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For educational purposes

Information on the basics of Invertebrate Zoology

 

 

                      Introduction                                                                                                                                                                                                                                                                   Platyhelminthes

 

An Introduction To The Study of Invertebrate Zoology

Animalia, Porifera, Ctenophora & Cnidaria

(Contact)

 

 

Kingdom:  ANIMALIA

SubKingdom:  Parazoa

Phylum:  Porifera -- Sponges

    Class:  Calcarea
   
Class:  Hexactinellida
   
Class:  Demospongia

 

[COELENTERATA -- obsolete category]

 

SubKingdom:  Metazoa  (Eumetazoa)

 

Phylum:  Ctenophora (Previously

     under Coelenterata)

 

    Class:  Tentaculata
    Class:  Nuda

Phylum:  Cnidaria (Previously under Coelenterata) -- Combs, corals, jelly fish

 

    Class:  Anthozoa

        Subclass:  Alcyonaria

        Subclass:  Zooantharia

    Class:  Hydrozoa

    Class:  Leptolinae
    Class:  Trachylinae

    Class:  Scyphozoa

Bibliography      Citations     Plates

Sample Examinations

Invertebrate Classification

 

     CLICK on underlined file names and included illustrations to enlarge:

 

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Kingdom:  ANIMALIA, SubKingdom:  Parazoa, Phylum:  Porifera -- Sponges

 

          Three classes of Porifera will be discussed as follows:  Class:  Calcarea that have a skeleton of calcium carbonate spicules and three body forms (Ascon, Sycon and Leucon); Class:  Hexactinellida that have 6-rayed spicules of silicon dioxide and a Leucon body type.  Most are deep-sea inhabitants and there are many elaborate and beautiful forms; Class:  Demospongia, the largest group whose skeletons are of silica, spongin or a combination of both.  Their spicules are never 6-rayed and they possess a Leucon body type.  Included here are the fresh water sponges.

 

          Porifera have their entire body perforated with tiny perforations.  They are multicellular with simple tissues.  There are neither organs nor organ systems.  The body is porous and the pores communicate with a system of channels that carries water currents through the body.  No muscle nor nerve cells are in evidence, there being only epithelia and connective tissue.  The adults are sessile.  Collar cells or choanocytes exist that line the interior of the chambers.

 

 

          There is an intracellular digestion as in the Protozoa where food is taken into cells directly.  They are primarily a marine group with one family living in fresh water.

 

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          An example of a typical Porifera is Scypha, which lives in shallow marine environments.  It is cylindrical with a prominent crown 2.5 cm. High.  There is radial symmetry and an interior cavity called a false cloaca.  Finger-like chambers radiate out of the chamber and others run in from the outside, all of which end blindly.  Connections between the chambers are minute incurrents and radial canals, the latter having flagellated chambers.  Extremely minute but specialized pores, called prosopyles, permeate the entire body and interconnect the chambers.  Apopyles occur, which are openings into the cloaca and these are larger than the prosopyles.  A water current is set up by the flagella of the collar cells.  The water is directed into the prosopyles, through the apopyles and into the cloaca from which it escapes via the osculum to the outside of the animal.  Calcium carbonate may arise from the mesoglea to form spicules.

 

   
   
         
Feeding and Digestion.-- Food is microscopic and occurs in the water.  It is picked-up by the collars on the choanocytes.  Three things that can happen to the food are (1) collar cells may digest the food, (2) food may diffuse into the mesoglea and (3) the collar cell may transfer the food to an amebocyte.  Cells in the epidermis obtain their food by diffusion from the amebocytes.

 

 

          Respiration and Excretion.-- These two function are accomplished by simple diffusion through any or all body surfaces.

 

          Support and Protection.-- The spicules give support and they may consist of calcium carbonate or silicon dioxide.  Protection is afforded by spongin, a horny protein, which forms an anastomosing network.  Both the spicules and spongin are secreted by the amebocytes.

 

          Locomotion and Movement.-- Porifera are sessile and immobile.  Every cell can change its shape to some degree.  If all cells contract at once, there will be a minute shrinkage of the entire body.

 

          Sensitivity and Conduction.-- There are no special organs for sensitivity and conduction, and cells respond to stimuli as individuals.  A response may be spread out slowly from cell to cell, e.g., one cell passes a stimulus on to the next and so forth.

