- They are multicellular
- Their cells are Euchariotic and lack cell wall
- They are non-photosynthetic and so feed Heterotrophically
- They have nerves and muscles except the sponges
- They are motile, that is they can move from place to place
Symmetry is the description of any object in relation to a point or line. All animals can be described in one or more ways in terms of symmetry. When an animal is moving, one part of the body travels first. This means an animal has
- An anterior end;
This is the body part that travels forward
- A posterior end;
This is the body part that comes after the anterior end.
- A ventral surface;
This is the under side of the animal, which is normally directed downward e.g. dog, cow, goat etc. in bipedal primate like human, it’s in the front
- A dorsal surface;
this is the part of the animal which faces upward. In bipedal primate, it is the back.
Most animals are bilaterally symmetrical; only a single line can divide their body into two equal halves examples include a rabbit. A few animals possess more than one line of symmetry. They are said to be radically symmetrical e.g. Hydra, sea anemones
A few other animals have no line of symmetry they are said to have no regular body shape. They are said to be asymmetric e.g. sponges.
Some animals have no head and are permanently fixed in one place. They are described as being sessile example are adult barnacles.
Others are slow moving. They are described as being sedentary. Example are the hydra and sea anemones. They have no definite head end, but they have a ring tentacle with special stinging cell to capture their prey. All sessile and sedentary animals are radially symmetrical.
Fig: symmetry section of a fish
Fig: Hydra showing radial symmetry
Most zoologist place animals under about 30 phyla. The common of them are;
|PHYLUM||EXAMPLES OF MEMBERS|
|Cnidarian||Hydra, jelly fish, sea anemones|
|Platyhelminthes||Tapeworm, planaria, flukes|
|Nematoda||Ascaris, hookworm, Guinea worm|
|Annelida||Earthworm, ragworm, leeches|
|Arthropoda||Cricket, centipede, grasshopper|
|Mollusca||Snails, clam, slugs|
|Echinodermata||Star fish, sea urchins|
|Chordata||Humans, frog, snakes|