Classification of the Phylum Porifera

Introduction

Sponges are regarded as most primitive metazoans with lower grade of body organizations. Body of sponges is made of cells, no tissues or organs found in their body. They are sedentary animals and look like plants. They can be differentiated from the protozoans in having cells and skeleton in the forms of spicules.

The described scheme of classification of the phylum Porifera follows scheme by Parker and Haswell (1964) and edited by Marshall and Williams (1974).

Definition of Porifera

A group of metazoan animal whose bodies are without any symmetry or radially symmetrical, without mouth and nervous system and whose bodies are provided with many pores, canals and chonaocyte cells, are recognized as Porifera or sponges.

Body skeleton, i.e., spicules and sponging fibres are the basis of the classification of Porifera.

General Characters

1. All sponges are aquatic, marine or freshwater; remain attached to some submerged substratum.

2. Body flower-vase like or tubular, with radial symmetry or without any symmetry.3. Multicullular body is provided with many pores or Ostia.4. The space in between ectoderm and endoderm is filled with mesenchyme, i.e., the animal

is diploblastic.5. Body is having canal system.6. Flagellated cells or choanocytes line the radial canal.7. Internal skeleton is made up of spongin fibre, siliceous or calcareous spicules.8. Reproduction sexual or asexual, asexual reproduction takes place by the formation of

gemmules.

Scheme of classification

Phylum – Porifera

Class                                       Subclass

I. Calcarea                                           (i) Calcaronea

(ii) Calcinea

II. Hexactinellida                           (i) Hexasterophora

(ii) Amphidiscophora

III. Demospongia                           (i) Tetractinomorpha

(ii) Ceractinomorpha

Classification with reasons and examples

Class I- Calcarea

Characters:-

1. All are marine, single in live in colony.2. Endoskeleton is composed of spicules made of calcium carbonate.3. Choanocyte cells are large.4. Osculum small surrounded by spicules.

Class Calcarea has been divided into two Sub Classes:

Sub Class (i) Calcaronea

Characters:-

1. Nucleus of the choanocyte lies at the base of the collar.2. Flagellum arises from the basal granule attached with the nucleus.3. Spicules triaxon or 3-rayed, one ray is larger.

Examples- Sycon (= Scypha), Leucosolenia

Sub Class (ii) Calcinea

Characters:-

1. Nucleus of the chonocyte lies at the base of the collar.2. Flagellum does not originate from the nucleus.3. Spicules tri-rayed, all rays are equal.

Examples- Clathrina, Petrobiona

Class II. Hexactinellida

Characters:-

1. Flowre-vase like body.2. Spicules are composed of silica.3. Choanocytes are restricted to flagellated chamber.

Class II. Hexactinellida has been divided into two Sub classes:

Sub Class (i) Hexasterophora

Characters:-

1. Spicules are smaller, triaxon and sixrayed.2. Spicules with pointed end.

Example: Euplectella

Sub Class (ii) Amphidiscophora

Characters:-

1. Spicules comparatively large.2. Both end of the spicule is provided with disc like plates.

Example: Hyalonema

Class III. Demospongia

Characters:-

1. Endoskeleton may be formed by siliceous spicules or sponging fibres or both.2. Choanaocytes are restricted to spherical chambers.3. Spicules are having one to four rays.

Class Demospongia has been divided into two Sub Classes as:

Sub Class (i) Tetractinomorpha

Characters:-

1. Spicules tetraxon types.2. No spongin fibre, only siliceous spicule present.3. Radially symmetrical body.

Examples: Cliona, Poterion

Sub Class (ii) Ceractinomorpha

1. Endoskeleton made of spongin fibres only.2. Siliceous spicules when present  monaxon type with one ray only.

Examples: Spongilla, Helichondra

Hartman and Goreau (1970) proposed addition of one more Class Sclerospongiae based on their researches on Jamaican sponges (Barnes- 1980). The characters of the Class are-

1. Endoskeleton made of siliceous spicules and spongin fibres.2. Exoskeleton made of calcareous spicules.3. Star shaped canals system.4. Many Oscula.