 

          Reproduction.-- There are two types of reproduction:  asexual and sexual.  Asexual reproduction may be by budding or gemmules.  Gemmules are a kind of budding found only in fresh water Porifera and where a group of amebocytes clump together in a spherical mass and then are surrounded by a wall.  These form in the mesoglea and they serve to carry-over the organism in winter, drought, etc.  New individuals are formed from each gemmule.

 

          In Sexual reproduction the source of gametes is in the amebocytes and they are formed by meiosis.  Eggs remain in place in the mesoglea of the parent.  Sperm break out of the mesoglea and leave the parent.  They are taken into the body of another sponge with the water-food current.  A sperm cell is then captured by a collar cell and taken into a vacuole and finally transferred to an amebocyte, which conveys it to the egg cell.  The fertilized egg develops into a flagellated larva in the mesoglea.  Later it escapes to the external environment as an amphiblastula.  Such a curious reproductive behavior removes this group from a direct evolutionary line in the animals.

 

          Body Forms.-- There are three body forms in Porifera as noted previously.  In the Ascon Type there is one large flagellated chamber.  There are no apopyles, incurrent or radial canals and the organisms are extremely small.

 

 

          The Sycon Type is similar but with a different shape:

 

 

          The Leucon Type allows for a much thicker body wall.  There is a reduction of the cloaca, which is divided into a complex series of channels.  This type is found in all of the larger sponges:

 

 

          All these body type occur in the Calcarea, but Hexactinellida and Demospongia have only the Leucon Type.  The economic importance of sponges has been almost entirely for the commercial bath sponge.  Porifera have been regarded as a blind alley in evolution.

 

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Please see following plates for Example Structures of the Porifera:

 

Plate 10 = Animalia: Parazoa: Porifera: Scypha sp.

Plate 11 = Animalia: Parazoa: Porifera: Calcarea & Hexactinellida (Scypha sp., Hyalonema sp. & Euplectella sp.)

Plate 33 = Phylum: Porifera: Noteus sp. & Asplancha amphora

 

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          SubKingdom:  Metazoa Phylum:  Ctenophora (Previously under Coelenterata).  Included here are the sea gooseberries, sea walnuts and sea cucumbers (= not to be confused with the Holothuroidea).  Two Classes are Tentaculata and Nuda.  Their name is derived from eight meridially arranged comb rows or ctene.  These are a row of locomotory structures quite similar to the membranelles of the Ciliata.  They consist of several rows of cilia fused together to form a plate or ctene.  There is a symmetry that is a kind of hybrid between radial and bilateral symmetry.  Two tentacles are present.  Lasso Cells or colloblasts exist, which are sticky in nature.  However, there are no nematocysts.  There is a jelly-like mesoglea and a digestive cavity or gastrovascular cavity with a canal system.  Luminescence allows these animals to glow in the dark.  There is no known economic importance and their evolution is probably another blind alley, which evolved from a stock similar to the Cnidaria.

 

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Please see following plates for Example Structures of the Ctenophora:

 

Plate 67 = Phylum: Ctenophora -- Transverse section to axis of Corallium sp.

Plate 68 = Phylum: Ctenophora -- Structure of an anemone.

 

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          SubKingdom:  Metazoa  Phylum:  Cnidaria (previously under Coelenterata).  The Cnidaria are primarily marine animals with radial symmetry.  Included are combs, corals, jelly fish Their tentacles are equipped with Nematocysts, which are stinging structures used in capturing their prey.  Their body form is a double walled sac with a large central cavity, which is called the gastrovascular cavity or coelenteron.  Three classes discussed here are Hydrozoa, Scyphozoa and Anthozoa. 

 

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          The Class Hydrozoa shows extremes in the presence or absence of various generations.  Some species possess conspicuous polyp stages and suppressed medusoid stages.  Some show both equally while others show only a pronounced medusoid stage; e.g., Hydra spp. has no medusae while Gonionemus spp. have mostly medusae.  The Order Siphonophora (Portuguese Man-of-War) affords the best example of polymorphism in the entire Animal Kingdom.  Several kinds of polyps and medusae are all together in one body.  Each serves a particular function (eg., locomotion, digestion, hunting, etc.).  Some corals, e.g., Elk-horn Coral, are also represented here.  The Class is characterized by having alternation of generations, medusae with a velum, gonads that are ectodermal and exteriorly situated, and a completely acellular mesoglea.

