Basis of classification and types of classification
- Features such as arrangement of cells, body symmetry, nature of coelom, patterns of digestive, circulatory or reproductive systems are the basis of animal classification.
Levels of Organisation
- Cellular level of organization- cells are arranged as loose cell aggregates. Example- Sponges
- Tissue level of organization- cells performing the same function are arranged into tissues. Example- coelenterates.
- Organ level of organization- tissues are grouped together to form organs, each specialized for a particular function. Example- Platyhelminthes
- Organ system level of organization- organs have associated to form functional systems, each system concerned with a specific physiological function. Example- Annelids, Arthropods, Molluscs, Echinoderms and Chordates.
- Complexities can be seen in animals with organ system level of organization.
For example-i) the digestive system in Platyhelminthes has only a single opening that serves as both mouth and anus, and is hence called incomplete digestive system, whereas a complete digestivesystem has two openings, mouth and anus.
- ii) open type circulatory system is the one in which the blood is pumped out of the heart and the cells and tissues are directly bathed in it or ,whereas closed type is the one in which the blood is circulated through arteries, veins and capillaries.
Fig. complete and incomplete digestive systems
- Asymmetrical- when any plane that passes through the center does not divide them into equal halves. Example- Sponges.
- Radial symmetry- When any plane passing through the central axis of the body divides the organism into two identical halves. Example- Coelenterates, Ctenophores etc.
- Bilateral symmetry- When the body can be divided into identical left and right halves in only one plane. Example- Annelids, Arthropods, etc.
Diploblastic and Triploblastic Organisation
- Diploblastic animals- Animals in which the cells are arranged in two embryonic layers, an external ectoderm and an internal endoderm. Example- coelenterates
- Triploblastic animals- Animals in which the developing embryo has a third germinal layer, mesoderm, in between the ectoderm and endoderm. Examples- platyhelminthes to chordates
Fig. diploblastic and triploblastic organization
- The body cavity, which is lined by mesoderm is called coelom.
- Animals possessing coelom are called coelomates. Examples- annelids, molluscs etc.
- 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. Examples- Aschelminthes
- The animals in which the body cavity is absent are called acoelomates. Examples- Platyhelminthes
- Segmentation is the process of dividing the body externally and internally into segments with a serial repetition of at least some organs.
For example- In earthworm, the body shows called metameric segmentation and the phenomenon is known as metamerism.
Fig. metameric segmentation in earthworm
- Notochord is a mesodermally derived rod-like structure formed on the dorsal side during embryonic development.
- Animals with notochord are called chordates and those animals which do not form this structure are called non-chordates. Examples- porifera to echinoderms.
Types of classification
- Animals are classified based on arrangement of cells, body symmetry, nature of coelom, patterns of digestive, circulatory or reproductive system.
- Animal kingdom is classified into various phylums such as Phylum Porifera, Phylum Coelenterata (Cnidaria), Phylum Ctenophore, Phylum Platyhelminthes, Phylum Aschelminthes, Phylum Annelida, Phylum Arthropoda, Phylum Mollusca, Phylum Echinodermata, Phylum Hemichordata and Phylum Chordata.