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For educational purposes only; do not review, quote or
abstract:-- Information on
the basics of Invertebrate Zoology |
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An Introduction To The
Study of Invertebrate Zoology Kingdom: Animalia, Phylum: Echinodermata (Contact)
CLICK on underlined file names and included illustrations to
enlarge: Echinodermata derives from the sane "spiny skin"
and includes such organisms as starfish and brittle stars. Parts of the skeleton project through the
body wall. These organisms that are
found only in the marine environment possess radial
symmetry where the rays occur in five or multiples of five. This contrasts to Ctenophora and Cnidaria
where there are 4, 6 or 8 rays. The
endoskeleton is calcareous and is derived from the mesoderm. A true coelom is present. There is a water vascular system called
the "Ambulacral System." This is a system of tubes containing
mostly water. And is modified toward a closed syncytium. It operates the tube
feet. These organisms are interesting for being quite
different from all other animal phyla, and their origin is obscure. They have the calcareous endoskeleton and
water vascular system. All members
have bilateral larvae that arose from some bilateral ancestor. The radial symmetry is advantageous to a
sessile animal. They also have a
close relationship to the Chordata, which is evidenced by the calcareous
skeleton, an embryonic development that includes a blastula and gastrula, and
considerable biochemical evidence. ------------------------------------ The Class Asteriodea includes
the starfish.
There is great uniformity in appearance among the various
members. They consist of a central disc
with five or multiples of five radiating arms and no sharp demarcation. The anus is not set off sharply from the
disc and the coelom extends out into the arms. Tube feet are for locomotion and one stomach is eversible.
A type genus is Asterias, which is found off the east coast of North
America. Most species live offshore
in shallow water among the rocks or any relatively hard substrate. However, some genera do live in deep water
or in mud. Body Plan.-- This consists of a disc and
surrounding arms. There is an oral
and an aboral surface but no actual dorsal or ventral surface. The madreporite is
an opening to the water vascular system on the aboral surface. A mouth occurs on the oral surface. The ambulacral groove is on the oral
surface from which tube feet project through the body wall. There is one eyespot on the tip of each
arm. The whole body surface is
studded with small, blunt spines, which are projections from the internal
skeleton or ossicles. Dermal branchiae, or gills,
contain small extensions of the coelomic cavity. These may be branched. Pedicellariae are pincher-like structures on
the body surface, which cluster around bases of larger spines. They function like avicularia in the Bryozoa in that they keep the body surface
clean. Body Wall.-- A ciliated epidermis covers the
whole body but these may become worn off of the older spines. The dermis is a
layer in the epidermis, which is a mixture of muscle and connective
tissue. Ossicles are borne in the
dermis and form the skeleton. Surface
spines project from them. A ciliated
peritoneum lines the coelom. A
coelomic fluid exists inside the coelom. Food & Digestion.-- There is an oral stomach
located closest to the mouth, which is called the cardiac
stomach. There is also a
second or aboral stomach located in back of the oral stomach, which is called
the pyloric stomach. Pyloric caecae are digestive glands that
lead off of each of the five points of the pentagon aboral stomach. An anus is present but it is barely
functional. A rectal
caecum occurs as a branch off of the intestine. Asterias
feeds on small worms, crustaceans and other echinoderms and mollusks. Food is taken directly into the oral
stomach. To feed on a clam the
starfish pulls open its valves by exerting a continuous pull with its tube
feet. When mollusk valves begin to
open the starfish extends its oral stomach into the opening and begins to
digest the contents in place. The
stomach is everted by contraction on the center point of the body, and
special muscles retract it. Digestion is partially extra cellular and
intracellular. Undigested material
leaves via the mouth. The anus is
practically nonfunctional. Haemal System.-- This is nonfunctional, and it
may be a remnant of a previously functioning circulatory system.
Circulation.--
movement of the coelomic fluid accomplishes this. Respiration.-- Dermal branchiae and any exposed
surface, especially the tube feet, may serve for respiration. Excretion.-- Amebocytes,
which occur in the coelomic fluid acquire crystalline wastes and migrate
through dermal branchiae to eventually end up in the ocean. These have been referred to as leucocytes.
There is also some diffusion through any exposed surface. Support & Protection.-- The skeletal
ossicles support the organism, as well as the heavy body wall. Pedicellariae keep the body surface clean. Locomotion.-- This is primarily a function of
the water vascular system. The
madreporite takes in water via a sieve.
