BRANCH ECHINODERMATA: STARFISHES,
SEA-URCHINS, SEA-CUCUMBERS
STARFISH (_Asterias_ sp.)
TECHNICAL NOTE.--The species of _Asterias_ are widely distributed
on both coasts of the United States and may be procured on almost
any rocky shore at low tide. Teachers in inland schools can obtain
preserved material from the dealers mentioned on p. 453. Most of
the specimens should be placed in alcohol or 4% formalin. If fresh
material can be had it is well to place at least one specimen for
each student in a 20% solution of nitric acid in water for two or
three hours, when all of the calcareous parts will have been
dissolved, and after a thorough washing the specimen will be ready
for use.
=External structure= (figs. 18 and 19.)--In a fresh specimen or one
which has been preserved in alcohol or formalin note the raying out of
parts of the body from a common centre. This is characteristic of the
body organization of all Echinoderms, and is known as _radial symmetry_.
The lower surface of the body is called the _oral_ (because the mouth is
on this surface), while the upper is called the _aboral_ surface. The
central part of the body is called the _disk_. Note on the aboral
surface of the disk a small striated calcareous plate, the _madreporite_
or _madreporic plate_. In the middle (or very nearly in the middle) of
this surface of the disk there is a small pore, the _anal opening_. The
entire aboral surface as well as a greater part of the oral side is
thickly studded with the calcareous _ossicles_ of the body-wall. These
ossicles support numerous short stout _spines_ arranged in irregular
rows. Note that some of the ossicles support certain very small
pincer-like processes, the _pedicellariæ_. In the interspaces between
the calcareous plates are soft fringe-like projections of the inner
body-lining, the _respiratory cæca_. Note at the tip of each arm or ray
a cluster of small calcareous ossicles and within each cluster a small
speck of red pigment, the eye-spot or _ocellus_.
[Illustration: FIG. 18.--Dissection of a starfish (_Asterias_ sp.).]
Make a drawing of the aboral surface showing all these parts.
On the oral surface note the centrally-located _mouth_, the
_ambulacral grooves_, one running longitudinally along each ray, and
in each groove two double rows of soft tubular bodies with sucker-like
tips. These are called the _tube-feet_ and are organs of locomotion.
Make a drawing of the oral surface.
=Internal structure= (figs. 18 and 19).--TECHNICAL NOTE.--Take a
specimen which has been immersed for some time in the nitric acid
solution, and with a strong pair of scissors, or better,
bone-cutters, cut away all the aboral wall of the disk except that
immediately around the madreporite and the anus. Now begin at the
tip of each ray and cut away the aboral wall of each, leaving,
however, a single arm intact. When the roof of each arm has been
carefully dissected away the specimen should appear as in fig. 18.
Note the large _alimentary canal_, which is divided into several
regions. Note the short _œsophagus_ leading from the _mouth_ on the
oral surface directly into a large membranous pouch, the _cardiac_
portion of the _stomach_. By a short constriction the cardiac portion
is separated from the part which lies just above, i.e., the _pyloric_
portion of the stomach. From the pyloric portion large, pointed,
paired glandular appendages extend into each ray. These are the
_pyloric cæca_. Their function is digestive, and oftentimes they are
spoken of as the _digestive glands_ or "livers." The pyloric cæca, as
well as the cardiac portion of the stomach, are held in place by
paired muscles which extend into each arm. Note two sets of these
muscles, one set for thrusting the cardiac portion of the stomach out
through the mouth and another for pulling it back, the _protractor
muscles_ and _retractor muscles_, respectively. The starfish obtains
its food by enclosing it in its everted stomach and then withdrawing
stomach and food into the body. Note that the pyloric portion of the
stomach opens above into a short _intestine_ terminating in the
_anus_, and observe that there is attached to the intestine a
convoluted many-branched tube, the _intestinal cæcum_.
Carefully remove a pair of pyloric cæca from one of the rays and note
the short duct which connects them with the pyloric chamber of the
stomach. Note in the angle of each two adjoining rays paired glandular
masses which empty by a common duct on the aboral surface. These
glands are the _reproductive organs_. Note the small bulb-like
bladders extending in two double rows on the floor of each ray. These
are the water-sacs or _ampullæ_, and each one is connected directly
with one of the locomotor organs, the tube-feet.
