Classification of Animals
Classification of Animals Synopsis
Synopsis
Basis of Classification
- The animal kingdom includes all metazoans.
- Metazoans are multicellular animals with holozoic nutrition.
- All the animals have certain features in common which form the basis of animal classification.
- These different features are
Phylum Porifera
- Phylum Porifera includes pore-bearing animals.
- They are also called sponges.
- They are marine, diploblastic, either radially symmetrical or asymmetrical animals.
- They show cellular level of organisation.
- Body has a number pores called ostia.
- The central cavity called spongocoel open outside through the osculum.
- In the canal system, the water current enters through ostia, passes through the spongocoel and exits the body through the osculum.
- Choanocytes or collar cells line the spongocoel.
- The body skeleton is made up of spicules or sponging fibres.
- Sponges are hermaphrodites.
- They reproduce asexually by fragmentation and sexually by the formation of gametes.
- Fertilisation is internal and the development in indirect.
- Examples: Sycon, Euspongia
Phylum Cnidaria
- The old name of this phylum is Coelenterata.
- Aquatic mostly marine. Only hydra is a freshwater form.
- They are sessile or free swimming.
- They are radially symmetrical and diploblastic.
- They show the tissue level of organisation.
- There is a gastro-vascularcavity with a single opening called hypostome.
- Digestion is extracellular and intracellular.
- Cnidoblasts are the stinging cells of cnidarians found around the mouth and on the tentacles. They are used for the defense and catching prey. The name ‘Cnidaria’ is derived from the cnidoblasts.
- Cnidarians show two body forms, a polyp and a medusa. Cnidarians exhibiting both these body forms show alternation of generation.
- Polyps reproduce medusa asexually, and medusa produce polyps sexually.
- Examples: Hydra, Physalia
Phylum Ctenophora
- These animals were earlier placed in the phylum Cnidaria but later separated in different phylum because of lack of cnidoblasts.
- They are commonly known as seawalnuts or comb jellies.
- They are exclusively marine,radially symmetrical and diploblastic animals.
- They show tissue level organisation.
- Ctenophores bear cilia which are arranged in eight external rows called comb plates which help in locomotion.
- Digestion is both extracellular and intracellular.
- Bioluminescence is commonly seen in ctenophores. It is the property of living organisms to emit light.
- Examples: Pleurobranchia, Beroe
Phylum Platyhelminthes
- They are also called flatworms because of their dorsoventrally flattened body.
- Flatworms are endoparasites.
- They are bilaterally symmetrical, triploblastic and acoelomates.
- They show organ level of organisation.
- In parasitic forms, there are organs of adhesion such as hooks, suckers and spines. Some also absorb nutrients from the host by their body surface.
- They show the presence of flame cells which help in excretion and osmoregulation.
- Fertilisation is internal and the life cycle show many larval stages.
- Examples: Liver fluke, Taenia
Phylum Aschelminthes
- Phylum Aschelminthes is also known as Phylum Nematoda or Nemathelminthes.
- Their body in the cross-section looks round and hence, they are also known as round worms.
- They are free-living or parasitic, aquatic or terrestrial.
- They show organ level of body organisation.
- They are bilaterally symmetrical, triploblastic, pseudocoelomate animals.
- Alimentary canal is complete and a muscular pharynx is present.
- Sexes are separate. The male is smaller than the female, and its posterior end is curved.
- Fertilisation is internal. Development may be direct or indirect.
- Examples: Ascaris, Wuchereria
Phylum Annelida
- Animals of this phylum have a body made of ring-like segments, and hence, the phylum is named Annelida. ‘Annulus’ is a Latin word which means ‘little ring’.
- They may be terrestrial or aquatic. They are free-living but some are parasitic.
- They show organ level of body organisation.
- They are bilaterally symmetrical, triploblastic, metamerically segmented, coelomate animals.
- Nephridia act as excretory organs.
- The nervous system shows the presence of paired ganglia connected by the lateral nerves to a double ventral nerve cord.
- Some animals are hermaphrodites and some are unisexual, i.e. sexes are separate.
- Reproduction is sexual.
- Examples: Earthworm, Nereis
Phylum Arthropoda
- Phylum Arthropoda is the largest phylum of Kingdom Animalia.
- They can be terrestrial (found on land, in the soil) or aquatic (either marine or freshwater).
- They show organ level of organisation.
- They are bilaterally symmetrical, triploblastic, segmented and coelomate animals.
- A thick, tough exoskeleton covers the body which is formed chitin.
- The body of arthropods is divided into the head, thorax and abdomen. In crustaceans, the head and thorax fuse to form the cephalothorax.
- Arthropods have jointed legs.
- Respiration is by trachea or book lungs in terrestrial forms. In aquatic forms, it is by gills or book gills.
- They have an open type circulatory system.
- They have well developed sense organs such as antennae and compound eyes.
- Excretion is by malpighian tubules.
- Sexes are separate.
- Fertilisation is external in some aquatic forms and internal in terrestrial forms.
- They are oviparous. Development may be direct or indirect.
- Examples: Crab, Cockroach
Phylum Mollusca
- Phylum Mollusca is the second largest animal phylum.
- Molluscs are marine animals. Some are also found in freshwater or damp soil.
- They show tissue level of organisation.
- They are bilaterally symmetrical, triploblastic, unsegmented and coelomate animals.
- Body is divided into a distinct head, muscular foot and visceral hump.
- The visceral hump is further covered by the mantle.
- The cavity between the hump and the mantle is called the mantle cavity.
- Respiration is by ctenidia or gills present in the mantle cavity.
- The anterior region of the body i.e. head possesses tentacles.
- The mouth contains a rasping organ which is a tongue-like structure called radula.
- Sexes are separate (dioecious).
- They are oviparous and the development is indirect.
- Examples: Octopus, Oyster
Phylum Echinodermata
- They are called echinoderms because they bear spines on their body.
- They are marine.
- They show the organ level of body organisation.
- Adults are radially symmetrical. Larvae are bilaterally symmetrical.
- They are triploblastic and coelomates.
- Digestive tract is complete.
- The water vascular system is a unique characteristic of echinoderms. It helps in locomotion, capture and transport of food.
- Excretory system is absent.
- Sexes are separate.
- Fertilisation is external. Development is indirect, with a free-swimming bilateral larval stage.
- Examples: Star fish, Sea urchin.
Phylum Hemichordata
- Phylum Hemichordata was considered a sub-phylum under Chordata.
- Recently, it was placed under non-Chordata as a separate phylum.
- They are marine, bilaterally symmetrical and triploblastic animals.
- They show organ level of organisation.
- Body is composed of anterior proboscis, a collar and a trunk.
- The circulatory system is of open type.
- Respiration is by gills.
- Proboscis gland is the excretory organ.
- Sexes are separate.
- Fertilisation is external and development is indirect.
- Example: Balanoglossus
Phylum Chordata
- Animals which belong to this phylum have three basic characteristics:
- They are bilaterally symmetrical, triploblastic, coelomate animals.
- They show organ level of body organisation.
- The circulatory system is of closed type.
Comparison between Non-chordates and Chordates
Classification of Sub-phylum Vertebrata
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