Learning Objectives
After studying or reading this, you should be able to:
1) Outline the major phyla and classes of Kingdom Animalia.

2) Identify and describe the characteristics of the major phyla and classes of Kingdom Animalia.

There are about eight phyla under the kingdom Animalia. These are; Cridaria; platyhelminthes; (flatworms), Nematoda (roundworms) Annelida (segmented worms) Mollusca (mollusks) Arthropoda (arthropods); Echinodermato (echinoderms) and Chordate (chordates).

PHYLUM: Cnidaria

The body is sack-like, radially symmetrical, soft and jelly-like. The body wall consists of two cell layers (Diploblastic) an outer ectoderm and an inner endoderm. The two cells are separated by mesogloea, a non-cellular jelly.

The mesogloea surrounds a hollow which has only one opening, the mouth. A ring of tentacles bearing stinging cells called cridoblast surround the mouth. The tentacles are also used for feeding. There is absence of anus, excretory organs, brains and spinal cord. Two basic body types are recognized in the cridarians, these are the polyp and medusa. The polyp is cylindrical and sessile, whiles the medusa is umbrella shaped and free swimming or floating.  They show polymorphism. Examples: Hydra; Obelia; jelly fish; Sea anemone and Corals.
jelly fish

PHYLUM: Platyhelminthes (flatworms)
They are aquatic. They are dorso-ventrally flattened and leaf-like in shape. They are bilaterally symmetrical. They have thin and soft bodies. The body consists of three layers that are Tripoblastica that is, ectoderm, mesoderm and endoderm. They are unsegmented and have no cavity (acoelamate). They have a mouth but have no anus and vascular system. They have excretory organs called name cells. Most are hermaphrodites. They have a large surface area to volume ratio for gaseous exchange. Three main classes are recognized. These are: Trematoda; Turbellaria; and Cestoda.

CLASS: Trematoda (flukes)
   They are endoparasitic covered with a thick cuticle. They have ventral mouth and suckers for attachment to host. They have no cilia and no eyes eg. Fasciola hepatica (liver fluke), and schistosoma.
fasciola hepatica

CLASS: Turbellaria (turberiliarians)
    These are small flattened and free-living. Their elongated bodies are covered with cilia. They have a relatively broad anterior head with a pair of eyes on the dorsal surface and a posterior end that is clearly tapered. The mouth is ventrally placed towards the posterior end of the body. Suckers are rarely present. They are mainly carnivorous feeding on small worms; crustacean and in dead bodies of larger organisms. They are found mainly in the sea (marine), fresh water and in moist places on land. Examples include: Planaria; Dendrocoelum.

CLASS: Cestoda (tapeworms)
    They are endoparasites. They have elongated body divided into proglottides which are capable of breaking off. The proglottides is flattened and is segmented. The head (scolex) has suckers on it and sometimes hooks for attachment onto host. The proglottides (psendosegment) each has a complete reproductive organ. They have a thick resistant cuticle. They live in the alimentary canal of man and in pig's muscle. Examples include; Taenia Solium; Taenia taeniformis; Taenia seginatta and Taenia serrata.

PHYLUM: Nematoda (roundworms) They have elongated, cylindrical unsegmented body tapering at each end. They are triploblastic with no blood system like Platyhelminthes. The body has thick cuticle and is bilaterally symmetrical. There are also free-living forms as well as parasite, on human host. They are neither acoelomate nor coelomate. They have a gut with a mouth and anus. Sexes are separate.
Examples include; Onchoarca; Ascaris Ancylostoma (hookworm) and Wuchereria (elephantiasis).

PHYLUM: Annelida (segonented worms)
   They are aquatic or terrestrial. They are triploblastic and coelomate. The body is cylindrical with bilateral symmetry. They are segmented and pointed at both ends. They have chaetae which aid locomotion with the exception of leeches. They have a single preoral segment called Prostomium. It is a lip-like extension of the first segment situated above the mouth. They are divided into three classes which are: Polychaeta; Oligochaeta; and Hirudinea.