Make a drawing of the organs in the dissection which have so far been
studied.
TECHNICAL NOTE.--For a careful study of the locomotor organs a fresh
starfish should be injected. This can usually be accomplished by
cutting one ray off squarely, and inserting the needle of a
hypodermic syringe (which has been previously filled with a watery
solution of carmine or Berlin blue), into the end of the radial
water-tube which runs along the floor of the ray. By injecting here,
the whole system of vessels, tube-feet, and ampullæ are filled.
Note a ring-shaped canal which passes around the alimentary canal near
the mouth from which radial vessels run out beneath the floor of each
ray and from which a hard tube extends to the madreporite. This hard
tube is the _stone canal_, so called because its walls contain a series
of calcareous rings, while the circular tube is the _ring canal_ or
_circum-oral water-ring_ from which radiate the _radial canals_. In some
species of starfish there are bladder-like reservoirs, _Polian
vesicles_, which extend interradially from the ring canal.
Note that the ampullæ and tube-feet are all connected with the radial
canals. By a contraction of the delicate muscles in the walls of the
ampullæ the fluid in the cavity is compressed, thereby forcing the
tube-feet out. By the contraction of muscles in the tube-feet they are
again shortened while the small disk-like terminal sucker clings to
some firm object. In this way the animal pulls itself along by
successive "steps." This entire system, called the _water-vascular
system_, is characteristic of the branch Echinodermata. In addition to
the fluid in the water-vascular system there is yet another
body-fluid, the _perivisceral fluid_, which bathes all of the tissues
and fills the body-cavity.
[Illustration: FIG. 19.--Semi-diagrammatic figure of cross-section of
the ray of a starfish, _Asterias_ sp.]
TECHNICAL NOTE.--Take a drop of the perivisceral fluid from a
living starfish and examine under high power of microscope, noting
the amœboid cells it contains.
The perivisceral fluid is aerated through outpocketings of the thin
body-wall which extend outward between the calcareous plates of the
body. These outpocketings have already been mentioned as the
respiratory cæca (see p. 109). Surrounding the stone canal is a thin
membranous tube, and within it and by the side of the stone canal is a
soft tubular sac. The function of these organs is not certainly known.
Work out the _nervous system_; note, as its principal parts, a
nerve-ring about the mouth, and nerves running from this ring beneath
the radial canals along each arm.
=Life-history and habits.=--The starfishes are all marine forms. They
hatch from eggs, and in their early stages are very different in
appearance from the adults. At first they are bilaterally symmetrical,
their radial symmetry being acquired later. Thousands of eggs and
sperm-cells are extruded into the sea-water, where fertilization and
development take place. The young swim freely in the open sea, feeding
on microscopic organisms, and then undergo very radical changes in the
course of their development. The adults are for the most part
carnivorous, feeding on crabs, snails, and the like. The live prey is
surrounded by the extruded stomach which secretes fluids that kill it,
after which the soft parts are digested. (See general account of the
life-history of Echinoderms on p. 119.)
THE SEA-URCHIN (_Strongylocentrotus_ sp.)
=External structure.=--TECHNICAL NOTE.--If fresh or alcoholic
specimens or even the dry "tests" of the sea-urchin (fig. 20) are
to be had, the general characteristics of the external structure
can be made out.
How does the external surface of the sea-urchin differ from that of
the starfish? Can you find the very long _tube-feet_? Where is the
mouth-opening? With what is it surrounded? Each tooth is enclosed in a
calcareous framework. The whole structure is known as "_Aristotle's
lantern_."
TECHNICAL NOTE.--Remove the spines from the underlying shell or
test (fig. 21) and wash the test until perfectly clean, or place
in a solution of lye for a short time and then wash.
[Illustration: FIG. 20.--A sea-urchin, _Strongylocentrotus
franciscanus_. (From specimen from Bay of Monterey, Calif.)]