Then leading down the madreporite to the oral side is a stone canal with calcium deposited on its
walls. The stone canal runs into the ring canal, which runs all the way around the
mouth. At intervals the radial canals
emanate into the arms. Lateral
branches of these are called transverse canals. The transverse canals connect with ampullae, which are enlargements at the inner ends
of the tube feet. The whole system is
full of seawater. There are valves that guard the junctions with the
ampullae. Suckers occur at the distal
end of the tube feet and retraction of tube feet is by muscles. Forward motion is accomplished by "pole vaulting" on tube feet. Loss of water from the system is by
leakage under pressure. Associated with
the ring canal are five pair of vesicles called "Tiedemann's Bodies, whose function is not
fully understood. Nervous System.-- There is a poorly developed
nervous system present, which is very simple with nerves being located
adjacent to the epidermis. These
parallel the ambulacral system with a ring around the mouth and one branch
under each radial canal with subsequent branches. There is no central organ or brain. The only sense organs occur at the tips of the arms. However, all the body surfaces are
generally sensitive, with the tube feet being most sensitive. It is thought that little specialization
is needed because these animals are radially symmetrical and slow moving. Reproduction.-- Sexes are separate and there is
no sexual dimorphism. There are five
gonads, each one is branched and joined with the adjacent one.
Eggs and sperm
are shed directly into seawater.
Fertilization occurs entirely in the open water and there is a
tremendous number of gametes produced. There is no
actual asexual reproduction, but there is a high degree of regenerative
ability. Development.-- This involves a blastula that
gives rise to a gastrula and finally a ciliated larva called a bipinnaria, which is bilaterally
symmetrical.. The bipinnaria possess
a complete digestive tract and feed immediately upon hatching, which is very
rapid. The larva attaches to the
bottom of the environment and becomes sessile for a few weeks. Later it swims off as a mature starfish. Economic Importance.-- Starfish can be slightly
destructive to commercial oyster beds. ------------------------------------ Please see following plates for Example Structures of the Asteroidea: Plate 39 = Phylum: Echinodermata:
Asteroidea: Asterius sp. Plate 40
= Phylum: Echinodermata: Asteroidea: Asterius
sp. -- Cross-section Plate 43
= Phylum: Echinodermata -- Example Classes:
Echinoidea, Blastoidea, Ophiuroidea ------------------------------------ The Class Echinoidea includes
the sea urchins and sand
dollars. Type Genera are Strongylocentrotus and Arbacia. Ossicles are fused to form a rigid test. Aristotle's Lantern is present. Pedicellariae are 3-jawed stalked and sub
ocular shaped and variously flattened.
A Pluteus larva is present. Habitat.-- Being offshore to 100 fathoms on hard
or rocky substrates, this is similar to the starfish.
Body Plan.-- Sea urchins may be thought of as
starfish with the five arms brought up over the back. They are basically similar to
starfish. Dermal ossicles of the
skeleton are regularly arranged in rows and are fused into an immobile, rigid
structure called the Test or Corona. Spines are quite long and movable by
possessing ball and socket joints on
the test. The rows of ossicles are
given names because they are quite regularly arranged. There are altogether 20 rows extending
from the mouth to the aboral point.
The ambulacral rows have tube feet while the interambulacral rows are
without tube feet. A periproct occurs around
the anus at the aboral end. Genital plates are located around the periproct
and occur at the ends of the interambulacral rows. A genital pore occurs on each plate. The plate bears the madreporite in addition to a genital pore. An ocular plate occurs on the ends of ambulacral rows,
which corresponds to the ocular plats of a starfish. Oral tentacles,
which are modified tube feet, surround the mouth to function in feeding.
There are 10 dermal branchiae around the periphery of the peristome. The pedicellariae differ in two ways from a
starfish: (1) they occur on long
stalks and (2 they have three jaws. Body Wall.-- The construction is similar to a
starfish in that an epidermis covers the entire surface in early stages. A dermis lies under the spines. There are no muscles and connective tissue
that serves to secrete the skeleton is practically gone. A peritoneum lines the large coelom. Food & Digestion.-- Food consists of both
dead or living plant and animal material.
The digestive tract begins at the mouth. Aristotle's Lantern is
present, which is a group of ossicles that have been highly modified into a
feeding mechanism. It appears as
a 5-sided pyramid with the point
projecting through the mouth. It is
operated by muscles and consists of about 40 parts. Five teeth project through the bottom and pick up and chew
food. The mouth and Aristotle's Lantern lead to an
esophagus. This in turn leads to a
large intestine. The intestine runs
around and doubles back on itself to give way to the rectum and finally to
the anus. A Siphon is present that is a
diverticulum emanating from the esophagus.
It parallels the digestive tract and finally reenters the tract near
the anus. Its function is to carry a
fresh supply of water. There are no
digestive glands present.
