large wave literally breaks on a
submerged rock 15 fathoms beneath the surface. Speaking generally, it is
useless to look for Echinoderms on an exposed coast, and the same species,
which in the sheltered waters of the Clyde are exposed at low water, must
be dredged up from 20 to 30 fathoms outside Plymouth Sound.
The ordinary collector is attracted to the group chiefly by the regularity
and beauty of the patterns produced by the radial symmetry, but to the
scientific zoologist they are interesting from many other points of view.
Differing widely nevertheless from the higher Invertebrata in their
symmetry when adult, they have as larvae a marked bilateral symmetry, and
the secondary development of the radial symmetry constitutes one of the
most remarkable life-histories known in the animal kingdom.
Then again, owing to the possession of ossicles, the Echinodermata are one
of the few groups of Invertebrata of which abundant remains occur
fossilised. In attempting, therefore, to {430}decipher the past history of
life from the fossil record, it is necessary to have an exact and detailed
knowledge of Echinoderm skeletons and their relation to the soft parts.
Lastly, the internal organisation of Echinoderms throws valuable light on
the origin of the complicated systems of organs found in the higher
animals.
Echinodermata are divided into two great sub-phyla, which must have very
early diverged from one another. These are:—
(1) Eleutherozoa,
(2) Pelmatozoa.[439]
The sub-phylum PELMATOZOA, to which the living Feather-stars (CRINOIDEA)
and the majority of the known fossil species belong, is characterised by
the possession of a fixing organ placed in the centre of the surface
opposite the mouth—the aboral surface as it is called. Ordinarily this
organ takes on the form of a jointed stalk, but in most modern species it
is a little knob with a tuft of rooting processes, termed cirri. In the
other sub-phylum, the ELEUTHEROZOA, no such organ is found, and the animals
wander about freely during their adult life, though for a brief period of
their larval existence they may be fixed by a stalk-like protuberance
arising from the _oral_ surface.
SUB-PHYLUM I. ELEUTHEROZOA
The ELEUTHEROZOA are divided into four main classes, between which no
intermediate forms are found amongst the living species, though
intermediate types have been found fossil.
The four classes into which the Eleutherozoa are divided are defined as
follows:—
(1) ASTEROIDEA (Starfish).—"Star"-shaped or pentagonal Eleutherozoa with
five or more triangular arms, not sharply marked off from the central disc.
The mouth is in the centre of one surface, called from this circumstance
the "oral"; the anus is in the centre of the opposite surface, termed the
"aboral." From the mouth a groove runs out on the under surface of each
{431}arm towards its tip, termed the "ambulacral" groove. Projecting from
the ambulacral groove are found the podia or tube-feet, the organs of
movement and sensation of the animal.
(2) OPHIUROIDEA (Brittle Stars).—Eleutherozoa, in which the body consists
of a round disc with long worm-like arms inserted in grooves on its under
surface. No anus is present, and the ambulacral grooves are represented by
closed canals. The podia are merely sensory and respiratory, locomotion
being effected by muscular jerks of the arms.
(3) ECHINOIDEA (Sea-urchins).—Globular or disc-shaped Eleutherozoa, in
which the skeleton forms a compact cuirass except for a short distance
round the mouth (peristome) and round the anus (periproct). The ambulacral
grooves are represented by canals which, like meridians of longitude on a
school-globe, run from the neighbourhood of the mouth to near the aboral
pole of the body. The spines are large and movably articulated with the
plates. The animals move by means of podia and spines, or by means of the
latter only. The anus is usually situated at the aboral pole, but is
sometimes displaced towards the side, or even on to the ventral surface.
(4) HOLOTHUROIDEA (Sea-cucumbers).—Sausage-shaped Eleutherozoa, in which
the skeleton is represented only by isolated nodules of calcium carbonate,
and in which the body-wall is highly muscular. The mouth and anus are
situated at opposite ends of the body, and the ambulacral grooves
(represented by closed canals) run from near the mouth to the proximity of
the anus. Movement is accomplished by means of the podia, aided by
worm-like contractions of the body.
CLASS I. ASTEROIDEA[440] (STARFISH)
The Starfish derive their name from their resemblance in shape to the
conventional image of a star. The body consists of broad triangular arms
(generally five in number) which coalesce in the centre to form a disc. The
skin is soft and {432}semi-transparent, permitting the skeleton to be
easily detected; this consists of a mesh-work of rods or plates, leaving
between them intervals of soft skin. In a living Starfish it can be seen
that many of these soft places are raised up into finger-like outgrowths,
which are termed "papulae" or "dermal gills," through the thin walls of
which an active interchange of gases with the surrounding water takes
place, and the animal obtains in this way the oxygen necessary for its
respiration.
Very few and feeble muscle-fibres exist in the body-wall, and the movements
of the arms, as a whole, are very slow and limited in range. There is a
membranous lip surrounding the mouth, from which five broad grooves run
outwards, one on the underside of each arm. These are termed the
"ambulacral grooves." Each groove is Λ-shaped, and its sides are stiffened
by a series of rod-like ossicles called the "ambulacral ossicles."
The animal progresses by the aid of a large number of translucent
tentacles, termed "tube-feet" or "podia," which are attached to the walls
of the ambulacral grooves.
ANATOMY OF A STARFISH.—As an introduction to the study of the anatomy not
only of Starfish but of Echinodermata as a whole, we select _Asterias
rubens_, the common Starfish of the British coasts, which in many places
may be found on the beach near low-water mark.