 

 

          The genus Obelia is a colonial representative only a few inches high.  There is a branched, plant-like stalk and the animal is common in coastal North America where it may be found attached to any submerged surface in salt water.  The gastrovascular cavity is continuous throughout the colony.

 

          Food and Digestion.-- Members of the genus are carnivorous.  They use nematocysts to catch their food.  These are elaborate inclusions within cells.  There is an explosion of a thread from the nematocyst when there are physical or chemical stimuli.  The force in ejection probably involves an osmotic process.  The thread contains a poisonous substance that may kill, paralyze or occasionally trap the prey in a sticky secretion.  Nematocysts are located in greatest numbers on the tentacles, but they may also be scattered throughout the body.

 

          The food is stuffed through the mouth and into the gastrovacular cavity.  Digestion commences in the gastrovascular cavity, and proteineous enzymes are secreted by the gastrodermis.  Digestion is completed intracellularly.  The gastrodermal cells can send out pseudopodia, which engulf small solid food particles.  Egestion is also through the mouth.

 

          Circulation.-- flagellated cells that line the gastrovascular cavity and create a current accomplish this.  The epidermis gets its food by diffusion across the mesoglea. 

 

          Respiration.-- There is a simple diffusion on any or all body surfaces.

 

          Support.-- They may possess a secreted cuticle, which surrounds the stalks and hydranths of the colony or the mesoglea supports the animal.

 

          Protection.-- This is afforded by the nematocysts, which can sting and paralyze other organisms.

 

          Movement.-- Epithelio-muscular cells are present.  Usually the basal portion of the cell extends arms of striated muscles, which are simply extensions of the base of the cell.

 

 

          In the course of evolution the muscle cell came into being and served solely as such.  Muscles are arranged in layers under the epidermis or gastrodermis.

 

 

          Many hydroids are sessile, so they do not move, with the genus Hydra being an exception.  Jellyfish and related forms are moved by the current, although their tentacles and body as a whole may supply some motive force.

 

          Sensitivity.-- A Nervous System is present with nerve and sensory cells which are sensitive to all standard environmental stimuli. 

 

 

          The system consists of (1) a plexis of nerve fibers without any organization into nerve cords or centralization into ganglia, and (2) transmission of nerve impulses is not polarized and the impulse can move in any direction.

 

          There are Reflexes, which are primarily concerned with feeding movements.  For example, in Hydra spp. all tentacles contract over a food particle and the mouth opens.  Sense Organs are found only in medusae.  These are statocysts and simple eyes, which are organizations of several cells into a very simple organ.

 

          Reproduction.-- Alternation of Generations is common:

 

 

          Polymorphism occurs where the same species can exist in more than one body form.  The genus Obelia has hydranths, reproductive polyps and medusae.

 

          Medusoid Body Form.-- This is a free-swimming form that is not too fundamentally different from a polyp, except that a medusa has much more mesoglea than a polyp.  The principal radii are those that emanate from the four corners of the stomach and are called per-radii.  Additional radial canals may occur between them.  A velum is found only in the Hydrozoa.  Gonads hang down from the radial canals and are derived from the epidermis and located exteriorly.  Eggs and sperm are shed directly into the water.

 

 

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Please see following plates for Example Structures of the Hydrozoa:

 

Plate 12 = Animalia: Cnidaria: Hydrozoa: Obelia sp.

Plate 13 = Animalia: Cnidaria: Hydrozoa: Gonionemus sp.

Plate 14 = Animalia: Cnidaria: Hydrozoa: Campanula sp., Hydra sp., Eudendrium sp., Gonionemus sp.

Plate 15 = Animalia: Cnidaria: Hydrozoa: Tubularia sp., Bougainvilla sp.

 

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          The Class Scyphozoa includes the cup animals or jellyfish.  They are characterized by a medusa without a velum, gonads that are endodermal in origin and located internally.  The mesoglea does have some cells and fibers (= loose connective tissue).  Internal tentacles serve as gastric filaments.  The polyps are a very small, inconspicuous and short-lived larval form.  All are marine species, and their size varies up to about two meters in diameter.