Classification of the Phylum Cnidaria

There are differences between single celled animals the Protozoa and multicellular animals- the Metazoa. Typical metazoan characteristics are first found in Cnidaria, tissue grades of construction first appeared in Cnidaria. From taxonomic point of view members of the Phylum Cnidaria are regarded as first metazoan animal.

Hyman (1940) presented the classification of Coelenterata. Recently the name Cnidaria in place of Coelenterata is used because Ctenophora has been separated from it and given the status of a Phylum.

Definition

A group of diploblastic animals with radial symmetry, without organ system and anus and having single tubular gastrovascular cavity and with tentacles and nematocysts is recognized as Cnidaria.

General Characters of Cnidaria

1. Cnidarians are multicellular animals and first metazoan with tissue grade of construction.2. Most of the Cnidarians are marine, though some are fresh water forms.3. Body radially symmetrically, some are biradial.4. A single coelenteron or gastrovascular cavity is present which opens through mouth, anus

is absent.5. Small and narrow tentacles surround the mouth in circlet. Tentacles are used to procure

food and for defence.6. Body is covered by epidermis, gastrovascular cavity is lined by endodermis and in

between the two there is a layer of non cellular matrix known as mesoglea.7. Body diploblastic and so coelom is present.8. The colonies show polymorphism with polyp and medusa as basic Zooids. Polyp

represent asexual form and medusa represents sexual zooid.9. A special type stinging cells known as nematocysts are present. These cells help in

defence and food capture.10. Ciliated planula larva is found.11. In the life cycle alternation of generation and metagenesis are found.

Classification of Phylum Cnidaria with reasons and examples

The basis of classification is the adult forms whether polyp or medusa.

Phylum Cnidaria has been classified into three Classes-

Class I: Hydrozoa

Class II: Scyphozoa

Class III: Anthozoa

Class I- Hydrozoa

Characters:-

1. Fresh water members of this class live singly, but the marine forms live singly or form polymorphic colony.

2. Though only polyp or medusa is fund but sometimes both polyp or medusa may be fond in the same animal.

3. Gastrovascular cavity is directly connected with mouth.4. Alternation of generation is found in the life cycle.5. Ciliated planula larva is found.6. No stomodeum, velum present in medusa.7. Radial canals in medusa are not branched.8. Gametes develop from ectoderm.

Class II- Scyphozoa

Characters:-

1. Larger medusa, free swimming, all are marine.2. Polyp absent or reduced.3. Gastrovascular cavity branched.4. Well developed nematocyst very active.5. Gastric tentacles and gametes developed from endodermis.6. Mesoglea thick, fibrous and contains amoebocytes.7. Sense organs as tentaculocyst.8. Many tentacles hang from the velum.

Class III- Anthozoa

Characters:-

1.      Well developed stomodeum.

2.      Gastrovascular cavity subdivided by complete or incomplete septum.

3.      Mesenteries carry nematocyst and gonad.

4.      Mesoglea thick and cellular.

This class has two subclasses as:

Sub Class I – Octocorallia

Characters:-

1. Colonial marine forms.2. Polyp with 8 hollow, pinnate tentacles.3. 8 complete unpaired mesenteries.4. Polyps are dimorphic.

Sub Class II – Hexacorallia

Characters :-

1. Solitary or colonial marine forms.2. Tentacles are mesenteries are in multiple of six.3. Tentacles are simple, unbranched, hollow and conical.4. Polyps are usually monomorphic.

Classification of the phylum Platyhelminthes

The phylum Platyhelminthes include free-living as well as parasitic animals. The free-living flatworms live in springs, sands, streams or in bodies of salt water. On the other hand, parasitic form of each species has its own particular habitat. They are widely distributed among human beings and other vertebrates.

Definition: A group of flat or ribbon like animals having bilateral symmetry, without any segmentation, coelom, blood vascular system and anus and whose internal organs are surrounded by parenchyma tissues, are known as platyhelminthes.

Classification: The classification system described here follows the system which as appeared in Invertebrate Zoology Vol. I, by Parker and Haswell and edited by Marshall and Willim (1974).

