Classification of Multicellular Animals, Biology tutorial

Metazoan Branches: Mesozoa, Parazoa and Eumatazoa:

Kingdom Animalia includes all the multicellular animals (metazoans). These are split in three branches: 1) Mesozoa: 2) Parazoa and 3) Eumetazoa. Mesozoa include a small single phylum (Phylum Mesozoa): Parazoa include two phyla: a small single phylum Placozoa and rest frame phylum Porifera including sponges. Many metazoans come under major branch Eumetazoa. Cellular layers of Porifera (sponges) are not homologous to germ layers of Eumetazoa: Their developmental pattern is also rather different from that of Eumetazoa. Parazoa are considered as early evolutionary side branch of animal kingdom, that didn't give rise to any other group of animals.

Parazoa - Phylum Porifera - Sponges:

Parazoa include two phyla: tiny phylum. Placozoa and Phylum Porifera including the sponges.


As the name of group indicates, surface of body presents the large number of minute pores known as ostia (sing. ostium). Through the ostia, water current enters body cavity. These pores really lead in system of channels which, after permeating almost whole body, open to exterior by one or more large exhalent openings known as oscula (sing. osculum). This system of spaces connecting inhalent pores or ostia with exhalent oscula, is canal system. Sponge body is covered externally by the epithelial layer known as Pinacoderm. This layer is composed of flattened cells known as pinacocytes. Smaller food particles are engulfed by choanocytes while larger particles are passed on to archaeocytes or amoebocytes which are principal sites of digestion. Between epithelial layer and choanocyte layer, is a mesohyal layer consisting gelatinous protein matrix that has amoeboid cells (archaeocytes) and skeletal elements.

Classification of Phylum porifera:

Phylum porifera is split in four classes: Calcarea, Hexactinellida, Demospongiae and Sclerospongiae

1) Class Calcarea (Calcispongiae): These are characterized by presence of spicules of calcium carbonate. Spicules are needle shaped, or three- or four rayed. All these sponges are small, marine, mainly less than 111 cm in height. These may be asconoid, syconoid or leuconoid. Examples Leucosolenia

2) Class Hexactinellida (Hyalo-spongiae): These are usually called as glass sponges. Skeleton is composed of six-rayed siliceous spicules. Mainly deep sea sponges, radially symmetrical, having vase - or funnel - like bodies joined by stalks to substratum. Examples: Euplectella (Venus's flowerbasket). Mostly syconoid, 10-3ft cm. No pinococytes, but outer layer is composed of netlike syncitium derived from interconnecting pseudopodia of amoebocytes.

3) Class Demospongiae: This comprises largest class, with more than 90% of all sponge species. Skeleton is composed of spicules of spongin or which are siliceous, or both; spicules not six-rayed. Bath sponges belonging to family Spongidae e.g. Spongia have skeleton of spongin only.

4) Class Sclerospongiae: These comprise of very small group of sponges that secrete massive calcareous skeleton. Compound skeleton of siliceous spicules and spongin fibres is also frequently present. Living tissue is restricted to thin superficial layer on skeleton. All are leuconoid. They dwell hidden in crevices, caves and tunnels among coral reefs like Merlia.

Phylum Cnidaria:

Phylum Cndaria, along with phylum Ctenophora, together comprising Radiata, is sometimes referred to as coelenterates. They comprise more than 9000 living species, all aquatic, mainly marine but also some fresh-water forms. Their fossils date back to over 700 million years. They are successful group of animals, though of great structural and functional simplicity among metazoans. They comprise hydroids, jelly fishes, sea anemones and corals etc.


1. All are aquatic animals.

2. Radial or biradial symmetry around oral-aboral axis, but no head.

3. Diploblastic with the epidermis and gastrodermis, and less cellular or noncellular, gelatinous mesoglea in between.

4. A gastrovascular cavity with the single opening, mouth; tentacles encircling oral region.

5. No coelom, or separate excretory or respiratory system.

6. Nerve net, for diffuse conduction only, present. Few sensory organs also take place.

7. No distinct muscle tissue, but epithelio - muscular system is there.

Body wall:

The body wall of cnidarian comprises of three layers, outer layer called as epidermis, a middle layer known as mesoglea, and inner layer referred to as gastrodermis. Epidermis has collection of different types of cells. These comprise epitheliomuscular cells which contract and allow movement, interstitial cells which give rise to several other cell types like egg and sperm, cnidocytes that are specialized cells unique to cnidarians that in some cnidarians have stinging structures, mucus-secreting cells which glandular cells which secrete mucus, and receptor and nerve cells that gather and pass on sensory information.