Circulation.-- The coelomic fluid functions in
circulation, and the haemal system is more developed than in the starfish. Respiration.-- Dermal branchiae are inflated and
deflated so that the coelomic fluid becomes pumped in and out of the
branchiae. Tube feet and the siphon
also function in respiration. Excretion.-- This process is not entirely
understood but is believed to be similar to that of the starfish with dermal
branchiae and amebocytes involved. Support & Protection.-- The test, spines and
pedicellariae serve to support and protect the organism. Locomotion.-- Tube feet, which are quite long, and
the water vascular system are involved in locomotion just as in the
starfish. The ampullae are more leaf
like and not as bulbous as in starfish.
Each tube foot has two holes by which it emerges through the skeleton. Nervous System.-- This is similar to the
starfish and of the same degree of low complexity. Sense Organs.--Ocular plats serve as sense
organs. Reproduction.-- There are five gonads that lie
under inter ambulacral areas. The
gonads may be connected with one another to form a ring. They open to the outside via genital pores
on genital plates. Gametes are shed
into the open water. A Pluteus Larva is
present, which has long arms that provide locomotion. (both sea urchins and
starfish have been used extensively in embryology).
Morphological Variation.-- There is a high
degree of variation found in the Echinoidea.
Flattening has been carried out extensively. In the san dollar the anus occurs on the edge of the
dollar. Heart urchins, sea biscuits
and other groups have distinctive shapes.
Economic Importance.-- The group has academic
interest and the gonads are consumed as a delicious and nutritious human
food, often served cold and with lemon as in Chile and other countries. ------------------------------------ Please see following plates for Example Structures of the Echinoidea: Plate 41
= Phylum: Echinodermata: Echinoidea: Strongylocentrotus sp. Plate 42 = Phylum: Echinodermata:
Echinoidea: Strongylocentrotus sp. -- Aristotle's Lantern Plate 43
= Phylum: Echonodermata -- Example Classes:
Echinoidea, Blastoidea, Ophiuroidea ------------------------------------ The Class Ophiuroidea or brittle stars and
basket stars are similar in shape to starfish
except that their whole body appears more star shaped. This is because their arms are sharply
marked off from the main disc and supported by a long series of ossicles (=
vertebrae) for muscle attachment. The
digestive tract is limited to the disc region. The coelom is practically limited to the disc region also. They
do not contain caeca of the alimentary canal. The madreporite is on the oral side and the ambulacral groove
is covered. The size is small to moderate save for basket stars
that have arms branched of subdivided.
Their tube feet do not possess suckers and there are no
pedicellariae. These animals do not
move by means of tube feet but rather by pushing and pulling on surrounding
objects with their arms. To
accomplish the arms are sharply distinct from and freely movable on the main
disc. They are armored with skeletal
plates. The epidermis is vestigial
but there is a strong cuticle. Spines
occur on the side plates that allow for a grip. The ambulacral ossicles of each pair fuse to form a series of
vertebrae that articulate by an arrangement of knobs and sockets and can be
moved in various directions by four muscles.
The vertebrae reduce the perivisceral cavity in the arm to a canal in
which there is no room for caeca of the alimentary canal. The nerve cord has ganglia that correspond
to the muscles between the vertebrae.
The tube feet have no suckers and no ampullae and are usually provided
with warts of sense cells. The alimentary canal is reduced to a simple bag that
cannot be protruded through the mouth.
The mouth is armed with spines that serve as teeth. As in the Echinoidea there is a Pluteus larva. No
economic importance is attached to this group. ------------------------------------ Please see following plates for Example Structures of the
Ophiuroidea: Plate 43 = Phylum: Echonodermata
-- Example Classes: Echinoidea,
Blastoidea, Ophiuroidea ------------------------------------ The Class Holothuroidea are
the sea cucumbers. They have a reduced skeleton, which consists of tiny ossicles
imbedded in a heavy body wall. The
body is elongated along the oral-aboral axis (anterior & posterior
ends). There is a tendency to return
to bilateral symmetry. The dorsal and
ventral sides are flattened, and the tube feet are relatively large.
Habitat.-- These animals lie on the bottom of
the marine environment or they are buried in sand with their tentacles and
anus exposed. Body Wall.-- This is pliable but very
tough. Oral tentacles occur at the
anterior end and there are highly modified tube feet. Genital Opening.-- There is a single genital
opening, not five as in the previous classes. It appears as a wart like projection just back of the oral
tentacles. A type Genus is Cucumaria. Here the anus is located at the posterior
end. Tube feet are arranged in
regular five rows typical of Echinoidea and Asteroidea. However, the ventral rows are better
developed than in the other classes.