CLASS: Polychaeta (marine worms)
They are found in the sand of seashore; in tunnels and under stones. They have a head which bears jaws and tentacles. They have lateral projections on the body called parapodia which bears a tuft of chaetae. The head consists of an anterior prostomium, the first segment and posterior peristomium, the second segment. Examples include: Arenicola (lugworm); Nereis (rag worm).

CLASS: Oligochaeta (earthworms)
These include ' terrestrial and freshwater species. They have no distinct head and there is absence of parapodia. The anterior end of
the body is tapered while the posterior end is dorsoventrally flattened. The mouth and anus are at opposite ends of the body.
There is usually a distinct prostomium in front of the mouth but does not bear appendages. There is presence of a few chaetae per segment which are arranged singly. They are hermaphrodite. A reproductive structure called clitellum is ' dorsally and laterally swollen with gland cells that form a very noticeable saddle. The clitellum helps in the processes of copulation and cocoon formation.
Example include: Lumbricus terrestris (earthworm).

CLASS: Hirudinea
      These are ectoparasites with no distinct head,no parapodia and no chaetae. They have small " number of segments. Each segment is broken up externally in a number Of rings called annuli. The anterior and posterior ends of the body have suckers. They have no clitellum. Example; Hirudo

PHYLUM: Mollusca (mollusks)
      The phylum is the second largest in the animal kingdom. They are unsegmented, triploblastic coclomates. They are usually bilaterally symmetrical. The body is well organized into a well developed head, muscular foot for locomotion and a dorsal body mass called visceral hump. The body is soft and covered with a mantle cavity which secretes the shell. The shell can also be a handicap to locomotion, and hence the more active mollusks Show a reduction or loss of the shell. There are six classes in the phylum but the three most prominent ones are: _ Class: Gastropoda; Pelecypoda:Cephalopoda.

CLASS: Gastropoda and (gastropods)
    These are found in marine terrestrial and freshwater. The body is asymmetrical. It has a Shell which is in one piece, usually coiled or twisted due to rotation of hump during early growth. There is large flat foot that secretes slimy fluid to help in locomotion; It has well developed head with sensory tentacles and eyes. It has radula which is a tongue-likestructure enclosed with teeth which is used for feeding. The anus is anteriorly placed. Examples include: Helix sp. (land snail); Patella (limpet) Buccinum (whelk) and Limax [slug].

CLASS: Pelecypoda (bivalves) .
       The body is' laterally compressed. The body is bilaterally symmetrical. The 'shell consists of two halves called valves which are held together by a hinge. The foot is greatly reduced in size and often used for burrowing in sand or mud. The head is also reduced in size without tentacles. The . mantle contains two large flattened gills which help in filter feeding. They are mainly aquatic. Examples include: Ostrea (Oyster); Mussel; and Egeria.

CLASS: Cephalopoda (Cephalopods)
They show bilateral symmetry. They are mostly aquatic and probably the largest and most complex mollusks. There is presence of a highly developed head ,with complex eyes surrounded by many tentacles which are large.
The shell is greatly reduced and internal or absent. The mantle cavity communicates with the outside world by a siphon through which water can be expelled for faster locomotion. There is presence of suckers for seizing prey. They have radula. The anus is posterior placed. Examples include: Loligo (squid); octopus ‘vulgaris (octopus); Sepia (Cuttlefish)

PHYLUM: Echinodermata (echinoderms)
   They are mostly marine. The body has a five way radial symmetry. They are characterized by having spiny skin some also have wart-like outgrowths. They have tube feet with sucker for locomotion. They have no head and the mouth is generally on lower surface of body and anus on upper surface. They have calcareous exoskeleton.
Two classes are recognized in the phylum.
These are stelleroidea (starfish) and the class Echinoidea (sea Urchins).

CLASS: Stelleroidea (starfish)
   They are flattened dorsoventrally and are star shaped. They have live arms which radiate from the central disc. They have few calcareous plates in body wall, with movable spines.
Example: Asterias (starfish).

CLASS: Echinoidea (Sea urchins)
Body may be flattened with long spines or globular heart-shaped. It does not have arms. Numerous calcareous plates in body wall attached to each other to form a rigid structure relatively long and movable.
Example: Echinocardium (sea urchin).

