9.3.2 Phylum Cnidaria (Gk. Kindle, nettle,+L. aria connected with)
The name Cnidaria is given to this phylum due to the presence of specialized stinging cells called cnidocytes all over the body which give rise to nematocysts. The nematocyst is a capsule with paralyzing venom which acts as an offensive and defensive organ. Phylum Cnidaria exhibits the following characteristics.
Habit and Habitat:
Cnidarians are either sessile e.g., hydra or e.g., jellyfish). Many are colonial (e.g., Obelia). Most of them are carnivores. All cnidarians are aquatic (both marine and freshwater).
Size: may be from microscopic (Hydra) to very large (e.g., Brachioceranthus, up to two meters (in length.) living motile.
Body:
Cnidarians are radially symmetrical, most are diploblastic i.e., they have an outer ectoderm and inner endoderm between these two layers, and there is a gelatinous cementing substance, called mesoglea. A single hollow internal cavity known as a Structure motile enteron or gastrovascular cavity with a single opening called a mouth (acts both as mouth. and anus). The mouth is surrounded by tentacles.
The nervous system consists of an irregularly placed network of neurons with few ganglia. Some colonial cnidarians exhibit Polymorphism. They can reproduce both asexually and sexually. Asexual diploblastic. using budding and regeneration (such as in hydra), in sexual reproduction gametes are formed.
Polyho phism In cnidarian colonial forms exhibit polymorphism and alternation of generation, both polyp and medusa are diploid and diploblastic. Some colonies (like corals) grow to a great size, living in their own secreted skeleton, made of Calcium carbonate (lime) which they get from seawater by ectodermal cells.
These skeletons form coral reefs (water ridges or rocks) and even islands. Mostly found in Florida, the West Indies, Austria, east coast of Africa. In cnidarian, most systems still do not appear such as transport, excretory, and respiratory, thus only diffusion helps in respiration and excretion. Although a sac-like gastrovascular cavity is present. Importance of Cnidarians: Cnidarians are both
Evolutionary Adaptation in Cnidarians:
beneficial and harmful such as coral reefs protect the shores from erosion by tidal waves. Many jewelry items are made of corals and used as decoration in aquariums and rock gardens. Some Cnidarians sting humans (Jellyfish and sea Tentacle Gastroomscular anemones). In Pakistan, they are common at the shores of Mekran and Karachi. Many people are affected by poisonous cnidarians while they swim.
9.3.3 Phylum Platyhelminthes (Gk. Platy: flat, helminth: worm)
Platyhelminthes are also called flatworms or acoelomate worms. The characteristics of phylum Platyhelminthes are:
Habit and Habitat:
Platyhelminthes are either free-living (e.g., planaria) or parasitic (e.g., tapeworm). These worms are recorded both in fresh water and marine, while parasitic worms are also found in the liver and gut of human beings.
Body:
Flatworms are triploblastic, bilaterally symmetrical, and soft-bodied animals. Their body is dorsoventrally flattened and acoelomate. Free-living forms possess incomplete digestive systems i.e., with a single opening known as a mouth while less developed or absent in parasitic forms. The excretory system is well well-developed protonephiridial system containing flame cells.
The nervous system consists of a network of nerves, with two longitudinal nerves and an anterior cerebral ganglion. Locomotion occurs using underside cilia maria.
Reproductive system:
Asexual by fission (regeneration) sexual reproduction is also present. These worms are bisexual with well-developed gonads, ducts, and accessory organs, fertilization is internal, and development may be direct or indirect. Examples: Planaria (Dugesia), liver fluke (Fasciola), and Tapeworm (Taenia).
0 Comments