Note the characteristic radial symmetry of the _shell_ or _test_. Note
on the aboral aspect, diverging from the medial anal aperture, five
double rows of pores. What are these for? Each of the five divisions
set with pores is called an _ambulacral area_, while the intervening
segments which support the long spines are called the _interambulacral
areas_. Note on the aboral surface, surrounding the median-placed anal
aperture, a series of small plates. Those which are located in the
interambulacral areas are the _genital plates_. Through these plates
the ducts from the reproductive organs open by small pores. Note a
very much enlarged plate with a striated appearance. This is the
_madreporite_, which, as in the starfish, is the external opening of
the stone canal and water-vascular system. Note the small _ocular
plate_ at the tip of each ambulacral area. The ocular plates contain
small pigment-cells and communicate with the nervous system.
[Illustration: FIG. 21.--"Test" of sea-urchin, _Strongylocentrotus
franciscanus_, with spines removed. (From specimen.)]
From a general inspection of the sea-urchin's shell the Echinoderm
characteristics, namely, radial symmetry and the presence of the
water-vascular system, are readily seen. While at first glance there
is apparent little similarity between the starfish and sea-urchin,
nevertheless careful examination shows that the two animals are alike
in their fundamental structure. Both are radially symmetrical. The
position of the anal opening makes both starfish and sea-urchin
slightly asymmetrical. In both the madreporite and anus are on the
aboral side, while the mouth is centrally located on the oral side. In
the starfish we noted five ambulacral areas, one on the under side of
each arm; similarly we find five in the sea-urchin. In both cases also
we find the ocular spots at the tips of the ambulacral areas. The
genital apertures are situated interradially in the starfish. In the
sea-urchin they are similarly placed. The dissimilarity between the
two forms is largely due to the very much developed outer spines and
the dorso-ventral thickening of the disk in the sea-urchin. The
starfish is carnivorous, while the sea-urchin lives on vegetable
matter consisting for the most part of green algæ and the red
sea-weeds. Correlated with this difference in food-habits there are
certain differences in the structure of the internal organs. For
example, the alimentary canal in the sea-urchin winds in about two and
one-half turns within the body-cavity before it reaches the anus.
OTHER STARFISHES, SEA-URCHINS, SEA-CUCUMBERS,
ETC.
Without exception all the Echinoderms, under which term are included
the starfishes, sea-urchins, brittle-stars, feather-stars, and
sea-cucumbers, live in the ocean. Some of them, the starfishes and
sea-urchins, are among the most common and familiar animals of the
seashore. Most of them are not fixed, but can move about freely,
though slowly. Some of the feather-stars are fixed, as the sponges and
polyps are.
=Shape and organization of body.=--The body-shape of the Echinoderm
varies from the flat, rayed body of the starfish to the thick,
flattened egg-shape of the sea-urchin, the melon-like sac of the
sea-cucumber and the delicate many-branched head of the sea-lily
sometimes borne on a slender stalk. But in all these shapes can be
seen more or less plainly a symmetrical, radiate arrangement of the
parts of the body. The Echinoderm body has a central portion from
which radiate separate arm or branch-like parts, as in the starfishes
and sea-lilies, or about which are arranged radiately the internal
body-parts, although the external appearance may at first sight give
no plain indication of the radiate arrangement. This is the case with
the sea-urchins and sea-cucumbers, yet, as has been seen in the
sea-urchin, the radiate arrangement can be readily perceived by closer
examination of the surface of the egg- or sac-like body. The radiating
parts of the body are usually five. In the body of an Echinoderm can
be usually recognized an upper or dorsal surface and a lower or
ventral surface. The mouth is usually situated on the ventral side and
the anal opening on the dorsal. Echinoderms agree also in having a
calcareous outer skeleton or body-wall usually in the condition of
definitely-shaped plates or spicules fitted either movably or rigidly
together. This outer body-wall or exoskeleton may bear many tubercles
or spines. These spines are sometimes movable. The body-wall of the
sea-urchin shows very well the exoskeleton composed of plates on which
are borne movable strong spines.
=Structure and organs.=--As has been learned from the dissection of
the starfish, the Echinoderms have well-developed systems of organs.
The body-structure in its complex organization presents a marked
advance beyond the structural condition of the polyps and jellyfishes.