PHYLUM: Chordata (Chordates) These are vertebrate animals which have at some stage in their life history or throughout their life time a notochord. It is a flexible rod of tightly packed, vacuolated cells held together with a firm sheath. During the cause of vertebrate evolution, the notochord became encaged in a bony or cartilaginous vertebrate which form the axis of the skeleton. They are bilaterally symmetrical. They have pharyngeal clefts which may develop into fins in fishes. They have tubular dorsal central nervous system.

They have a post-anal tail (tail starts posterior to anus). They also have segmented muscle blocks (myotomes) on either side of the body. Limbs are formed from more than one body segment.

There are six main classes in the phylum. These are: Chondrichthyes; Osteichthyes; Aves and Mammalia.

CLASS: Chondrichthyes (cartilaginous fish)
They have skeletons composed entirely of cartilage‘and almost all are marine. They have at least five pairs of gill slits, which are not covered by operculum. That is, they do not have gill covers. They have streamline body with lateral line system which is sensitive to pressure changes in the sea. They have paired, fleshy pectoral and pelvic fins. Sharks have tooth-like (placoid) scales that render the skin to be like sandpaper.The mouth is ventrally placed. It has a heterocercal tail (tail not equally‘ divided). This is effective in maintaining buoyancy since it has no air bladder and swim bladder, for that, fishes in this class are also referred to as Elasmobranches. They include: sharks; skates; rays. There is internal fertilization.

CLASS: Osteichthyes (bony lish) Bony fishes are also called teleosts. Their skeleton is mainly made of bones with patches of cartilage at a few places such as the nostrils. They have paired pectoral and pelvic fins supported by bony rays, giving greater manoeuvrability. The mouth is terminally placed. They have a symmetrical tail fin. The skin is covered with flat overlapping cycloid scales. The gills used for gaseous exchange are covered with operculum. There is external fertilization. There is presence of swim bladder.
Examples include: Tilapia, herrings, etc. 

CLASS: Amphibia (amphibians) .
      The amphibians are characterized by having a soft, moist skin which can be used for gaseous exchange. They have no scales covering the body, ‘(no feathers; and no hairs). They have no external ear but have a middle ear. They have two pairs of pentadactyl limbs. Visceral clefts are present in aquatic larvae (tadpole) only, but lungs in adults which live mainly on land. Adults return to water for reproduction and there is external fertilization. Eggs laid have no shells but are in membranes and jelly-like material. They have tiny teeth at the upper jaw for holding and not for chewing. They are prikilothermic.
Examples include: frog; toad; newts and salamanders.

CLASS: Reptilia (reptiles)
They have bony endoskeleton. They have a dry horny scale. covering the body. This resists dehydration during high temperatures on the terrestrial habitat. Fertilization is internal. The eggs are heavily yolked with calcareous shell. Gaseous exchange is by lungs and visceral clefts never develop into gills. They have two pairs of pentadactyl limbs but absent in snakes. They have no external ear. They have homodont dentition and possess doaca. Some are ovoviviparous [lay eggs in the body which hatch and give birth but the hatching does not depend on the body eg. Viper. Some are also oviparous (lay the eggs). Examples include crocodiles; snakes; tortoise; turtles; and alligators.

CLASS: Aves (birds)
Their skin is covered with feathers except on the legs which have scales on them. They have two pairs of pentadactyl limbs, with the front pair modified to form wings. They have hollow light bones (adaptation for flight). They have horny beak that bear no teeth. They have well developed cleidoic eggs with large yolk, They also possess four-chambered heart. Birds have lungs as :visceral clefts never develop gills. Birds are homoeothermic. Examples include: Vulture, Ostrich, Domestic fowl; parrot, etc.

CLASS: Mammalia (mammals) . The body is covered with ham fur. The skin/body has sweat gland and sebaceous gland. They have endoskeleton. They have two pairs of pentadactyl limbs. They have mammary glands and the young ones are suckled with milk secreted by mammary glands, They have heterodont dentition.
Fertilization and embryonic development is internal in the female reproduction; is mostly viviparous (a few Australian species lay eggs). Mammals have lungs for gaseous exchange and four chambered heart for blood circulation.
They have external ear in addition to middle and inner ears. They are homoeothermic with heterodont dentition. Examples include: man dog; cat; elephant; bat; etc.