General Characters of Phylum- Platyhelminthes

i)                    Bilaterally symmentrical body, dorsoventrally flattened, triploblastic, and without any body skeleton.

ii)                   Unsegmented body, laceolet or ribbon like, body covered with cilia or cuticle.

iii)                 Anterior end with a head and sucker.

iv)                 With the sucker and hooks they remain anchored to some organs.

v)                  Coelom absent but the interior of the body is filled up with parenchyma derived from mesenchyme.

vi)                 Alimentary canal either imcomplete or branched, anus absent.

vii)               Definite respiratory and blood vascular system absent.

viii)              Excretory canal with flame cells.

ix)                 Reproduction asexual and sexual.

x)                  Hermaphrodite and life cycle is complicated with sexual larval stages.

Phylum Platyhelminthes has been divided into 5 classes namely-

Ø      Class I- Turbellaria

Ø      Class II- Digenia

Ø      Class III- Aspidogastrea

Ø      Class IV- Monogenea

Ø      Class V- Cestoda

Classification with reasons and examples

Ø                  Class I- Turbellaria

Characters:

i) Most members are free-living, some are symbiotic, some are commensals, some are parsites.

ii)  Free-living forms may be marine, fresh water or terrestrial in habit.

iii) Unsegmented body without any sucker.

iv)  Ectodermal cilia mucous and rhabdite may be present. Mucous glands may be found in mesenchyme.

v) Mouth is anterior and ventral anus absent.

vi) Intestine may or may not be present.

Class Turbellaria has two subclasses: as-

Subclass 1. Archopora

Characters:

i.      All are marine.

ii.      Ovary divided.

Example- Convulata

Subclass 2. Neophora

Characters:

i.      Most of them are aquatic, some are terrestrial.

ii.      Ovary undivided.

Examples- Planaria notoplana

Ø                  Class II- Digenea:

Characters:

i.      Leaf-like or lanceolet body form.

ii.      To complete the life cycle two hosts are required, all are endoparasites.

iii.      Two suckers, one near the mouth and other posterior to it.

iv.      Bipartite intestine.

v.      Many larval forms in the life history.

vi.      Cilia in the ectoderm absent.

This class has no sub-class.

Examples: Fasciola hepatica, F. buski

Ø                  Class III- Aspidogastrea:

Characters:

i.      All are small and endoparasites.

ii.      Mouth funnel-like without sucker.

iii.      Intestine undivided.

iv.      Sucker present on the ventral side.

This class has no sub class.

Examples: Aspidogaster, Conchiola

Ø                  Class IV- Monogenea:

Characters:

i.      Live as ectoparasite on the skin or gill of fishes.

ii.      Adult without cilia and rhabdite.

iii.      Pharynx and divided intestine present, except Gyrocotyle.

iv.      Mouth may be surrounded by sucker, sucker may be absent in many cases, posteroventral sucker present.

v.      A pair of anterolateral excretory pore present.

vi.      Free living and free swimming larvae possess 10-16 lateral hooks, life cycle simple.

This class has no sub class.

Examples: Entobdella, Polystome etc.

Class V- Cestoda :

Characters:

i.      Elongated, flat, ribbon-like segmented body consisting of many segments, each segment is known as proglottid.

ii.      The head is like the pinhead and known as scolex.

iii.      Scolex with rostellum having hooks ad suckers.

iv.      Digestive system absent.

v.      Body is covered by a cuticle, sense organs absent.

vi.      They are endoparasites of vertebrate.

vii.      Hermaphrodite, life cycle complicated.

This class has been divided into 2 sub classes as-

Sub class I: Cestodaria

Characters:

i.      Small body, with a single segment, scolex absent.

ii.      Sucker presents at the anterior side.

iii.      Larva contains 10 hooks.

iv.      Parasite of fishes.

Example- Amphilina

Sub Class II: Eucestoda

Characters:

i.      Ribbon like body with many proglottids.

ii.      Scolex with 4 suckers and many hooks.

iii.      Larva hexacanth, i.e., with six hooks.

Examples- Taenia, Echinococcus etc.

Classification of the Phylum Annelida

The term annelida has been coined from either Greek word annulus meaning small rings of French word annular meaning arranged like small single rings. They are the first coelomate animals and as regarded as metamerically segmented animals.

Definition: The animal group who are triploblastic, coelomate and whose bodies are metamerically segmented  and whose bodies are covered with soft cuticle is known as annelida.

