Cnidocytes : Animals from the phylum Cnidaria have stinging cells called cnidocytes. Cnidocytes contain large organelles called a nematocysts that store a coiled thread and barb. When hairlike projections on the cell surface are touched, b the thread, barb, and a toxin are fired from the organelle.
An example of the polyp form is Hydra spp. Polyp forms are sessile as adults, with a single opening to the digestive system the mouth facing up with tentacles surrounding it. Medusa forms are motile, with the mouth and tentacles hanging down from an umbrella-shaped bell. Cnidarian morphology : Cnidarians have two distinct body plans, the medusa a and the polyp b. All cnidarians have two membrane layers, with a jelly-like mesoglea between them.
Some cnidarians are polymorphic, having two body plans during their life cycle. An example is the colonial hydroid called an Obelia. The sessile polyp form has, in fact, two types of polyps. The first is the gastrozooid, which is adapted for capturing prey and feeding; the other type of polyp is the gonozooid, adapted for the asexual budding of medusa.
When the reproductive buds mature, they break off and become free-swimming medusa, which are either male or female dioecious. The male medusa makes sperm, whereas the female medusa makes eggs. After fertilization, the zygote develops into a blastula and then into a planula larva.
The larva is free swimming for a while, but eventually attaches and a new colonial reproductive polyp is formed. Types of polyps in Obelia : The sessile form of Obelia geniculate has two types of polyps: gastrozooids, which are adapted for capturing prey, and gonozooids, which bud to produce medusae asexually. All cnidarians show the presence of two membrane layers in the body that are derived from the endoderm and ectoderm of the embryo.
The outer layer from ectoderm is called the epidermis and lines the outside of the animal, whereas the inner layer from endoderm is called the gastrodermis and lines the digestive cavity.
Between these two membrane layers is a non-living, jelly-like mesoglea connective layer. In terms of cellular complexity, cnidarians show the presence of differentiated cell types in each tissue layer: nerve cells, contractile epithelial cells, enzyme-secreting cells, and nutrient-absorbing cells, as well as the presence of intercellular connections.
However, the development of organs or organ systems is not advanced in this phylum. The nervous system is primitive, with nerve cells scattered across the body.
This nerve net may show the presence of groups of cells in the form of nerve plexi singular: plexus or nerve cords. The nerve cells show mixed characteristics of motor as well as sensory neurons. The predominant signaling molecules in these primitive nervous systems are chemical peptides, which perform both excitatory and inhibitory functions.
Despite the simplicity of the nervous system, it coordinates the movement of tentacles, the drawing of captured prey to the mouth, the digestion of food, and the expulsion of waste.
The cnidarians perform extracellular digestion in which the food is taken into the gastrovascular cavity, enzymes are secreted into the cavity, and the cells lining the cavity absorb nutrients. The gastrovascular cavity has only one opening that serves as both a mouth and an anus; this is termed an incomplete digestive system. Cnidarian cells exchange oxygen and carbon dioxide by diffusion between cells in the epidermis with water in the environment, and between cells in the gastrodermis with water in the gastrovascular cavity.
The lack of a circulatory system to move dissolved gases limits the thickness of the body wall, necessitating a non-living mesoglea between the layers.
There is no excretory system or organs; nitrogenous wastes simply diffuse from the cells into the water outside the animal or in the gastrovascular cavity. There is also no circulatory system, so nutrients must move from the cells that absorb them in the lining of the gastrovascular cavity through the mesoglea to other cells.
The phylum Cnidaria contains about 10, described species divided into four classes: Anthozoa, Scyphozoa, Cubozoa, and Hydrozoa. The anthozoans, the sea anemones and corals, are all sessile species, whereas the scyphozoans jellyfish and cubozoans box jellies are swimming forms. Members of the class Anthozoa display only polyp morphology and have cnidocyte-covered tentacles around their mouth opening.
The class Anthozoa includes all cnidarians that exhibit a polyp body plan only; in other words, there is no medusa stage within their life cycle. Examples include sea anemones, sea pens, and corals, with an estimated number of 6, described species.
Sea anemones are usually brightly colored and can attain a size of 1. These animals are usually cylindrical in shape and are attached to a substrate. Anthozoans : The sea anemone a , like all anthozoans, has only a polyp body plan b.
The mouth of a sea anemone is surrounded by tentacles that bear cnidocytes. They have slit-like mouth openings and a pharynx, which is the muscular part of the digestive system that serves to ingest as well as egest food. It may extend for up to two-thirds the length of the body before opening into the gastrovascular cavity.
This cavity is divided into several chambers by longitudinal septa called mesenteries. Each mesentery consists of one ectodermal and one endodermal cell layer with the mesoglea sandwiched in between. Mesenteries do not divide the gastrovascular cavity completely; the smaller cavities coalesce at the pharyngeal opening.