PHYLUM: Arthropoda (arthropods)
      The arthropods have jointed appendages used for locomotion, feeding or sensory in function. They all have chitinous exoskeleton, (i.e. tough flexible cuticle). They show metameric segmentation and bilateral symmetry.

They are divided into live classes: which are Crustacean; Chilopoda; Diplopoda; Arachnida and lnsecta.

CLASS: Crustacea {Crustaceans}
     The crustaceans have two body divisions, Cephalothorax and abdomen. The head and thorax are fused together to form the Cephalothorax. The Cephalothorax is covered with a carapace. They have two pairs of antennae on the head. One pair of the antennae is larger than the other. They have five pairs of thoracic appendages which act as jaws and walking legs, modified also for swimming. They are mainly aquatic and breathe by means of gills, except terrestrial woodlice. Examples include; Daphnia (water flea); Astacus (Cray fish); Prawns; crabs; lobsters and woodlice.

CLASS: Diplopoda (millipedes)
They are mainly terrestrial found in damp dark places under rotten leaves; in the soil, etc. The body is divided into two that is, a distinct head and a slender segmented body. The head has a pair of short antennae but has no poisonous claws. The trunk (abdomen) has two pairs of walking legs per ' each segment (many). Gaseous exchange is by means of trachea or spiracle. Have cylindrical body with simple eyes. They are herbivorous. Example, lulus (millipede)

CLASS: Chilapoda (centipede)
    They are mainly terrestrial and carnivorous.  They are found at moist places. There are two body divisions a distinct head and a slender segmented trunk. The head has a pair of antennae and a pair of poisonous claws. It has many identical legs, one pair per segment. Gaseous. exchange is by tracheae. The body is torso-ventrally flattened, with simple eyes.
Example Lithobius (centipede).

CLASS: Arachnida (arachnids)
     They are mainly terrestrial. The body is divided into two; cephalothorax (prosoma) and abdomen (episthoma). The fused head and thorax (cephalothorax) is separated by a narrow waist-like constriction. They have no antennae. The cephalothorax bears four pairs of walking legs (limbs), a pair of pedipalps and simple eyes only with no compound eyes. They have no true mouth parts but one pair of appendages used in capturing prey (chelicerae). Gaseous exchange is by living hooks or gill hooks or tracheae.
Examples include: Scorpio (scorpion) Epiera (ureb-spinning spider) mites, ticks, etc.

three pairs of jointed legs, each pair on each of the thoracic segment. They have a Well developed compound and simple eyes on the head. They have usually one or two pairs of wings on the thorax. Gaseous exchange is by means of tracheal system, adult forms have no gills. Life cycles involves either complete or incomplete metamorphosis. Excretion is by means of malpighian tubules. Examples include: Periplaneta; (cockroach); Pieris (white butterfly), bugs, beetles, fleas, termites, grasshoppers, etc.

The class has several orders; eg. Odonata, Orthoptera, Coleoptera, Hymenoptera, Hemiptera, Diptera, isopteran and Lepidoptern.

Are you sure you have UNDERSTOOD what you have just read?

To make sure you have understood what you have just read, ANSWER the  Questions below:


1. a. Give the classes of prawn and centipede. b. For each of them, state any two observable characteristics of the class to which each belongs.

2. Give the class of millipede, centipede and shrimp and for each of them, state any two observable characteristics of the class to which they belong.

3. a. Give the class of the domestic fowl and a bony fish. b. State any two observable features on each of them to support your conclusion.

4. State four characteristics common to nematodes.

5. Give three reasons why an amphibian and a reptile are classified in the same phylum.

6. Write down two characteristics each of the following groups of organism and give one example of each.
i. Aves.  ii. Osteichthyes. iii. Mammal. iv. Amphibians.

7. a. Which of the animal phylum contains the most developed animals?
b. Give the six main classes of this phylum.

8. State five differences between ostaichltyes and chondrichtyes.

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