 

          A representative species is Aurelia aurita, which is found in inshore waters of coastal North America.  The species has radial symmetry and no velum.  There is a canal system that is much more complex than that found in the Hydrozoa.  There are oral lobes where the corner of the mouth is pulled far out and bears nematocysts on tentacles.  A subgenital pit has no obvious function, but may serve to make a closer contact between seawater and the stomach. 

 

          Food and Digestion.-- Aurelia is carnivorous and may catch some larger fish.  It has a stomach that is subdivided into four lobes:

 

 

          Digestion begins extracellularly and terminates intracellularly as in the Hydrozoa.  Gastric tentacles secrete enzymes and bear nematocysts; they also create a water current.

 

          Respiration and Excretion.-- These are accomplished by simple diffusion as in the Hydrozoa.

 

          Support.-- Only the mesoglea, which may be rather stiff, supports the organism.

 

          Movement.-- There are muscles of the epithelio-muscular variety that allow for movement.  The muscles occur in strong bands around the circular portion of the body.  However, most movement is planktonic.

 

          Sensitivity and Conduction.-- These are very similar to the Hydrozoa except that the sense organs are more elaborate and are called Tentaculocysts.  Each contains a statocyst, eyespot and olfactory pits.  There is a diffuse conduction through a network of neurons without any coordination.

 

          Life History.-- Auralia is dioecious.  Eggs and sperm are borne internally in the endodermis.  Fertilization occurs in the digestive cavity.  The development of eggs begins in the gastrovascular cavity and is completed at the tips of the mesenteries where they give rise to a hollow planula larva, which possesses an archenteron.  The larva develops into a small polyp called a scyphistoma, which is very inconspicuous.

 

 

          A peculiar budding type occurs where a linear arrangement of buds is assumed.  This is called the Strobila Stage.

 

 

          Buds detach to form free ephyra, which are actually juvenile adults.  These are a free-swimming stage that gradually grows into the adult.

 

 

          An alternation of generations exists but is inconspicuous.

 

          Commensals and Symbionts.-- Some fish are able to live inside the tentacle mass and are not injured by the nematocysts.  Also, some jellyfish have symbiotic algae in their cells.

 

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          The Class Anthozoa includes the "flower animals," which includes the sea anemones, horny corals and calcareous corals.  There is only a polyp stage.  The oral end of a polyp is expanded into a disc, which bears tentacles either on the rim or scattered over the disc.

 

 

          They have a stomodaeum or gullet, which is ectoderm that has turned in at the mouth.  The gastrovascular cavity is subdivided by vertical partitions called mesenteries or septa.  These greatly increase the surface area in the gastrovascular cavity.  Gastric filaments exist that are equipped with nematocysts and they begin along the sides of the free edges of the mesenteries, terminating freely in the gastrovascular cavity.  A mesoglea serves as a sort of connective tissue and it consists of cells and fibers.  These are more specialized than in the Scyphozoa where there are only scattered cells.  The gonads are endodermal in origin and located internally.

 

          Two Subclasses divide this group as follows:

 

          The Subclass Alcyonaria includes the sea pens, sea pansies, sea fans and sea whips.  All species of Alcyonaria are colonial and possess a horny skeleton consisting of proteinaceous material.  There is a relatively simple body construction of eight pinnate tentacles and eight mesenteries.  The polyp organization is simple, but elaborate colonies are formed.

 

          The Subclass Zooantharia includes the sea anemones and stony corals.  Their structure is more complicated than that of the Alcyonaria.  Sea anemones are solitary while corals are mostly colonial.  There are six pairs of septa as in the genus Metridium, and a general tendency towards bilateralism. 

 

          The stony corals have a calcareous skeleton.  They occur in the form of cups in which resides a polyp.  A partition of skeletal material has pushed into the body wall but does not penetrate the wall, and therefore it remains external and is secreted by the epidermis.

