The kingdom Animalia is broadly divided into the following phyla:
1- Phylum Porifera
This phylum contains sponges. Most of them are marine while some live in freshwaters. Leucosolenia and Euplectella (Venus' flower basket) are marine sponges. Spongilla is a common freshwater sponge.
A commercial sponge is prepared by drying, beating, and washing a sponge until all cells are removed.
Sponges do not have tissue level organization. Most sponges are asymmetrical but some have radially symmetry. They do not have nervous system. There are numerous pores in body wall called ostia. Through ostia, water enters the body. The larger pore through which water leaves the body is called osculum. The outer layer of body is made of thin, flat cells called pinacocytes. The second layer is jelly-like and is called mesohyle. It contains amoeboid cells. The third layer, which lines the spongocoel, is made of choanocytes or collar cells. They have skeleton in the form of minute needles of calcium carbonate or silica. Most sponges reproduce asexually by budding or regeneration. Some sponges form resistant capsules, called gemmules. When parent sponge dies, it releases its gemmules. In favourable environment, amoeboid cells come out of the gemmules and form a new sponge.
2- Phylum Cnidaria
Almost all cnidarians are marine, although a few are found in freshwater e.g., hydra and jellyfish. Most cnidarians are colonial e.g., obelia, corals, sea fans etc. Most of them are sessile e.g., hydra, coral, obelia etc. Some cnidarians are motile e.g., jellyfish.
They are radially symmetrical animals and are diploblastic. It means that the adult body contains two tissue layers i.e., the epidermis and the gastrodermis, derived from ectoderm and endoderm respectively. Between the epidermis and gastrodermis, a jelly-like mesoglea is present. It contains amoeboid cells that have originated either from ectoderm or endoderm.
Corals are colonial cnidarians. They produce hard exoskeleton of Calcium carbonate. The skeleton makes coral islands and coral reefs.
Between the epidermis and gastrodermis, a jelly-like mesoglea is present. It contains amoeboid cells that have originated either from ectoderm or endoderm. They possess special cells, called cnidocytes. A cnidocyte contains a special organelle, called nematocyst. Nematocysts defend the body and captures prey. Cnidarians have a blind-ending cavity, called gastrovascular cavity or enteron. It opens outside by a single opening, the mouth. Mouth also acts as anus for the removal of undigested material. Mouth is surrounded by a series of projections, called tentacles. This types of digestive system in which there is a single opening for the entry of food and removal of undigested matter, is called sao like digestive system.
The nervous system is in the form of a network of neurons in the body wall. There is no central nervous system (brain and spinal cord). They do not have respiratory,excretory and transport systems. There are two body forms in cnidarians i.e., polyps and medusae. Polyps are cylindrical and are attached to a substrate at the aboral end. They reproduce asexually. Medusae are umbrella-like and are free-swimming. They reproduce sexually.
3- Phylum Platyhelminthes
They are called "flatworms". They are unsegmented and body is soft and dorsoventrally compressed. Most of them are free-living e.g., planaria. Some are endoparasites of humans and other animals e.g., liver fluke, tapeworm, and blood-fluke. They are triploblastic i.e., the tissues of the body are derived from three embryonic layers; ectoderm, mesoderm and endoderm. They are acoelomates.
A loose connective tissue called parenchyma fills space between the body wall and body organs. They have bilateral symmetry with distinct left and right sides as well as dorsal and ventral sides. They do not have respiratory and circulatory (transport) systems. They have a network of tubular protonephridia. These tubules have numerous branches. Each branch ends in a bulb-like cell called flame cell. The cilia of flame cells beat to suck surrounding fluid into the tubules. The tubules filter the waste materials from fluid and release them out of body wall through a small opening called a nephridiopore.
They release them out of body wall through a small opening called a nephridiopore. They have a network of neurons. There are cerebral ganglia in the anterior end (head). These ganglia are attached to longitudinal nerve cords that are interconnected across the body by transverse branches. Most free-living flatworms have two simple eyespots at their anterior end. Flatworms reproduce asexually by "fission" in which the animal constricts in the middle and then divides into two pieces. Each piece then regenerates the missing part. The sexually-reproducing flatworms are hermaphrodites (bisexual).
4- Phylum Nematoda
They are roundworms with elongated worm-like (round) body with pointed ends. Some roundworms are free-living (in water and soil) e.g., Caenorhabditis elegans. Many are parasites e.g., ascaris, hookworm, pinworm, and whipworm.
The pseudocoelomates are classified in seven phyla. These phyla are grouped as a unit called Aschelminths. Phylum Nematoda is the representative phylum of this group.
They are triploblastic, bilateral symmetrical, and possess unsegmented body. They are pseudocoelomates because they possess a false body cavity called pseudocoelom filled with fluid. They possess tube-like digestive system. It consists of an alimentary canal with two openings; mouth at anterior end and anus at posterior end. The parasitic roundworms have simplified digestive systems.
Their excretory system consists of protonephridia and two excretory canals, which unite at the anterior end to form a single canal. The single canal then opens outside through a nephridiopore on the ventral surface. They possess a network of neurons in body. There is a nerve ring around the pharynx, which is attached to four longitudinal nerve cords. They have raised hair-like sense organ called sensory papillae, present on lips. They do not have defined respiratory and circulating systems. They are unisexual i.e.; male nematodes have testes and female nematodes have ovaries.
Post a Comment