There is a very tough dermis and a circular muscle is located
underneath the dermis. There are five
longitudinal muscles appearing as very heavy bands running the length of the
body. Peritoneum lines the coelom. The skeleton consists of ossicles imbedded
in the dermis. They are microscopic
in size and serve no function other than to strengthen the dermis. Food & Digestion.-- Holothuroidea are
passive feeders. Food settles on
their tentacles or it is scooped up from the substrate by the tentacles. A short esophagus follows the mouth. A pharyngeal
collar surrounds the esophagus, which is homologous to Aristotle's
Lantern. But here it is just a
strengthening ring. Circulation.-- The principal medium for
circulation is coelomic fluid. A haemal system exists that consists of vessels
containing red haemoglobin in some cases.
There are no muscular walls and distribution of the blood is
inefficient. Oxygen and other
materials are distributed. Respiration.-- A respiratory
tree branches off the rectum and eventually takes up a lot of the
space in the coelomic cavity. The
rectum is equipped with muscles for dilation and constriction, and a valve
can close the anus. Water can be
pumped in and out of the respiratory trees and thus dissolved oxygen is
distributed. Additional respiratory
surfaces are found on the tube feet and oral tentacles. Excretion.-- The respiratory tree has been
thought to be involved in most excretion, although positive verification is
lacking. Support & Protection.-- The toughness and
thickness of the body affords support and protection. Some species possess cuvierian organs, which secrete a material
that is passed through the anus. The
substance is sticky and may be ejected to entangle small predators.
Locomotion.--
Movements of the body wall in a wormlike fashion and/or tube feet enable
locomotion. Water Vascular System.-- The system functions in
a similar pattern as other Echinodermata.
The madreporite opens into the coelom, but the system is not filled
with seawater but rather with coelomic fluid. The polian vesicle is a
reservoir for the fluid of the system.
It appears as a large sac off of the ring canal. Five radial canals run underneath the five
longitudinal muscles. The tentacles
are operated by the water vascular system. Nervous System & Sensitivity.-- This is the
same as that found in other Echinodermata.
It consists of a simple ring next to the water vascular ring with
subsequent branches. There are no
sense organs but the body is generally sensitive. Especially sensitive are the tentacles, anal region and tube
feet. Reproduction.-- In the sexual reproduction only
one gonad is located between two ambulacral rows. A genital duct runs along a mesentery. Fertilization is external and gametes are
fed into the open water. An Auricularia larva is formed, which is
bilaterally symmetrical. There is no actual asexual reproduction, but these
animals possess a powerful regenerative ability that is referred to as evisceration.
Here the entire viscera are delivered to a predator by rupturing the
body wall at the oral and anal ends.
The remaining torn portions regenerate themselves. The process is accomplished by internal
pressure that ruptures the body wall. Economic Importance.-- In some parts of the
Orient the dried body wall is consumed as food, which is called Tripang or Leche-de-mer. ------------------------------------ Please see following plates for
Example Structures of the Holothuroidea: Plate 44 = Phylum: Echinodermata:
Holothuoroidea: Cucumaria sp. Plate 45 = Phylum: Echinodermata:
Holothuoroidea: Cucumaria sp. -- Cross-section Plate 46 = Phylum: Echinodermata:
Holothuoroidea: & Crinoidea -- Examples ------------------------------------ The Class Crinoidea includes
the sea lilies and stone
lilies. They are a remnant of
a very large group of Echinodermata that are mostly represented by
fossils. Most of their life is spent
in attachment by a stalk consisting of a group of ossicles. They have pinnately branched tentacles and
skeletal ossicles make up support of the branched arms. The calyx, or
disc of other Echinodermata, is the site of most internal organs. Tube feet are not locomotory but serve as
sensory structures. The madreporite
is and water combines with the coelomic fluid in the water vascular system
via a series of holes. In some ways
these animals are considered to be very primitive whereas in others they are
advanced.
Habits.-- Some Crinoidea remain sessile
throughout their life but in others movement is accomplished by alternate
beating of the arms. The sessile
stage develops more extensively. Feeding.-- A mucous
scheme is present. All of the
arms are covered with mucous and a groove runs down each arm. There is a constant stream of mucous,
which is carried toward the mouth.
The mouth and anus are located on the same side, with the mouth being
in the center and the anus off to one side. Reproduction.-- The sexes are separate and
fertilization is external. Bilateral
larvae are formed. ------------------------------------ Additional Echinodermata: Plate 43 = Phylum: Echonodermata -- Example Classes: Echinoidea, Blastoidea, Ophiuroidea ============== |