There is a well-organized digestive system with mouth, alimentary
canal, and anal opening. The alimentary canal is either a simple
spiral or coiled tube, or it is a tube in which can be recognized
different parts, namely, œsophagus, stomach, intestine, cæca, and
special glands secreting digestive fluids. This alimentary canal is
not, as in the polyps, simply the body-cavity, but it is an inclosed
tubular cavity lying within the general body-cavity. At the
mouth-opening there is in some Echinoderms, notably the sea-urchins, a
strong masticating apparatus consisting of five pointed teeth which
are arranged in a circle about the opening. The nervous system
consists of a central ring around the œsophagus or mouth, from which
branches extend into the radiately arranged arms or regions of the
body. There is no brain as in the higher animals, but the central
nerve-ring is composed of both nerve-cells and nerve-fibres as in the
nerve-centres of higher forms. Of organs of special sense there are
special tactile or touch organs in all the Echinoderms, and the
starfishes have very simply composed eyes or eye-like organs at the
tips of the rays.
While some of the Echinoderms breathe simply through the outer
body-wall, taking up by osmosis the air mixed with the water, some of
them have special, though very simple, gill-like respiratory organs.
These organs consist of small membranous sacs which are either pushed
out from the body into the water, or lie in cavities in the body to
which the water has access. There is also a distinct circulatory
system, but the "blood" which is carried by these organs and which
fills the body-cavity consists mainly of sea-water, although
containing a number of amœboid corpuscles containing a brown pigment.
There is no organ really corresponding to the heart of the higher
animals. There are distinct organs for the production of the germ or
reproductive cells. The sexes are distinct (except in a few species),
each individual producing only sperm-cells or egg-cells, but the
organs or glands which produce the germ-cells are very much alike in
both sexes. There is no apparent difference between male and female
Echinoderms except in the character or rather in the product of the
germ-cell producing organs. A few species are exceptions, certain
starfishes showing a difference in color between males and females.
As all of the Echinoderms except some of the feather-stars can move
about, they have organs of locomotion, and well-defined muscles for
the movement of the locomotory organs. The external organs of
locomotion, the tube-feet (in the sea-urchins the dermal spines aid
also in locomotion), are parts of a peculiar system of organs
characteristic of the Echinoderms, called the ambulacral or the
water-vascular system. This system is composed of a series of radial
tubular vessels which rise from a central circular or ring vessel and
which give off branches to each of the tube-feet. The water from the
outside enters the ambulacral system through a special opening, the
madreporic opening, and flowing to the tube-feet helps extend them.
The tube-feet usually have a tiny sucking disk at the tip, and by
means of them the Echinoderm can cling very firmly to rocks.
=Development and life-history.=--Differing from the sponges and the
polyps and jellyfishes, the reproduction of the Echinoderms is always
sexual; young or new individuals are never produced by budding, or in
any other asexual way. The new individual is always developed from an
egg produced by a female and fertilized by the sperm of a male. The eggs
are usually red or yellow, are very small (about 1/50 in. in diameter in
certain starfishes), and are fertilized by the sperm-cells of the males
after leaving the body of the female. That is, both sperm-cells and
unfertilized egg-cells are poured out into the water by the adults, and
the motile sperm-cells in some way find and fertilize the egg-cells.
From the egg there hatches a tiny larva which does not at all resemble
the parent starfish or sea-urchin. It is an active free-swimming
creature, more or less ellipsoidal in shape and provided with cilia
for swimming. Soon its body changes form and assumes a very curious
shape with prominent projections. The larvæ of the various kinds of
Echinoderms, as the starfishes, sea-urchins, sea-cucumbers, etc., are
of different characteristic shapes. The naturalists who first
discovered these odd little animals did not associate them in their
minds with the very differently shaped starfishes and sea-urchins, but
believed them new kinds of fully developed marine animals, and gave
them names. Thus the larvæ of the starfishes were called Bipinnaria,
the larvæ of the sea-urchins Pluteus, and so on. These names are still
used to designate the larvæ, but with the knowledge that Bipinnaria
are simply young starfishes, and that a Pluteus is simply a young
sea-urchin. From these larval stages the adult or fully developed
starfish or sea-urchin develops by very great changes or
metamorphoses. The Echinoderms have in their life-history a
metamorphosis as striking as the butterflies and moths, which are
crawling worm-like caterpillars in their young or larval condition.