EXTERNAL FEATURES.—In this species (Fig. 185) the skeleton is a net-work of
rod-like plates, leaving wide meshes between them, through which protrude a
perfect forest of transparent papulae. From the points of junction of the
rods arise short blunt spines surrounded by thick cushions of skin. The
surfaces of these cushions are covered with a multitude of whitish specks,
which, on closer inspection, are seen to have the form of minute pincers,
each consisting of two movable blades crossing each other below and
articulated to a basal piece. These peculiar organs are termed
"pedicellariae" (Fig. 186), and their function is to keep the animal clean
by seizing hold of any minute organisms which would attempt to settle on
the soft and delicate skin. When irritated the blades open and then snap
together violently, and remain closed for a long time.[441] These actions
are brought about by appropriate muscles attaching the blades to the basal
piece.
{433}[Illustration: FIG. 185.—_Asterias rubens_, seen from the aboral
surface, × 1. _mad_, Madreporite.]
The last-named ossicle increases the certainty of the grip by fixing the
lower parts of each blade in the same vertical plane, and preventing
lateral slipping, so that it serves the same purpose as the pivot in a pair
of scissors. Each blade, in fact, fits into a groove on the side of this
piece. The muscles which close the blades arise from the lower ends
(handles) of the blades, and are united below to form a common muscular
string which attaches the whole organ to one of the plates of the skeleton.
An attempt of the victim to tear the pedicellaria out is resisted by the
contraction of this string, which thus brings about a closer grip of the
blades. In order that the blades may open they must first be lifted out of
the grooves on the basal piece—this is effected by special lifting muscles.
The opening is {434}brought about by muscles extending from the "handle" of
one blade to the upper part of the other.
Scattered about amongst the papulae between the cushions are other
pedicellariae of a larger size in which the blades do not cross one another
(Fig. 186, B).
In the space or "interradius" between two arms, on the aboral surface,
there is found a button-shaped ossicle. This is covered with fine grooves,
and from a fancied resemblance between it and some forms of coral it has
received the name "madreporite" (Fig. 185, _mad_). The bottoms of the
grooves are perforated by capillary canals lined by flagella, through the
action of which water is constantly being introduced into the
water-vascular system.
The anus is situated near the centre of the upper surface of the disc, but
it is so minute as to require careful inspection in order to discover its
position (Fig. 185).
[Illustration: FIG. 186.—View of pedicellariae of _A. glacialis_. A,
Crossed form, × 100. 1, Ectoderm covering the whole organ; 2, basal piece;
3, auxiliary muscle closing the blades; 4, muscle lifting right blade out
of the groove; 5, handle of left blade; 6, muscles closing the blades, and
uniting to form 7, the muscular string attaching the pedicellaria to the
skeleton. B, straight form, × 10. 1, Basal piece; 2, blades; 3 and 4,
muscles closing the blades; 5, muscle opening the blades. (From Cuénot.)]
On the under side of the animal the most conspicuous features are the five
ambulacral grooves which radiate out from the "peristome," a thin
membranous area surrounding the central mouth. The grooves are filled with
the tube-feet, which are closely crowded together and apparently arranged
in four rows.
SKELETON.—The sides of the ambulacral grooves are stiffened by the rod-like
"ambulacral ossicles." To the outer ends of these are articulated a set of
shorter rods termed the "adambulacral ossicles" which carry each two or
three rod-like spines, the "adambulacral spines," the skin covering which
bears numerous pedicellariae (Fig. 187, B). When the animal is irritated
the edges of the groove are brought together, and these {435}spines then
form a trellis-work covering and protecting the delicate tube-feet; the
numerous pedicellariae are then in a position to make it unpleasant for any
intruder. The closure of the groove is effected by means of powerful
muscles connecting each ambulacral ossicle with its fellow. There are also
feebler muscles connecting these plates with their successors and
predecessors, which enable the arm to be bent downwards in a vertical
plane. It is raised by a muscular band running along the dorsal wall of the
coelom to the point of the arm.
[Illustration: FIG. 187.—A, _Asterias rubens_, seen from the oral surface,
drawn from a living specimen, × 1. B, an adambulacral spine, showing three
straight pedicellariae; C, a tube-foot expanded and contracted.]
When the series of ambulacral and adambulacral ossicles is followed inwards
towards the mouth it is seen that the first ambulacral ossicle is closely
fixed to the second, but is widely {436}separated from its fellow,
remaining, however, connected with the latter by a powerful adductor
muscle. In consequence of the separation of this pair of ossicles each is
brought into closer contact with the corresponding ossicle in the adjacent
radius, to which it is connected by a muscle called the abductor. The first
adambulacrals in adjacent radii are also brought into closer contact and
carry long spines which, when the ambulacral grooves are contracted,
project like a grating over the mouth. In the order of Asteroidea to which
_Asterias_ belongs, the adambulacrals themselves do not project much, but
in all other cases they form prominent mouth-angles, so that the opening of
the mouth becomes star-shaped (Fig. 211, p. 483).
Except in the case of the ambulacral and adambulacral plates little regular
arrangement is to be detected in the ossicles of the skeleton which, as has
already been mentioned, form a mesh-work. If, however, the arm be cut open
and viewed from the inside it will be seen that the edge is strengthened
above and below by very thick, powerful, rod-like plates. These are called
the "supero-marginal" and "infero-marginal" ossicles; they are not visible
from the outside, since they are covered by a thick layer of the body-wall
containing other smaller plates (Fig. 190, _marg_). In many genera,
however, they are exposed, and form a conspicuous edging to the arm above
and below. In many genera, also, there are three conspicuous series of
plates on the back of each arm, viz. a median row, called "carinals"
(_car._, Fig. 191), and two lateral rows, termed "dorso-laterals"
(_d.lat._, Fig. 191). These three rows, with the two rows of marginals, one
of ambulacrals, and one of adambulacrals on each side (11 rows in all),
constitute the primitive skeleton of the arm, and appear first in
development.
