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By the end of this section, you will be able to:
  • Compare structural and organization characteristics of Porifera and Cnidaria
  • Describe the progressive development of tissues and their relevance to animal complexity

Phylum Cnidaria    includes animals that show radial or biradial symmetry and are diploblastic, that is, they develop from two embryonic layers. Nearly all (about 99 percent) cnidarians are marine species.

Cnidarians contain specialized cells known as cnidocytes (“stinging cells”) containing organelles called nematocysts (stingers). These cells are present around the mouth and tentacles, and serve to immobilize prey with toxins contained within the cells. Nematocysts contain coiled threads that may bear barbs. The outer wall of the cell has hairlike projections called cnidocils, which are sensitive to touch. When touched, the cells are known to fire coiled threads that can either penetrate the flesh of the prey or predators of cnidarians (see [link] ) or ensnare it. These coiled threads release toxins into the target and can often immobilize prey or scare away predators.

The illustration shows a nematocyst before (a) and after (b) firing. The nematocyst is a large, oval organelle inside a rectangular cnidocyte cell. The nematocyst is flush with the plasma membrane, and a touch-sensitive hairlike projection extends from the nematocyst to the cell’s exterior. Inside the nematocyst, a thread is coiled around an inverted barb. Upon firing, a lid on the nematocyst opens. The barb pops out of the cell and the thread uncoils.
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.

View this video animation showing two anemones engaged in a battle.

Animals in this phylum display two distinct morphological body plans: polyp    or “stalk” and medusa    or “bell” ( [link] ). An example of the polyp form is Hydra spp.; perhaps the most well-known medusoid animals are the jellies (jellyfish). 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.

The illustration compares the medusa (a) and polyp (b) body plans. The medusa is dome-shaped, with tentacle-like appendages hanging down from the edges of the dome. The polyp looks like a tree, with a trunk at the bottom and branches at the top. Both the medusa and polyp have two tissue layers, with mesoglea in between. The mesoglea is thicker in the dome of the medusa than in the polyp. Both also have a central body cavity.
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, that is, they have 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, shown in [link] . 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, which develops into a planula larva. The larva is free swimming for a while, but eventually attaches and a new colonial reproductive polyp is formed.

Illustration a shows Obelia geniculata, which has a body composed of branching polyps of two different types.
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.

Questions & Answers

what is mutation
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Sifune Reply
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Sifune
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mahase Reply
classification of plants, gymnosperm features.
Linsy Reply
what is the features of gymnosperm
Linsy
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Samuel
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urine is formed in the nephron of the renal medulla in the kidney. It starts from filtration, then selective reabsorption and finally secretion
onuoha Reply
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how is urine formed in human
Konadu
how is urine formed in human
Rahma
what is the diference between a cavity and a canal
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Diamond
malaria is caused by an insect called mosquito.
Naomi
Malaria is cause by female anopheles mosquito
Isaac
Malaria is caused by plasmodium Female anopheles mosquitoe is d carrier
Olalekan
a canal is more needed in a root but a cavity is a bad effect
Commander
what are pathogens
Don Reply
In biology, a pathogen (Greek: πάθος pathos "suffering", "passion" and -γενής -genēs "producer of") in the oldest and broadest sense, is anything that can produce disease. A pathogen may also be referred to as an infectious agent, or simply a germ. The term pathogen came into use in the 1880s.[1][2
Zainab
A virus
Commander
Definition of respiration
Muhsin Reply
respiration is the process in which we breath in oxygen and breath out carbon dioxide
Achor
how are lungs work
Commander
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Achiri Reply
in the mouth
EZEKIEL
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Rashima Reply
stimulates the follicle to release the mature ovum into the oviduct
Davonte
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Chinaza
endocrine secrete hormone and regulate body process
Achor
while pituitary gland is an example of endocrine system and it's found in the Brain
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Egbodo Reply
Biology is the study of living organisms, divided into many specialized field that cover their morphology, physiology,anatomy, behaviour,origin and distribution.
Lisah
biology is the study of life.
Alfreda
Biology is the study of how living organisms live and survive in a specific environment
Sifune
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Source:  OpenStax, Animals. OpenStax CNX. Mar 20, 2015 Download for free at https://legacy.cnx.org/content/col11771/1.1
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