Animals can be categorized on the basis of their symmetry.
Animals in which the cells are arranged in two embryonic layers, external ectoderm, and internal endoderm, are called diploblastic animals.
An undifferentiated layer, mesoglea, is present in between the ectoderm and the endoderm.
Those animals in which the developing embryo has a third germinal layer, mesoderm, in between the ectoderm and endoderm, are called triploblastic animals
The presence or absence of a cavity between the body wall and the gut wall is very important in classification.
The body cavity, which is lined by mesoderm is called coelom. Animals possessing coelom are called coelomates
In some animals, the body cavity is not lined by mesoderm, instead, the mesoderm is present as scattered pouches in between the ectoderm and endoderm.
Such a body cavity is called pseudocoelom and the animals possessing them are called pseudocoelomates.
The animals in which the body cavity is absent are called acoelomates.
The notochord is a mesodermally derived rod-like structure formed on the dorsal side during embryonic development in some animals.
Members of this phylum are commonly known as sponges.
Examples: Sycon (Scypha), Spongilla (Fresh water sponge) and Euspongia (Bath sponge).
They are aquatic, mostly marine, sessile or free-swimming, radially symmetrical animals.
Examples: Physalia (Portuguese man-of-war), Adamsia (Sea anemone), Pennatula (Sea-pen), Gorgonia (Sea-fan) and Meandrina (Brain coral).
Ctenophores, commonly known as sea walnuts or comb jellies are exclusively marine, radially symmetrical, diploblastic organisms with tissue level of organisation.
Examples: Pleurobrachia and Ctenoplana.
They have dorso-ventrally flattened body, hence are called flatworms.
Examples: Taenia (Tapeworm), Fasciola (Liver fluke).
The body of the aschelminthes is circular in cross-section, hence, the name roundworms.
Examples : Ascaris (Roundworm), Wuchereria (Filaria worm), Ancylostoma (Hookworm).
They may be aquatic (marine and fresh water) or terrestrial; free-living, and sometimes parasitic.
Examples : Nereis, Pheretima (Earthworm) and Hirudinaria (Blood sucking leech).
This is the largest phylum of Animalia which includes insects.
Examples: Economically important insects – Apis (Honey bee), Bombyx (Silkworm), Laccifer (Lac insect) Vectors – Anopheles, Culex and Aedes (Mosquitoes) Gregarious pest – Locusta (Locust) Living fossil – Limulus (King crab).
This is the second largest animal phylum.
Examples: Pila (Apple snail), Pinctada (Pearl oyster), Sepia (Cuttlefish), Loligo (Squid), Octopus (Devil fish), Aplysia (Seahare), Dentalium (Tusk shell) and Chaetopleura (Chiton).
These animals have an endoskeleton of calcareous ossicles and, hence, the name Echinodermata (Spiny bodied).
Examples: Asterias (Star fish), Echinus (Sea urchin), Antedon (Sea lily), Cucumaria (Sea cucumber) and Ophiura (Brittle star).
Hemichordata was earlier considered as a sub-phylum under phylum Chordata.
Examples: Balanoglossus and Saccoglossus.