The structure of all these elements of the skeleton is the same. They may
be described as scaffoldings of carbonate of lime, interpenetrated by a
mesh-work of cells fused with one another, by which the carbonate of lime
has been deposited. The matrix in which the ossicles lie is a jelly-like
substance traversed by a few bands of fibres which connect the various rods
with one another. This jelly is almost fluid in the fresh state, but when
heated forms a hard compound, possibly allied to mucin, which will turn the
edge of a razor.
When the covering of the back is dissected off the COELOM is {437}opened.
This is a spacious cavity which apparently surrounds the alimentary canal
and extends into the arms. It has, however, its own proper wall, which is
called the "peritoneum," both on the outer side, where it abuts on the
skin, and on the inner side, where it comes in contact with the wall of the
alimentary canal. The outer wall is called the "somatic peritoneum," and it
is possible to dissect off the rest of the body-wall and leave it intact;
the inner wall, from its close association with the alimentary canal, is
termed the "splanchnic peritoneum." This wall can only be distinguished in
microscopic sections from that of the alimentary canal, to which it is
closely applied.
The coelom is filled with a fluid, which is practically sea water with a
little albuminous matter in solution. Through the thin walls of the papulae
oxygen passes into this fluid, whence it easily reaches the inner organs,
since they are all in contact with some part of the coelomic wall.
Similarly CO_{2} is absorbed by the coelomic fluid from all parts of the
body, and diffuses through the papulae to the surrounding water.
The Starfish possesses no definite KIDNEY for getting rid of nitrogenous
waste. In most of the higher animals with a well-developed coelom it has
been proved that the kidney is simply a specialised portion of the coelom,
and in many cases some parts of the coelomic wall still retain their
excretory functions, which apparently the whole originally possessed. In
the Starfish and in Echinodermata generally this primitive state of affairs
is still retained. From the cells forming the coelomic wall, cells are
budded off into the fluid, where they swim about. These cells from their
movements are called amoebocytes. If a substance such as indigo-carmine,
which when introduced into the tissues of the higher animals is eliminated
by the kidney, is injected into the Starfish, it is found soon after to be
vigorously absorbed by the amoebocytes. These later accumulate in the
dermal branchiae, through the thin walls of which they make their way[442]
to the outside, where they degenerate.
The coelom is indented by five folds, which project inwards from the
interradii. These folds are called the "interradial septa"; they are
stiffened by a calcareous deposit, which is not, however, sufficiently
dense to constitute a plate. In one of the {438}septa the axial sinus and
stone-canal (see below) are embedded. These septa are to be regarded as
areas of lateral adhesion between the arms.
[Illustration: FIG. 188.—View of upper half of a specimen of _Asterias
rubens_, which has been split horizontally into two halves. _ax.c_, Axial
sinus; _g.d_, genital duct; _oe_, cut end of the oesophagus, the narrow
neck of the stomach; _py_, pyloric sac; _py.c_, pyloric caeca; _r_, rectum;
_r.c_, rectal caeca; _sept_, interradial septum; _st.c_, stomach lobe.]
The ALIMENTARY CANAL consists of several distinct portions. The mouth leads
by a narrow neck called the "oesophagus" into a voluminous baggy sac termed
the "stomach," which is produced into ten short pouches, two projecting
into each arm. The stomach leads in turn by a wide opening into a
pentagonal flattened sac, the "pyloric sac," which lies above it. Each
angle of the pyloric sac is prolonged into a tube—the so-called "pyloric
duct"—running out into the arm, where it immediately bifurcates into two
forks, each beset by a large number of small pouches {439}and attached to
the dorsal wall of the coelom by suspensory bands of membrane called
mesenteries. These ten forks are called "pyloric caeca"; they are of a deep
green colour owing to the pigment in their wall. Beyond the pyloric sac the
alimentary canal is continued as the slender "rectum" to the anus. The
rectum gives off two small branched pouches of a brown colour called
"rectal caeca." This comparatively complicated form of alimentary canal is
related to the nature of the food of the animal and the method it employs
to capture its prey.
[Illustration: FIG. 189.—View of a Starfish (_Echinaster_) devouring a
Mussel. 1. The madreporite.]
The favourite FOOD[443] of _Asterias_ consists of the common bivalves of
the coast, notably of the Mussel (_Mytilus edulis_). There is, however, no
animal which it will not attack if it is fortunate enough to be able to
catch it. The Starfish seizes its prey by the tube-feet, and places it
directly under its mouth, folding its arms down over it in umbrella
fashion. The muscles which run around the arms and disc in the body-wall
contract, and the pressure thus brought to bear on the incompressible fluid
contained in the coelom, forces out the thin membranous peristome and
partially turns the stomach inside out. The everted edge of the stomach is
wrapped round the prey.
{440}Soon the bivalve is forced to relax its muscles and allow the valves
to gape. The edge of the stomach is then inserted between the valves and
applied directly to the soft parts of the prey which is thus completely
digested. When the Starfish moves away nothing but the cleaned shell is
left behind. If the bivalve is small it may be completely taken into the
stomach, and the empty shell later rejected through the mouth.
It was for a long time a puzzle in what way the bivalve was forced to open.