Categorization of Phylum Cnidaria:

Depending mostly on whether polyp or medusa is dominant form in life cycle, Cnidaria are divided in four classes (i) Hydrozoa; (ii) Scyphozoa; (iii) Cubozoa and (iv) Anthozoa.

1) Class Hydrozoa. They may be lone or colonial forms. There are asexual polyps and sexual medusas, although one kind may be suppressed. Feeding zooids (hydranths) don't have mesenteries. Hydra is a common fresh water form, but atypical, being lonely and without the medusoid form, Obelia is a colonial form. It has feeding polyps with tentacles (gastrozooids or trophozooids), and reproductive zooids without tentacles, that bud off medusae (gonozooids). Then the gonozooids are called gonophores.

2) Class Scyphozoa. These are solitary cnidarians, with polyp stage decrease absent. Medusae don't have velum. Gelatinous mesoglea is extremely much enlarged. Margin of umbrella has generally eight notches with sense organs. All are marine. Aurelia is typical example. Medusa is dominant individual, 10-30 cn diameter. Zygote grows in planula larva that attaches to substratum and develop polypoid scyphistoma.

3) Class Cubozoa. These are also solitary medusoid forms with reduced polyp stage. But the medusa is square in cross section. Tentacles or groups of Animal tentacles hang from tough flattened pedalia which occur at the corners of the umbrella. All are marine. Example Carybdea

4) Class Anthozoa. All are polyps and marine. They are lonely or colonial. Mesenteries or septa subdivide enteron. Septa bear nematocysts. Gonads are gastrodermal. Anthozoan polyps are larger and more complex. Flattened oral disc bears tentacles and slit like mouth. Mouth leads into flattened stomodaeum. A ciliated groove called syphonoglyph extends from the mouth into the stomodaeum along one or both the edges.

Coral Reefs:

The coral reefs are built mostly of stony corals (scleractinians or madreporarians). Examples are Fungia, Diploria (brain coral), Agaricia (lettuce coral), Montastrea, Favia, Porites etc (over eighty genera). But several other organisms are also comprised in coral-reef formation.

Certain fast growing calcareous red and green algae saturated with lime grow on coral colonies and contribute significantly to formation of reefs. Foraminifera shells, calcareous sponges, alcyonarians, gorgonians, Millepora, Tubipora and Heliliopora etc. also form main part of such Coral reef is very productive ecosystem, and comprises of large formations stone in shallow sea. Reefs of open seas are washed by waves having relatively poor nutrients.

Kinds of Coral Reefs:

Fringing or Shore Reef: This is the platform of reef skirting shore seawds rather suddenly front which there is a steep slope down. There may not be the lagoon or only narrow lagoon between and shore.

Barrier Reef: Barrier reef runs parallel to shore and resembles the fringing reef except that it is divided from shore by the channel, which is frequently of great width and depth.

Atolls: Atoll comprises of ring shaped reef, enclosing an open lagoon but not an island.

Phylum Ctenophora:

These are commonly known as Comb jellies, sea walnuts or sea gooseberries, and to about 100 species, all being marine. They derive their name from comb plates for locomotion.


i) All are medusoid. No polymorphism.

ii) Gastrovascular cavity comprises of mouth, pharynx, stomach, series of canal: anal pores, carnivorous.

iii) Mesoglea with amoebocytes and smooth muscle fibres.

iv) Lack of nematocysts. Instead, they contain adhesive cells (colloblasts or lass cells).

v) Locomotion through comb plates, that are comb-like fused ciliary plat, set radially in 8 meridional rows (combrows).

vi) Subepidermal nerve plexus concentrated under comb rows. The aboral organ, that is a statocyst, is there.

Categorization of Phylum ctenophora:

Phylum ctenophora is divided into 2 classes:

l. Class Tentaculata: These contain two tentacles. Example: Pleurobrachia Velamen. Body is so much laterally flattened that it appears to be transparent ribbon. Ctenoplana is flattened along oral-aboral axis.

2. Class Nuda: Lack of tentacles, with wide mouth and expanded stomodaeum. Somewhat conical; example Beroe.

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