are flagellated collar cells that create and maintain water flow through the sponge thus facilitating respiratory and digestive functions. Animals such as sponges lack nervous tissue and muscle tissue, what does this tell you about sponges? Tissue Level of Organisation In some animals, cells that perform similar functions are aggregated to form tissues. The cells of a tissue integrate in a highly coordinated fashion to perform a common function, due to the presence of nerve cells and sensory cells.
This tissue level of organisation is exhibited in diploblastic animals like cnidarians. The formation of tissues is the first step towards evolution of body plan in animals ( Hydra - Coelenterata). Organ Level of Organisation Different kinds of tissues aggregate to form an organ to perform a specific function. Organ level of organisation is a XI Std Biology-Zoology Chapter- .
. Diploblastic and Triploblastic Organisation During embryonic development, the tissues and organs of animals originate from two or three embryonic germ layers. On the basis of the origin and develop- ment, animals are classified into two cate- gories: Diploblastic and Triploblastic. Animals in which the cells are arranged in two embryonic layers (Figure .
), the external ectoderm, and internal endoderm are called diploblastic animals . In these animals the ectoderm gives rise to the epidermis (the outer layer of the body wall) and endoderm gives rise to gastrodermis (tissue lining the gut cavity). An undifferentiated layer present between the ectoderm and endoderm is the mesoglea. (Corals, Jellyfish, Sea anemone) Animals in which the developing embryo has three germinal layers are called triploblastic animals and consists of outer ectoderm (skin, hair, neuron, nail, teeth, etc), inner endoderm (gut, lung, liver) and middle mesoderm (muscle, bone, heart).
Most of the triploblastic animals show organ system level of organisation (Flat worms to Chordates). axis are identical. An animal’s body plan results from the animal’s pattern of devel- opment. The simplest body plan is seen in sponges (Figure .
). They do not display symmetry and are asymmetrical . Such animals lack a definite body plan or are irregular shaped and any