Schiemenz[444] has, however, shown that when the Starfish folds itself in
umbrella-like form over the prey it holds on to the substratum by means of
the tube-feet of the distal portions of the arms, whilst, by means of the
tube-feet belonging to the central portions, it drags apart the valves by
main force. He has shown experimentally: (1) that whilst a bivalve may be
able to resist a sudden pull of 4000 grammes it will yield to a pull of 900
grammes long continued; (2) that a Starfish can exert a pull of 1350
grammes; (3) that a Starfish is unable to open a bivalve unless it be
allowed to raise itself into a hump, so that the pull of the central
tube-feet is at right angles to the prey. A Starfish confined between two
glass plates walked about all day carrying with it a bivalve which it was
unable to open.
The lining of the stomach is found to consist very largely of mucus-forming
cells, which are swollen with large drops of mucus or some similar
substance. It used to be supposed that this substance had some poisonous
action on the prey and paralysed it, but the researches of Schiemenz show
that this is incorrect. If when an _Asterias_ is devouring a bivalve
another be offered to it, it will open it, but will not digest it, and the
victim shows no sign of injury but soon recovers. The cells forming the
walls of the pyloric sac and its appendages are tall narrow cylindrical
cells crowded with granules which appear to be of the nature of digestive
ferment. This substance flows into the stomach and digests the captured
prey.
A very small amount of matter passes into the rectum and escapes by the
anus, as the digestive powers of the Starfish are very complete. The rectal
caeca are lined by cells which secrete from the coelomic fluid a brown
material, in all probability an excretion, which is got rid of by the anus.
{441}When the meal is finished the stomach is restored to its former place
by the action of five pairs of retractor muscles, one pair of which
originates from the upper surface of the ambulacral ossicles in each arm
and extends to the wall of the stomach, where they are inserted (Fig. 190,
_ret_).
The tube-feet, which are at once the locomotor and the principal sensory
organs of the Starfish, are appendages of that peculiar system of tubes
known as the WATER-VASCULAR SYSTEM, which is derived from a part of the
coelom cut off from the rest during the development of the animal. This
system, as already mentioned, consists of (1) a narrow "ring-canal,"
encircling the mouth and lying on the inner surface of the membranous
peristome; (2) a radial canal leaving the ring-canal and running along the
under surface of each arm just above the ambulacral groove; (3) a vertical
stone-canal running from the madreporite downwards to open into the
ring-canal in the interspace between two arms. The madreporite is covered
externally by grooves lined with long cilia, and is pierced with narrow
canals of excessively fine calibre, the walls of which are also lined by
powerful cilia. Most of these narrow canals open below into a main
collecting canal, the stone-canal, but some open into a division of the
coelom termed the axial sinus, with which also the stone-canal communicates
by a lateral opening. The cavity of the stone-canal is reduced by the
outgrowth from its walls of a peculiar Y-shaped projection, the ends being
rolled on themselves in a complicated way (Fig. 190, B). The walls of the
canal consist of a layer of very long narrow cells, which carry powerful
flagella, and outside this of a crust of calcareous deposit, which gives
rigidity to the walls and has suggested the name stone-canal.
The tube-feet are covered externally by ectoderm, inside which is a tube in
connexion with the radial water-vascular canal. This latter is lined by
flattened cells, which in the very young Starfish are prolonged into
muscular tails; in the older animal these tails are separated off as a
distinct muscular layer lying between the ectoderm and the cells lining the
cavity of the tube. The tube-foot is prolonged inwards into a bulb termed
the "ampulla," which projects into the coelom of the arm and in consequence
is covered outside by somatic peritoneum. Just where the ampulla passes
into the tube-foot proper the organ passes downwards between two of the
powerful ambulacral ossicles which support {442}the ambulacral groove, and
a little below this spot a short transverse canal connects the tube-foot
with the radial canal which lies beneath these ossicles (Fig. 191).
[Illustration: FIG. 190.—A, view of the under half of a specimen of
_Asterias rubens_, which has been horizontally divided into two halves. B,
enlarged view of the axial sinus, stone-canal and genital stolon cut
across. _amb.oss_, Ambulacral ossicle; _amp._ ampullae of the tube-feet;
_ax.s_, axial sinus; _gon_, gonad; _g.stol_, genital stolon; _marg_,
marginal ossicle; _nerv.circ_, nerve ring; _oe_, cut end of oesophagus;
_pst_, peristome; _ret_, retractor muscle of the stomach; _sept_,
interradial septum; _stone c_, stone-canal; _T_, Tiedemann's body; _w.v.r_,
water-vascular ring-canal.]
The tube-feet are, therefore, really a double row of lateral branches of
the radial canal. The appearance of being arranged in four rows is due to
the fact that the transverse canals connecting them with the radial canal
are alternately longer and shorter so as to give room for more tube-feet in
a given length of the arm. Each tube-foot ends in a round disc with a
slightly thickened edge. The radial canal terminates in a finger-shaped
{443}appendage, called the median tentacle, at the base of which is the
eye.
The manner in which this complicated system acts is as follows:—When the
tube-foot is to be stretched out the ampulla contracts and drives the fluid
downwards. The contraction of the ampulla is brought about by muscles
running circularly around it. The tube-foot is thus distended and its broad
flattened end is brought in contact with the surface of the stone over
which it is moving and is pressed close against it. The muscles of the
tube-foot itself, which are arranged longitudinally, now commence to act,
and the pressure of the water preventing the tearing away of the sucker
from the object to which it adheres, the Starfish is slowly drawn forward,
whilst the fluid in the tube-foot flows back into the ampulla.
