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Information on the basics of Invertebrate Zoology
An Introduction To The Study of Invertebrate Zoology
Kingdom: Animalia, Phylum: Echinodermata
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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.
Plate 43 = Phylum: Echonodermata -- Example Classes: Echinoidea, Blastoidea, Ophiuroidea