These are also known as sponges. They are usually found in marine habitat. Most of the poriferans are asymmetrical. Cellular level organization is present in these animals. ‘Pores on the body’ is the main feature of these animals.
Water Transport System in Sponges: Water transport system is a distinguishing feature of sponges. Water enters the body through minute pores on the body wall. Water enters into a central cavity (spongocoel) and then goes out through the osculum. The water transport system helps in gathering food, in exchange of gases and in excretion. The spongocoel and canals are lined by collar cells or choanocytes. Digestion is intracellular. A skeleton made up of spicules or sponging fibres supports the body.
Reproduction: These are hermaphrodite animals, i.e. sexes are not separate. While sexual reproduction is by gamete formation, asexual reproduction is by fragmentation. Fertilization is internal and development is indirect.
These are aquatic animals and most of them live in marine habitat. They can be sessile or free-swimming. The body is radially symmetrical. Body is composed of a cavity which has only one opening. Stinging capsules or nematocytes are present on the tentacles. Cnidoblasts are used for anchorage, defense and for capturing the prey.
Tissue level organization is present in cnidarians. They are diploblastic animals. Digestion is extracellular and intracellular. Some of the cnidarians have a skeleton composed of calcium carbonate, e.g. corals.
There are two basic body forms in the cnidarians, viz. polyp and medusa. Polyp is sessile and cylindrical form, e.g. Hydra, Adamsia, etc. Medussa is umbrella-shaped and free-swimming, e.g. Aurelia (Jelly Fish). Some cnidarians exist in both forms and exhibit alternation of generations, i.e. metagenesis. In this case, the polyp produces the medusa asexually and the medusa produces the polyp sexually.
Common Examples: Physalia (Portuguese man-of-war), Adamsia (Sea anemone), Pennatula (Sea-pen), Gorgonia (Sea-fan) and Meandrina (Brain coral).
These are commonly known as sea walnuts or comb jellies. These are exclusively marine animals. The body is radially symmetrical and diploblastic. Tissue level organization is present. There are eight eternal rows of ciliated comb plates on the body. These rows help in locomotion. Digestion is both extracellular and intracellular. Bioluminescence is well-developed in ctenophores.
Reproduction: These are hermaphrodite animals. Reproduction is only through sexual means. Fertilisation is external and development is indirect.
Common Examples: Pleurobrachia and Ctenoplana.
Habit and Habitat: These are also called flatworms because of dorso-ventrally flattened body. Most of the flatworms live as endoparasites in animals; including human bodies. They are bilaterally symmetrical, triploblastic and acoelomate animals. Organ level organization is present in them.
Hooks and suckers are present in the parasitic forms. Some of the flatworms absorb nutrients from the host directly through their body surface. Osmoregulation and excretion are carried on by specialized cells called flame cells.
Reproduction: Flatworms are hermaphrodite animals. Fertilization is internal and development is indirect. Some members have the ability for regeneration, e.g. planaria.
Common Examples: Taenia solium (Tapeworm), Fasciola (Liverfluke)
Habit and Habitat: These are also called roundworms because their body appears circular in cross-section. They can be free living, aquatic and terrestrial. Some of them are parasitic in plants and animals. Organ system level of organization is present. The roundworms are bilaterally symmetrical, triploblastic and pseudocoelomate.
Digestive System: Alimentary canal is complete. Well developed muscular pharynx is present. There is an excretory tube to remove wastes through excretory pore.
Reproduction: The roundworms are dioecious, i.e. sexes are separate. Females are usually longer than males. Fertilization is internal and development can be direct or indirect.
Common Examples: Ascaris (Round Worm), Wuchereria (Filaria worm), Ancylostoma (Hookworm).
Habit and Habitat: The annelids are aquatic or terrestrial. The aquatic annelids are found in marine and freshwater habitats. Most of them are free-living, while some can be parasitic. Organ system level organization is present. Body is bilaterally symmetrical, coelomate and triploblastic. The body is metamerically symmetrical. The body is distinctly divided into segments or metameres.
Longitudinal and circular muscles are present to help in locomotion. Aquatic annelida possess lateral appendages (parapodia) which facilitates swimming, e.g. Nereis. Closed circulatory system is present. Nephridia are present for excretion and osmoregulation. The neural system is composed of paired ganglia which are connected by lateral nerves to a double ventral nerve cord.
Reproduction: Earthworms and leeches are hermaphrodite, while nereis is dioecious.
Common Examples: Nereis, Pheretima (Earthworm) and Hirudinaria (Blood sucking leech).
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