[Illustration: FIG. 191.—Diagrammatic cross-section of the arm of a
Starfish. _adamb_, Adambulacral ossicle; _amb_, ambulacral ossicle; _amp_,
ampulla of tube-foot; _branch_, papula; _car_, carinal plate; _d.lat_,
dorso-lateral plate; _inf.marg_, infero-marginal plate; _p.br_,
peribranchial space; _ped_, pedicellaria; _s.marg_, supero-marginal plate.
The nervous ridge between the bases of the tube-feet and the two perihaemal
canals above this ridge are shown in the figure but not lettered.]
If each tube-foot were practically water-tight, then each would be entirely
independent of all the rest, and it would not be easy to suggest a reason
for the presence of the complicated system of radial canals and
stone-canal. Just at the spot, however, where the transverse canal leading
from the radial canal enters the tube-foot there is a pair of valves which
open inwards and allow fluid to pass from the radial canal into the
tube-foot but prevent any passing outwards in the reverse direction. The
presence of these valves renders it probable that the tube-foot is not
quite water-tight; that when it is distended under the pressure produced by
the contraction of the muscles of the ampulla, some fluid escapes through
the permeable walls; and {444}that the loss thus suffered is made up by the
entry of fresh fluid from the radial canal. The radial canal in turn draws
from the ring-canal, and this last is supplied by the stone-canal, the
cilia of which keep up a constant inward current.
In the fluid contained in the water-vascular system, as in the coelomic
fluid, there are amoebocytes floating about. These are produced in short
pouches of the ring-canal, nine in number, which are called after their
discoverer "Tiedemann's bodies" (Fig. 190, T). From the cells lining these
the amoebocytes are budded off.
The NERVOUS SYSTEM of the Starfish is in a very interesting condition. The
essential characteristic of all nervous systems is the presence of the
"neuron," a cell primitively belonging to an epithelium but which generally
has sunk below the level of the others and lies amongst their bases. This
type of cell possesses a round body produced in one direction into a long
straight process, the "axon," whilst in the other it may have several
root-like processes, or "dendrites," which may spring from a common stem,
in which case the neuron is said to be "bipolar." The axon is often
distinguished as a "nerve-fibre" from the round body which is termed the
"nerve-cell." This is due to the fact that for a long time it was not
recognised that these two structures are parts of a whole.
Now at the base of the ectoderm all over the body of the Starfish there is
to be found a very fine tangle of fibrils; these are to be found partly in
connexion with small bipolar neurons lying amongst them and partly with
isolated sense-cells scattered amongst the ordinary ectoderm cells. This
nervous layer is, however, very much thickened in certain places, so as to
cause the ectoderm to project as a ridge. One such ridge is found at the
summit of each ambulacral groove running along the whole under surface of
the arm and terminating in a cushion at the base of the median tentacle of
the water-vascular system. This ridge is called the radial nerve-cord. The
five radial nerve-cords are united by a circular cord, the nerve-ring,
which appears as a thickening on the peristome surrounding the mouth.
The sense-organs of the Starfish are chiefly the discs of the tube-feet.
Round the edges of these there is a special aggregation of sense-cells;
elsewhere, as in the skin of the back, only {445}isolated sense-cells are
found, and it becomes impossible to speak of a sense-organ.
A prolongation of the radial nerve-cord extends outwards along one side of
each tube-foot. This is often spoken of as the "pedal nerve," but the term
nerve is properly retained for a mere bundle of axons such as we find in
the higher animals, whereas the structure referred to contains the bodies
of nerve-cells as well as their outgrowths or cell-fibres and is therefore
a prolongation of the nerve-cord.
[Illustration: FIG. 192.—Diagrammatic longitudinal section through a young
Asteroid passing through the tip of one arm and the middle of the opposite
interradius. This diagram is generalised from a section of _Asterina
gibbosa_. _ab_, Aboral sinus; _ax_, axial sinus; _ax_^1, basal extension of
axial sinus forming the inner perihaemal ring-canal; _br_, branchia = gill
= papula; _g.r_, genital rachis; _mp_, madreporite; _musc.tr_, muscle
uniting a pair of ambulacral ossicles; _nerv.circ_, nerve-ring; _n.r_,
radial nerve-cord; _oc_, eye-pit; _oss_, ossicles in skin; _p.br_,
peribranchial sinus; _p.c_, pore canal; _perih_ (on the right), perihaemal
radial canal, (on the left), outer perihaemal ring-canal; _py_, pyloric
caecum; _rect_, rectum; _rect.caec_, rectal caeca; _sp_, spines; _st.c_,
stone-canal; _t_, median tentacle terminating radial canal; _w.v.r_,
water-vascular radial canal. The genital stolon (not marked by a reference
line) is seen as an irregular band accompanying the stone-canal, its upper
end projects into a small closed sac, also unmarked, which is the _right_
hydrocoele or madreporic vesicle.]
At the base of the terminal tentacle the radial nerve-cord ends in a
cushion. This cushion is called the "eye," for it is beset with a large
number of cup-shaped pockets of the ectoderm. Each pocket is lined partly
by cells containing a bright orange pigment and partly by visual cells each
of which ends in a small clear rod projecting into the cavity of the pit
(Fig. 193, A, _vis.r_). The pit is apparently closed by a thin sheet of
cuticle secreted by the most superficial cells.
An exposed nervous system and simple sense-organs such as the Starfish
possesses lend themselves admirably to the purposes {446}of physiological
experiment, and so Starfish have been favourite "corpora vilia" with many
physiologists.