Most of the Echinoderms have the power of regenerating lost parts.
That is, if a starfish loses an arm (ray) through accident, a new ray
will grow out to replace the old. And this power of regeneration
extends so far in the case of some starfishes that if very badly
mutilated they can practically regenerate the whole body. This amounts
to a kind of asexual reproduction. Some species, too, have the
peculiar habit of self-mutilation. "Many brittle stars and some
starfishes when removed from the water, or when molested in any way,
break off portions of their arms piece by piece, until, it may be, the
whole of them are thrown off to the very bases, leaving the central
disc entirely bereft of arms. A central disc thus partly or completely
deprived of its arms is capable in many cases of developing a new set;
and a separated arm is capable in many cases of developing a new disc
and a completed series of arms." In some of the sea-cucumbers "it is
the internal organs, or rather portions of them, that are capable of
being thrown off and replaced, the œsophagus ... or the entire
alimentary canal, being ejected from the body by strong contractions
of the muscular fibres of the body-wall, and in some cases, at least,
afterwards becoming completely renewed."
=Classification.=--The Echinodermata are divided into five classes,
viz., the Asteroidea or starfishes, "free Echinoderms with star-shaped
or pentagonal body, in which a central disc and usually five arms are
more or less readily distinguishable, the arms being hollow and each
containing a prolongation of the body-cavity and contained organs";
the Ophiuroidea, or brittle-stars, "star-shaped free Echinoderms, with
a central disc and five arms, which are more sharply marked off from
the disc than in the Asteroidea and which contain no spacious
prolongations of the body-cavity"; the Echinoidea, or sea-urchins,
"free Echinoderms with globular, heart-shaped, or disc-shaped body
enclosed in a shell or corona of close-fitting, firmly united
calcareous plates"; the Holothuroidea, or sea-cucumbers, "free
Echinoderms with elongated cylindrical or five-sided body, ... with a
circlet of large oral tentacles"; and the Crinoidea, or feather-stars,
"temporarily or permanently stalked Echinoderms with star-shaped body,
consisting of a central disc, and a series of five bifurcate or more
completely branched arms, bordered with pinnules."
=Starfishes (Asteroidea).=--The starfishes feed on other marine
animals, especially shell-fish and crabs. They are also reputed to
destroy young fish. By means of their sucking-tubes, or tube-feet with
sucker tips, they can seize and hold their prey firmly. They do much
injury to oyster-beds by attacking and devouring the oysters. When
attacking prey too large to be taken into the mouth the starfish
everts its stomach over the prey and devours it. The stomach is
afterward drawn back into the body-cavity by special muscles.
Starfishes vary much in size, color and general appearance, although
all are readily recognizable as starfishes (fig. 22). The number of
arms or rays varies from five to thirty or more in different species;
some have the interradial spaces filled out nearly to the tips of the
rays, making the animal simply a pentagonal disc. In size starfishes
vary from a fraction of an inch in diameter to three feet; in color
they are yellow or red or brown or purple.
[Illustration: FIG. 22.--A group of Echinoderms; the upper one, a
starfish, _Asterina mineata_, the one at the right a starfish,
_Asterias ocracia_, at the left a brittle-star, species unknown, and
at bottom two sea-urchins, _Strongylocentrotus franciscanus_. (From
living specimens in a tide-pool on the Bay of Monterey, California.)]
=Brittle-stars (Ophiuroidea).=--The brittle-stars, or serpent-stars
(fig. 22) as they are also called, resemble the starfishes in external
appearance, that is, they are flat and composed of a central disc with
radiating arms (always five in number, although each arm may be
several times branched). The central disc is always sharply
distinguished from the arms, and the arms are usually slender and more
or less cylindrical. The distinguishing difference between the
brittle-stars and the starfishes is that the body-cavity and the
stomach which extend out into the arms in the starfishes are in the
brittle-stars limited to the central disc, or to the disc and bases of
the arms. The tube-feet also have no suckers at the tips. More than
700 species of brittle-stars are known. They feed on marine
shell-fish, crabs and worms.