 

 

          The Genus Metridium represents the Zooantharia.  The species are from inshore waters where they may be found attached to any firm object underwater.  There is a cylindrical column with a basal disc and several rows of tentacles.  The tentacles are hollow and not as long as in the other classes of Cnidaria.  The body wall consists of an endodermis, ectodermis and cellular mesoglea.

 

          Food and Digestion.-- Any animal within range of the tentacles is attacked.  Small food particles are conducted down the tentacles toward the mouth in a regular stream, while larger food particles are passed into the extended mouth.  Digestion is the same as in other Cnidaria being extra- and intracellular.  Nematocysts occur in the gastric filaments, which sting the prey to death.

 

          Circulation.-- A highly ciliated siphonoglyph is a side chamber of the gullet that aids in the process of respiration by forcing a current of water down the gullet.  Its action also aids in keeping the food constantly stirred up in the gastrovascular cavity.

 

 

          There are six pairs of mesenteries, which are referred to as primary mesenteries, and two pairs of mesenteries that lie on the siphonoglyph and opposite to it, which are called directives.  Secondary mesenteries alternate between the primaries.  Tertiary and quaternary mesenteries may also be present.  All the mesenteries except for the primaries have free edges.  A mesentery filament is present, which is a vertical tube that joins the free ends of the mesenteries.

 

 

 

          Excretion.-- The main nitrogenous waste is ammonia and this is released by simple diffusion.

 

          Muscles.-- These are usually the epithelio-muscular cells.

 

          Movement.-- Sea anemones have very slow, creeping movement by use of muscles inside the basal disc, while other members of the group such as the corals are stationary.

 

          Sensitivity.-- There are no sense organs present in the polyp stage, which is the only stage present in the Anthozoa (only medusae have sense organs).  However, the polyp tentacles and oral disc are most sensitive but do not constitute organs.

 

          Reproduction.-- Both sexual and asexual reproduction are present.  In sexual reproduction there are separate sexes (dioecious).  Gonads are borne primarily on secondary mesenteries that are located in back of the free edge.  Gametes are shed into the gastrovascular cavity and sperm leaves the body to enter that of another polyp.  Fertilization takes place in the gastrovascular cavity where the eggs develop into the planula larva, which is hollow and which swims out and eventually settles on the ground.  The polyp reproduces sexually again and therefore there is no alternation of generations.

 

          Asexual reproduction involves pedal laceration, where pieces of the basal disc may break off and leave young polyps behind it.  However, continuous asexual reproduction results in abnormal individuals.

 

 

          In some cases the more normal type of budding results in the formation of large colonies.

 

          Economic Importance.-- There is little or no direct economic importance.  However, indirectly corals form islands and their presence allows for human habitation.  The horny skeletons of red corals have been used as jewelry.  Hydra have been used with some success to combat mosquitoes (See  Mosquito Suppression With Hydra).

 

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Review of Cnidaria Characteristics

 

          The origin of the Cnidaria is obscure, but they are probably derived from a colonial flagellate with a solid colony that had a planula larva.  Their Protista features are a final digestive phase being intracellular and the absence of specialized excretory or respiratory structures.  Their more advanced features include well-organized tissues with specialization within the tissues (the epidermis is more highly specialized than that of the Porifera).  The digestive cavity is the initial site of digestion.  There are the beginnings of structures that warrant the name of organ.  Muscle cells are present in the form of epithelio-muscular cells as well as distinct muscle cells.  Polymorphism and Alternation of Generations occur.  Polymorphism is a mechanism by which a very simple animal can obtain some division of labor without the need for highly specialized tissues.  After polymorphism came into existence, the alternation of generation probably arose.  For example, the polyp represents the original larval stage, which has become more and more prominent, and the medusa because of its sense organs suggests that this was the original adult form.

 

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Please see following plates for Example Structures of the Cnidaria

 

Plate 16 = Animalia: Cnidaria: Scyphozoa: Aurelia sp.

Plate 17 = Animalia: Cnidaria: Scyphozoa: Scyanea sp. & Aurelia sp.

Plate 18 = Cnidaria: Anthozoa: Zooantharia: Metridium sp. (Figs. A & B)

Plate 19 = Cnidaria: Anthozoa: Zooantharia: Metridium sp. (Fig. C)

 

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Bibliography

 

 

           Introduction                                                                                                                                                                                                                        Platyhelminthes