Every organism in Animal kingdom is multicellular.
The types of cellular organisations found in animals are:
1. Cellular level of organization: The organisms are made of cells which are formed or arranged as loose lumps. This is mainly seen in Sponges.
2. Tissue level of organization: A tissue refers to a group of cells performing a similar function. In this organisation cells cooperate with each other performing same function. It is mainly seen in coelenterate.
3. Organ level of organization: In this organisation the tissues are organised into organs to perform specific functions. It is mainly seen in platyheliminthes.
4. Organ system level of organisation: In this system of organisation, the organs associate and co-ordinate with each other to form functional systems. Each system is concerned with a specific physiological function. It is seen from Annelids to Chordates.
Patterns in circulatory, digestive and reproductive system:
1. Circulatory System:
· Open type: Blood is pumped out of the heart and cells and tissues are bathed in it. E.g. Arthropods, Mollusca.
· Closed type: Blood is circulated through series of vessels of different size and diameters. E.g. Vertebrates.
2. Digestive System:
· Complete: There are 2 openings to the outside of the body, the mouth and anus. E.g. Arthropods, Chordates.
· Incomplete: They have only one opening towards the outside of the body and it serves as both mouth and anus. E.g. Platyhelminthes.
3. Reproductive System:
· Internal fertilisation: Fertilisation takes place inside the body of some organisms. E.g. Platyhelminthes.
· External fertilisation: Fertilisation takes place outside the body of some organisms. E.g. Ctenophora.
1. Direct development: Embryo develops into a mature individual without involving a larval
stage. Metamorphosis is absent E.g. fishes, reptiles, birds etc.
2. Indirect development: Involves sexually immature larval stage, having different food requirements than adults. Metamorphosis is present. E.g. most of invertebrates and amphibians.
· Oviparous animals: They lay fertilised or unfertilised eggs. The chances of survival of young one is less.
· Viviparous animals: They give birth to young ones. The chances of survival of young one is more.
· Ovoviviparous animals: They produce eggs, but instead of laying eggs, the eggs develop within the maternal body and hatch within or immediately after release from the parent. E.g. Some sharks, snakes etc.