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Chapter

BIO2231: Phylum Cnidaria characteristics

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Department
Biology
Course
BIO2231
Professor
Various
Semester
Spring

Description
PHYLUM CNIDARIA Characteristics of Phylum Cnidaria: o Cnidocytes present, typically housing stinging organelles called nematocysts o Entirely aquatic, some in freshwater, but most marine o Radial symmetry or biradial symmetry around a longitudinal axis with oral and aboral ends; no definite head o Two types of individuals: polyps and medusa o Adult body two-layered (diploblastic) with epidermis and gastrodermis derived from embryonic ectoderm and endoderm, respectively. o Mesoglea, an extracellular matrix lies between body layers; amount of mesoglea variable; mesoglea with cells and connective tissue from ectoderm in some o Incomplete gut called gastrovascular cavity; often branched or divided with septa o Extracellular digestion in gastrovascular cavity and intracellular digestion in gastrodermal cells o Extensible tentacles usually encircle mouth or oral region o Muscular contractions via epitheliomuscular bells, which form an outer layer of longitudinal fibers at base of epidermis and an inner layer of circular fibers at base of gastrodermis; modifications of plan in hydrozoan medusa (independent ectodermal muscle fibers) and other complex cnidarians. o Sense organs include well-developed statocysts (organs of balance) and ocelli (photosensitive organs); complex eyes in members of Cubozoa o Nerve net with symmetrical and asymmetrical synapses; diffuse conduction; two nerve rings in hydrozoan medusa o Asexual reproduction by budding in polyps forms clones and colonies; some colonies exhibit polymorphism (different polyp types within a colony) o Sexual reproduction by gametes in all medusa and some polyps; monoecious or dioecious; holoblastic indeterminate cleavage; planula larval form o No excretory or respiratory system o No coelomic cavity A Fearsome Tiny Weapon: Most are sessile feeders; those that are attached, such as jellyfish, swim only feebly. None chase their prey. They are very effective predators that are able to kill and eat prey. They possess tentacles that bristle with tiny, sophisticated weapons called nematocysts. Like a cocked- gun, a completed nematocyst requires only a small stimulus to make it fire. It instantly penetrates its prey and injects a paralyzing toxin. Cnidarians are diploblastic, meaning that they have 2 embryonic cell layers, ectoderm and endoderm, form which adult structures develop. Two layers are produced as the embryo develops from a single-layers blastula to a gastrula. In adult diploblasts, the epidermis develops from ectoderm, and the gut cavity lining/gastrodermis develops from endoderm; this body plan is in marked contrast to that of adult sponges, where there are neither cell layers nor a gut cavity. A new stage of development, gastrulation, characterizes diploblasts and produced the cell layers of adult animals. They exhibit radial/biradial symmetry and are not cephalised. Only cnidarians produce cnidocytes. Phylum Cnidaria: The phylum takes its name from cells called cnidocytes, which contain organelles (cnidae) characteristics of the phylum. Cnidarians sometimes live symbiotically with other animals, often as commensals on the shell or other surface of their host. The presence of the algae in reef-building corals limits the occurrence of coral reefs to relatively shallow, clear water where there is sufficient light for the photosynthetic requirements of the algae. These kinds of corals are an essential component of coral reefs, and reefs are extremely important habitats for many other species. Although many cnidarians have little economic importance, reef- building corals are an important exception. Fish and other animals associated with reefs provide substantial amounts of food for people, and reefs are of economic value as tourist attractions. Precious coral is used for making jewelry and ornaments, and coral rock for constructing buildings. 4 classes of Cnidaria: - Hydrozoa - Scyphozoa - Cubozoa - Anthozoa Form and Function: All cnidarian forms fit into one of two morphological types (dimorphism): - A polyp or hydroid form, which is adapted to a sedentary/sessile life - A medusa, or jellyfish form, which is adapted for a floating/free- swimming existence. A medusa is an unattached polyp with the tubular portion widened and flattened into a bell shape. Most polyps have tubular bodies. A mouth surrounded by tentacles defines the oral end of the body. The mouth leads into a blind gut/gastrovascular cavity. Polyps may reproduce asexually by budding, fission/pedal laceration. In budding, a knob of tissue forms on the side of an existing polyp and develops a functional mouth and tentacles. If a bud detaches from the polyp that made it, a clone is formed. If a bud stays attached to the polyp that made it, a colony will form and food may be shared through a common gastrovascular cavity. Polyps that do not bud are solitary; polyps that do bud are clonal/colonial. Polymorphism occurs when a single genotype can express more than one body form; when an individual asexually produces other individuals with different morphologies. Other methods of asexual reproduction in polyps are fission, where an individual divides in half as one side of the polyp pulls away from the other side, or pedal laceration, where tissue torn from the pedal disc develops into tiny new pedals. Medusae are usually free-swimming and have bell-shaped/umbrella- shaped bodies. They often exhibit tetramerous symmetry where body parts are arranged in 4. The mouth is centered on the concave (subumbrellar) side, and it may be pulled downward into frilly lobes that extend a long way beneath the umbrella/bell. Tentacles extend from outward from the rim of the umbrella. They have sensory structures for orientation (statocysts) and light reception (ocelli). Sensory information is integrated with motor response by a nerve ring at the base of the bell; two such rings occur in hydrozoan medusa. Medusae of class Scyphozoa are often called scyphomedusae, whereas those of class Hydrozoa are hydromedusae. Hydromedusae differ from the scyphomedusae by the presence of a velum a shelf-life fold of tissue from the bottom of the bell that extends into the bell. By reducing the cross-sectional area at the bottom of the bell, the velum increases the exit velocity of water from the bell, making each pulsation more efficient. Life cycles: In the cnidarian life cycle, polyps and medusa play different roles. In general, a zygote develops into a motile planula larva. The planula settles on a hard surface and metamorphoses into a polyp. A polyp can make other polyps asexually, but eventually it produces free- swimming medusa by asexual production. Polyps produce medusa by budding, or by oth
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