[Illustration: FIG. 193.—A, longitudinal section of a single eye-pit of
_Asterias_. _s.n_, Nucleus of supporting cell; _vis.n_, nucleus of visual
cell; _vis.r_, visual rod. B, view of the terminal tentacle showing the
eye-pits scattered over it. (After Pfeffer.)]
The light-perceiving function of the eye is easily demonstrated. If a
number of Starfish be put into a dark tank which is illuminated only by a
narrow beam of light they will be found after an interval to have collected
in the space reached by the beam of light.[445] If all the median tentacles
but one be removed this will still be the case; if, however, they are all
removed the Starfish will exhibit indifference to the light.
If the under surface of a Starfish be irritated by an electric shock or a
hot needle, or a drop of acid, the tube-feet of the affected area will be
strongly retracted, and this irritation will be carried by the pedal nerves
to the radial nerve-cord, with the result that finally all the tube-feet in
the groove will be retracted and the groove closed by the action of the
transverse muscle connecting each ambulacral ossicle with its fellow. If,
on the other hand, the back of a Starfish be irritated this may produce a
contraction of the tube-feet if the irritation be strong, but this will be
followed by active alternate expansions and contractions, in a word, by
endeavours to move. Preyer[446] by suspending a Starfish ventral surface
upward, by {447}means of a small zinc plate to which a string was attached
which passed through a hole bored in the back and through the mouth, caused
movements of this description which lasted for hours. Irritation of the
back causes also activity of the local pedicellariae, which open their
valves widely and then close them with a snap in the endeavour to seize the
aggressor.
The uninjured Starfish in moving pursues a definite direction, one arm
being generally directed forwards, but this may be any one of the five. The
tube-feet of this arm are directed forwards when they are stretched out, by
the slightly unequal contraction of the longitudinal muscles of opposite
sides of the foot, which persists even when the circular muscles of the
ampulla are contracting. They thus may be said to swing parallel to the
long axis of the arm. The tube-feet of the other arms assist in the
movement, and hence swing obliquely with reference to the long axis of the
arm to which they belong, although they move parallel to the general
direction in which the Starfish is moving. A change in the direction of the
swing of the tube-feet will bring about a change in the direction of the
movement of the animal as a whole. If now the connexion of each radial
nerve-cord with the nerve-ring be cut through, each arm will act as a
separate Starfish and will move its tube-feet without reference to the
movement of those in the other arms, so that the animal is pulled first one
way and then another according as the influence first of one arm and then
of another predominates. Similarly, when a Starfish is placed on its back,
it rights itself by the combined action of the tube-feet of all the arms,
extending them all as widely as possible, those which first catch hold
being used as the pivot for the turning movement. If, however, the radial
nerve-cords are cut through, each arm tries to right itself and it is only
by chance that the efforts of one so predominate as to turn the whole
animal over. From these experiments it is clear that the nerve-ring acts as
co-ordinator of the movements of the Starfish, that is to say as its brain.
If a section be taken across the arm of a Starfish (Fig. 191), it will be
seen that between the V-shaped ridge constituting the radial nerve-cord and
the radial water-vascular canal there are two canals lying side by side and
separated from one another by a vertical septum. These canals are not mere
splits in the {448}substance of the body-wall, but have a well-defined wall
of flattened cells. They are termed, for reasons which will be explained
subsequently, PERIHAEMAL CANALS, and they open into a circular canal called
the "outer perihaemal ring," situated just beneath the water-vascular
ring-canal (Fig. 192, _perih_). These canals originate as outgrowths from
the coelom. From their upper walls are developed the muscles which connect
the pairs of ambulacral ossicles and close the groove, and also those which
connect each ossicle with its successor and predecessor and help to elevate
or depress the tip of the arm.
In most of the higher animals the processes of many of the ganglion-cells
are connected together in bundles called "motor nerves," which can be
traced into contact with the muscles, and thus the path along which the
stimulus travels in order to evoke movement can clearly be seen. No such
well-defined nerves can be made out in the case of the Starfish, and it is
therefore interesting when exceptionally the paths along which stimuli
travel to the muscles can be traced. This can be done in the case of the
muscles mentioned above. Whereas they originate from the dorsal walls of
the perihaemal canals, ganglion-cells develop from the ventral walls of
these canals, which are in close contact with the nerve-cord, so that the
nervous system of the Starfish is partly ectodermic and partly coelomic in
origin. Stimuli reaching the ectodermic ganglion-cells are transmitted by
them to the nervous part of the wall of the perihaemal canal and from that
to the muscular portion of the same layer of cells.
Besides the radial perihaemal canals and their connecting outer perihaemal
ring there are several other tubular extensions of the coelom found in the
body-wall. These are:—
(1) The "inner perihaemal canal," a circular canal in close contact with
the inner side of the outer perihaemal canal (Fig. 192, _ax_^1).
(2) The "axial sinus" (_ax_) a wide vertical canal embedded in the
body-wall outside the stone-canal. This canal opens into the inner
perihaemal canal below; above it opens into several of the pore-canals and
into the stone-canal. The separation of the axial sinus from the rest of
the coelom is the remains of a feebly marked metamerism in the larva.
(3) The "madreporic vesicle," a closed sac embedded in the dorsal body-wall
just under the madreporite. This sac by its {449}history in the larva
appears to be a rudimentary counterpart of the water-vascular system, since
this organ in correspondence with the general bilateral symmetry of the
larva is at first paired. Into this a special process of the genital stolon
projects.
