In this article we will discuss about:- 1. Levels of Organisation Found in Animals 2. Body Plans of Animals.
Levels of Organisation Found in Animals:
1. Protoplasmic or Acellular Level:
The organisation is at the level of protoplasm. It is present in protozoans.
2. Cellular Level:
The body consists of many cells which may be similar or show minor division of labour e.g., sponges.
3. Tissue Level:
The cells form poorly defined tissues, e.g. Cnidarians (=coelenterates), ctenophores.
4. Organ-System Level:
The cells are organised into tissues, tissues into organs and organs into organ systems. Except sponges and coelenterates, all the animals of the kingdom animalia have organ-system level of organisation.
Body Plans of Animals:
Animals have three types of body plans:
1. Cell Aggregate Plan:
The body consists of a cluster or aggregation of cells. It is found in sponges.
2. Blind Sac Plan:
The body has a single cavity with one opening to the outside. The single opening functions as both mouth for ingestion (intake of food) and anus for egestion (undigested food is passed out). It is found in Cnidarians (=coelenterates) and flatworms.
3. Tube-within-a-Tube Plan:
The body has two tubes, one formed by the body wall and the second formed within it by the digestive tract. Digestive tract is a continuous tube like structure that has two openings, a mouth for ingestion and anus for egestion. Food is digested and absorbed in the digestive tract (alimentary canal).
Tube-within-a tube plan is of two types:
(i) Protostomic Plan:
The mouth of the digestive tract develops first in the embryo and anus is formed later. This occurs in roundworms, annelids, molluscs and arthropods. The animals which have this body plan are called protostomes.
(ii) Deuterostomic Plan:
The anus of the digestive tract develops first in the embryo and the mouth is formed later. This is seen in echinoderms and chordates. Thus chordates are evolutionarily closer to the echinoderms. The animals which possess this body plan are called deuterstomes.