=Sea-urchins (Echinoidea).=--The sea-urchins (figs. 20, 21 and 22) of
which more than 300 species are known, have no arms or rays, and they
are usually not flat like the starfishes but globular, with poles more
or less flattened. As has been noted in the examination of the body-wall
or "shell," the radiate character of the body is shown by the five
radiating zones of tube-feet. The mouth, with its five strong "teeth,"
is on the ventral surface, and the anal opening and madreporic opening
are on the dorsal surface. The calcareous plates (seen distinctly in a
specimen from which the spines have been removed) which constitute the
firm part of the body-wall, are more or less pentagonal in shape and are
usually firmly united at the edges. The spines which are so
characteristic of the sea-urchins vary much in size and number and
firmness, but are present in some form on all of them.
While most of the sea-urchins live near the shore, being very common
in tide-pools, some live only on the bottom of the ocean at great
depths. Their food consists of small marine animals and of bits of
organic matter which they collect from the sand and débris of the
ocean floor. Many of the sea-urchins are gregarious, living together
in great numbers. Some have the habit of boring into the rocks of the
shore between tide-lines. I have seen thousands of small beautifully
colored purple sea-urchins lying each in a spherical pit or hole in
hard conglomerate rock on the California coast. How they are enabled
to bore these holes is not yet known. There is great variety in size
and color among the sea-urchins. The colors are brown, olive, purple
red, greenish blue, etc.
A few kinds of sea-urchins have a flexible shell or test. The
Challenger expedition dredged up from sea-bottom some sea-urchins, and
when placed on the ship's deck "the test moved and shrank from touch
when handled, and felt like a starfish." The cake-urchins or
sand-dollars are sea-urchins having a very flat body with short
spines. They lie buried in the sand, and are often very brightly
colored. Their hollow bleached tests with the spines all rubbed off
are common on the sands of both the Atlantic and Pacific coasts.
=Sea-cucumbers (Holothuroidea).=--The sea-cucumbers (fig. 23) show at
first glance little resemblance to the other radiate animals. The body
is an elongate, sub-cylindrical sac, resembling a thick worm or
sausage or cucumber in shape. At one end it bears a group of branched
tentacles which are set in a ring around the mouth-opening. The
body-wall is muscular and leathery, but contains many small separated
calcareous spicules. There are usually five longitudinal rows of
tube-feet. In some species, however, tube feet are wholly wanting; in
others they are scattered over the surface.
Although there are known about five hundred species of sea-cucumbers
many of which live along the shores, they are much less familiar to us
than the starfishes and sea-urchins. They usually rest buried in the
sand by day, feeding at night. Some of them attain a large size. A
great orange-red species of the genus _Cucumaria_, which is found in
the Bay of Monterey, California, is three feet long.
The people of some nations use sea-cucumbers as food. They are called
"trepang" in the orient. The trade of preparing the trepang is almost
entirely in the hands of the Malays, and every year large fleets set
sail from Macassar and the Philippines to the south seas to catch
sea-cucumbers.
[Illustration: FIG. 23.--A sea-cucumber, _Pentacta frondosa_. (After
Emerton.)]
=Feather-stars (Crinoidea).=--The feather-stars or sea-lilies or
crinoids (fig. 24), as they are variously called, differ from the
other Echinoderms in having the mouth on the upper side of the central
disc, and in the fact that all of the species are fixed, either
permanently or for a part of their life, being attached to rocks on
the sea-bottom by a longer or shorter stalk which is composed of a
series of rings or segments. The central disc is small and the
radiating arms are long, slender, sometimes repeatedly branched, and
all the branches bear fine lateral projections called pinnulæ. Most of
the feather-stars live in deep water and are thus only seen after
being dredged up. They feed on small crab-like animals, and on the
marine unicellular animals and plants.
[Illustration: FIG. 24.--A crinoid or feather-star, _Pentacrinus_ sp.
(After Brehm.)]