The scheme of classification of the phylum annelida described here follows the same as appeared in the “Invertebrate Zoology”, Vol. I, written by Parker and Haswell and edited by Marshall and William (1974).

General characters of the phylum Annelida:

1. Body  is metamerically segmented, each segment is known as annulus, generally external segment correspond with internal segmentation, annuli are arranged in a linear sequence from anterior to posterior direction.

2. Cuticle is soft and non chitinous.3. Body cavity is a true coelom, coelom is filled up with a fluid known as coelomic fluid.

Coelom is divided by septum into many chambers.4. Body bilaterally symmentrical and triploblastic.5. In most annelids sea or cheaeta and parapodia are locomotory are locomotory organs,

some move by muscles, i.e., seta or parapodia absent.6. Alimentary canal more or less straight, anus and mouth at opposite ends, digestion

extracellular.7. Well developed blood vascular system, blood vascular system closed.8. Excretion by nephridia, each segment bears a pair of nephridium.9. Most of them respire by body wall, some possess gills.10. Well developed nervous system with ganglia and nerve cords. A pair of supra

oesophageal ganglia, circum oesophageal connective and ventral nerve cord constitute the nervous system. Ventral nerve cord gives off ganglia in each segment.

11. Most of them are hermaphrodite, some are unisexual, development direct or through larval stage, larva is known as trochophore larva.

Classification with reasons and examples:

Parker and Haswell classified phylum annelida into 4 classes as-

Ø     Class-I : Polychaeta

Ø     Class-II:  Oligochaeta

Ø     Class-III: Hirudinea

Ø     Class- IV: Archiannelida

Class-I : Polychaeta

Characters:

1. Most of them are marine, 5-10 cm. in length and variously coloured.2. External segmentation and internal septum distinct.3. Each segment bears a lateral fleshy outgrowth help in swimming. These appendages are

known as parapodia and parapodium bears many setae.4. Body is with a distinct head, head with eyes, pulp and tentacles.5. Clitellum is absent.6. Generally unisexual, fertilization external.7. Development through trochophore larva.

The Class Polychaeta has been divided in to two subclasses.

Sub Class I- Errantia

Characters:

1. Body is divided into many segments, except head and anal segment. All segments are equal.

2. Parapodium is with an endoskeleton known as aciculum.3. Presence of mandible with teeth.4. Most of them are free swimmers, some may live in the tube.

Examples- Nereis, Aphrodite, Polynoe, Glycera, Syllus.

Sub Class I- Sedentaria

Characters:

1. Sedentary, tubicolous worms, living in calcareous or chitinous tubes secreted by epidermal glands.

2. Head poorly developed with tentacles, pulps and feeding appendages.3. Trunk segments differentiated into or three regions due to dissimilarity in the forms of

segments parapodia and setae.4. Pharynx not protrusible and devoid of jaws and teeth.

Examples- Chaetopterus, Arenicola, Amphitrite, Terebella, Sabella etc.

Class-II: Oligochaeta

Characters:

1. Either freshwater or terrestrial forms.2. Head indistinct and without head appendages.3. Parapodia and bristles absent and setae remain embedded in the skin.4. Clitellum present.5. Sexes united, gonads few, permanent, development direct

Examples- Tubifex, Dero, Pheretima, Megascolex etc.

Class-III : Hirudinea

Characters:

1. Mostly aquatic forms, either fresh water or marine, but a few may be terrestrial.2. Body dorsoventrally flattened, divides into definite number of segments which may be

divided externally into segments.3. Head distinct, without head appendages, may bear eyes.4. Parapodia, Setae, tentacles absent.5. Usually with two sucker, posterior one large and used for adhesion anterior one suctorial.6. Coelom filled with vacuolated parenchyma and botryoidal tissue.7. Blood vascular system haemocoelomic type.8. Fertilization internal, development direct, and are hermaphrodite.

Examples- Acanthobdella, Pontobdella, Hirudo.

Class-IV : Archiannelida

Characters:

1. Body simple, elongated vermiform and threadlike.2. Setae, parapodia, cirri absent.3. External segmentation indistinct, internal septum present.4. Hermaphrodite, gonads develop during reproductive season only.

Examples- Polygordius, Protodrilus etc.