(4) The "aboral sinus" (Fig. 192, _ab_), a tube embedded in the dorsal
body-wall running horizontally round the disc. The aboral sinus surrounds
the genital rachis (see p. 452) and gives off into each arm two branches,
the ends of which swell so as to surround the genital organs. It has no
connexion with the axial sinus though the contrary has often been stated by
Ludwig.[447]
(5) The "peribranchial spaces," circular spaces which surround the basal
parts of the papulae (Fig. 192, _p.br_).
Besides these, large irregular spaces have been described as existing in
the body-wall by Hamann[448] and other authors, but for various reasons and
especially because they possess no definite wall they appear to be nothing
more than rents caused by the escape of CO_{2} gas during the process of
decalcifying, to which the tissues of the Starfish must be subjected before
it is easy to cut sections of them.
The question as to whether or not there is a BLOOD SYSTEM in the Starfish
has an interesting history. It must be remembered that the examination of
the structure of Echinodermata was first undertaken by human anatomists,
who approached the subject imbued with the idea that representatives of all
the systems of organs found in the human subject would be found in the
lower animals also. So the perihaemal canals were originally described as
blood-vessels. Later, Ludwig[449] discovered a strand of strongly staining
material running in each septum which separates the two perihaemal canals
of the arm. Each of these radial strands could be traced into connexion
with a circular strand interposed between the outer and the inner
perihaemal ring-canals. This circular strand again came into connexion with
a brown, lobed organ, lying in the wall of the axial sinus, and this in
turn {450}joined at its upper end a circular cord of pigmented material
adhering to the dorsal wall of the coelom (lying in fact within the aboral
sinus), from which branches could be traced to the generative organs.
Ludwig concluded that he had at last discovered the true blood-vessels,
though the facts that the radial strands and the oral circular strand
absorbed neutral carmine strongly and that the vertical and aboral strands
were pigmented, constituted a very slender basis on which to found such a
conclusion. The colour apparently appealed to the imagination, and it is
undoubtedly true that the "plasma" or blood-fluid of other animals often
absorbs stain strongly.
The strands were accordingly named "radial blood-vessels," "oral
blood-ring," "aboral blood-ring"; and the brown vertical strand was called
the "heart," although no circulation or pulsations had ever been observed.
When later investigations revealed the fact that the so-called heart was
practically solid, the term "central blood-plexus" was substituted for
heart, although it was still regarded as the central organ of the system.
The name "perihaemal" was given to the spaces so called because they
surrounded the supposed blood-vessels.
In order to come to a satisfactory conclusion on the matter some general
idea as to the fundamental nature and function of the blood-vessels in
general must be arrived at. Investigations made on various groups of
animals, such as Annelida, Mollusca, Crustacea, Vertebrata, show that at an
early period of development a considerable space intervenes between the
alimentary canal and the ectoderm, which is filled with a more or less
fluid jelly. Into this cavity, the so-called "primary body-cavity" or
"archicoel," amoebocytes, budded from the ectoderm or endoderm or both,
penetrate. In this jelly with its contained amoebocytes is to be found the
common rudiment both of the connective tissue and of the blood system. The
resemblance of the archicoele and its contents to the jelly of a Medusa is
too obvious to require special insistence on, and therefore in the
Coelenterata it may be stated that there is to be found a tissue which is
neither blood system nor connective tissue but is the forerunner of both.
In the higher animals as development proceeds the jelly undergoes
differentiation, for some of the amoebocytes become stationary and
connected with their pseudopodia so as to form a protoplasmic network. A
portion of this network becomes {451}altered into tough fibres, but a
portion of each strand remains living, and in this way the connective
tissue is formed. In the interstices of the network of fibres a semi-fluid
substance (the unaltered jelly) is found, and this is traversed by free,
wandering amoebocytes. In other places the jelly becomes more fluid and
forms the plasma, or liquid of the blood, whilst the amoebocytes form the
blood corpuscles. The blood system thus arises from regions of the
archicoel where fibres are not precipitated.
Now in the Starfish the whole substance of the body-wall intervening
between the ectoderm and the coelomic epithelium really represents the
archicoel. The formation of fibres has, it is true, proceeded to a certain
extent, since there are interlacing bundles of these, but there are left
wide meshes in which amoebocytes can still move freely. Apart from the
skeleton, therefore, the tissues of the body-wall of the Starfish do not
exhibit much advance on those of a Jellyfish. If anything is to be compared
to the blood system of the higher animals it must be these meshes in the
connective tissue. From observations made on other Echinoderms it appears
probable that the colour of the skin is due to amoebocytes loaded with
pigment wandering outwards through the jelly of the body-wall and
disintegrating there. The strands regarded as blood-vessels by Ludwig are
specially modified tracts of connective tissue in which fibres are sparse,
and in which there are large quantities of amoebocytes and in which the
"jelly" stains easily. Cuénot[450] suggests that they are placed where new
amoebocytes are formed; this is quite possible, and in this case they ought
to be compared to the spleen and other lymphatic organs of Vertebrates, and
not to the blood-vessels.[451]
The organ regarded as the heart, however, belongs to a different category:
it is really the original seat of the GENITAL CELLS and should be termed
the "genital stolon." Careful sections show that at its upper end it is
continuous with a strand of primitive germ-cells which lies inside the
so-called aboral {452}blood-vessel, and is termed the "genital rachis"
(Fig. 192, _g.r_). The germ-cells are distinguished by their large nuclei
and their granular protoplasm. The genital organs are only local swellings
of the genital rachis, and from the shape of some of the germ-cells it is
regarded as highly probable that the primitive germ-cells wander along the
rachis and accumulate in the genital organs. The genital rachis itself is
an outgrowth from the genital stolon, and this latter originates as a
pocket-like ingrowth of the coelom into the wall separating it from the
axial sinus; when fully formed it projects into and is apparently contained
in this latter space.