The adaptive benefit of the mesenteries appears to be an increase in surface area for absorption of nutrients and gas exchange. The gastrovacular cavity is lined with a tissue called the gastrodermis. These cells release digestive enzymes into the gvc.
The enzymes break the animal down into food particles. The second step in food digestion is called intracellular digestion. This occurs within the gastrodermis cells. Food particles enter these cells by endocytosis, and food vacuoles are formed. These food molecules then enter the cytoplasm of the cell by diffusion, facilitated diffusion, or active transport.
The food molecules then are used by the gastrodermis cell or diffuse into the other tissue of the Hydra , the epidermis. All cells of the Hydra use food molecules in aerobic respiration or to build cell structures. The two tissue layers of the Hydra , the gastrodermis and epidermis are separated from each other by a jelly-like material called the mesoglea.
The processes of digestion and aerobic respiration produce waste materials that must be eliminated from animals. Food vacuoles release digestive waste into the gvc, and this digestive waste joins other digestive waste present in the gvc from extracellular digestion.
This waste material leaves the gvc through the mouth. The animal contracts gets shorter and wider and the waste shoots out of the mouth. Nitrogen waste ammonia is produced as a result of protein digestion.
The ammonia diffuses from the Hydra into the water surrounding the animal. Carbon dioxide waste is produced as a result of aerobic respiration. This too leaves the animal by diffusion. Oxygen is needed by animals to carry out aerobic respiration. This enters the cells of Hydra by diffusion from the water surrounding the animal into the cells of the animal. Members of the Cnidaria have several special features. These include the cnidocytes that are found in no other types of animals and are unique to the Phylum.
A second feature is the ability of these animals to undergo budding. Mitosis produces buds new individuals in Hydra , for example. The bud eventually becomes detached from the parent and then lives an independent existence. The gastrovascular cavity has only one opening that serves as both a mouth and an anus an incomplete digestive system.
Like the sponges, Cnidarian cells exchange oxygen, carbon dioxide, and nitrogenous wastes by diffusion between cells in the epidermis and gastrodermis with water.
The phylum Cnidaria contains about 10, described species divided into four classes: Anthozoa, Scyphozoa, Cubozoa, and Hydrozoa. The class Anthozoa includes all cnidarians that exhibit a sessile polyp body plan only; in other words, there is no medusa stage within their life cycle. Examples include sea anemones, sea pens, and corals, with an estimated number of 6, described species.
Sea anemones are usually brightly colored and can attain a size of 1. These animals are usually cylindrical in shape and are attached to a substrate. A mouth opening is surrounded by tentacles bearing cnidocytes [Figure 5]. Scyphozoans include all the jellies and are motile and exclusively marine with about described species.
The medusa is the dominant stage in the life cycle, although there is also a polyp stage. Species range from 2 cm in length to the largest scyphozoan species, Cyanea capillata , at 2 m across. Jellies display a characteristic bell-like body shape [Figure 6]. Identify the life cycle stages of jellies using this video animation game from the New England Aquarium.
Cubozoans are anatomically similar to the jellyfish. A prominent difference between the two classes is the arrangement of tentacles. Cubozoans have muscular pads called pedalia at the corners of the square bell canopy, with one or more tentacles attached to each pedalium. In some cases, the digestive system may extend into the pedalia. Cubozoans typically exist in a polyp form that develops from a larva.
The polyps may bud to form more polyps and then transform into the medusoid forms. Watch this video to learn more about the deadly toxins of the box jellyfish. Hydrozoa includes nearly 3, species, 1 most of which are marine.
Most species in this class have both polyp and medusa forms in their life cycle. Many hydrozoans form colonies composed of branches of specialized polyps that share a gastrovascular cavity.
Other species are solitary polyps or solitary medusae. The characteristic shared by all of these species is that their gonads are derived from epidermal tissue, whereas in all other cnidarians, they are derived from gastrodermal tissue [Figure 7] ab. Section Summary Animals included in phylum Porifera are parazoans and do not possess true tissues.
These organisms show a simple organization. Sponges have multiple cell types that are geared toward executing various metabolic functions. Cnidarians have outer and inner tissue layers sandwiching a noncellular mesoglea.
Cnidarians possess a well-formed digestive system and carry out extracellular digestion. The cnidocyte is a specialized cell for delivering toxins to prey and predators.
Cnidarians have separate sexes. They have a life cycle that involves morphologically distinct forms—medusoid and polypoid—at various stages in their life cycle. Describe the feeding mechanism of sponges and identify how it is different from other animals. The sponges draw water carrying food particles into the spongocoel using the beating of flagella in the choanocytes.
0コメント