Not all the cells forming the genital stolon become sexual cells. Many
degenerate and become pigment-cells, a circumstance to which the organ owes
its brown colour. In very many species of Starfish many of the cells of the
genital rachis undergo a similar degeneration, and hence is produced the
apparent aboral blood-vessel. Further, the rachis is embedded in connective
tissue which has undergone what we may call the "lymphatic" modification,
and this for want of a better name we call the "aboral" blood-ring.
The size of the genital organs varies with the season of the year; they are
feather-shaped, and attached to the genital rachis by their bases, but
project freely into the coelom of the arm. From their great variation in
size and also from the shape of some of the cells in the genital rachis,
Hamann concludes that as each period of maturity approaches fresh
germ-cells are formed in the rachis and wander into the genital organ and
grow there in size. It is probable that the aboral end of the genital
stolon is the seat of the formation of new germ-cells.
In the Starfish, therefore, as in other animals with a well-defined coelom,
the genital cells ultimately originate from the coelomic wall.
The genital ducts are formed by the burrowing outwards of the germ-cells.
When it is remembered that the fundamental substance of the body-wall is
semi-fluid jelly, this process will be better understood.
When the ova and spermatozoa are ripe, they are simply shed out into the
sea and fertilisation occurs there. The development is described in Chapter
XXI. The free-swimming larval period lasts about six weeks.
{453}Having described a single species with some degree of fulness, we must
now give some account of the range of variation of structure met with in
the group.
NUMBER OF ARMS.—In the overwhelming majority of Starfish the number of arms
is 5, but deviations from this rule are met with not only as individual
variations, but as the characteristics of species, genera, and even
families.
The number 5 is rarely diminished, but amongst a large collection of
specimens of _Asterina gibbosa_, belonging to the author, some 4-rayed
individuals are met with. One species of _Culcita_, _C. tetragona_, is
normally 4-rayed.
On the other hand the number 5 is often exceeded. The families
Heliasteridae and Brisingidae are characterised by possessing numerous
(19-25) arms. In the normally 5-rayed family Asteriidae _Pycnopodia_ has 22
arms; and in the Solasteridae the genera _Rhipidaster_ and _Solaster_ are
characterised by possessing 8 and 11-15 arms respectively; whilst
_Korethraster_ and _Peribolaster_ have only 5. The common Starfish of the
Gulf of St. Lawrence, _Asterias polaris_, is 6-rayed, whilst most of the
other species of the same genus are 5-rayed, though 6 rays are often met
with as a variation.
In some species the fact that the number of arms exceeds 5 seems to be
connected with the power of multiplication by transverse fission. Thus
Ludwig[452] has shown that in _Asterias tenuispina_ the number of arms is
usually 7, but sometimes 5, 6, or 8, and that in most cases the arms are
arranged in two groups—one consisting of small arms, the other of large.
SHAPE.—Apart from the varying number of arms, differences in the shape of
the Starfish are due to two circumstances:—
(1) The proportion of breadth to length of arm; and
(2) The amount of adhesion between adjacent arms.
The adhesion can go so far that the animal acquires the shape of a
pentagonal disc. This is the case for instance in _Culcita_. The fact that
the body of this animal is really composed of adherent arms is at once made
clear when the coelom is opened. This space is found to be divided up by
inwardly projecting folds called interradial septa, which are stiffened by
calcareous deposits and represent the conjoined adjacent walls of two arms.
{454}In the family Heliasteridae the mutual adhesion between the arms has
gone on merely to a slight extent, for the interradial septa are still
double.
SKELETON.—Most of the schemes of classification have been founded on the
skeleton, largely because the greater number of species have only been
examined in the dried condition, and little is known of their internal
anatomy or habits. There is, however, this justification for this
procedure, that the habits and food of the species (with the exception of
the Paxillosa) which have been observed in the living condition appear to
be very uniform, and that it is with regard to the skeleton that Asteroidea
seem to have split into divergent groups through adopting different means
of protecting themselves from their foes.
The description of the various elements of the skeleton will be arranged
under the following heads:—(_a_) Main framework; (_b_) Spines; (_c_)
Pedicellariae; (_d_) Ambulacral skeleton.
(_a_) MAIN FRAMEWORK.—The type of skeleton which supports the body-wall of
_Asterias_ is called reticulate. As already indicated it consists of a
series of rods bound together by bundles of connective-tissue fibres so as
to form a mesh-work. This is a very common type of aboral skeleton, but in
a large number of Starfish a different type occurs, consisting of a series
of plates which may fit edge to edge, leaving between them only narrow
interstices, as in the Zoroasteridae, or which may be placed obliquely (as
in _Asterina_) so that they imbricate or overlap one another. In a very
large number of Asteroidea the supero- and infero-marginal ossicles are
represented by squarish plates even when the rest of the skeleton is
reticulate; this is the so-called "phanerozonate" structure, the term
"cryptozonate" being used when the marginals are rod-like and
inconspicuous. In other cases (Ganeriidae) the whole skeleton of the
ventral surface is made of tightly fitting plates, whilst the aboral
skeleton is either reticulate or made of imbricating plates. Lastly, the
skeleton may be represented only by nodules forming the bases of paxillae
(see p. 455), as in the Astropectinidae, or may be entirely absent over
wide areas (Brisingidae).