Cnidarians have two distinct tissue layers. They also have structures like gonads and a nerve net. Why is this considered tissue level, and not organ-level organization?
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Answer:
Cnidarians have two distinct morphological body plans known as polyp, which are sessile as adults, and medusa, which are mobile; some species exhibit both body plans in their lifecycle.
All cnidarians have two membrane layers in the body: the epidermis and the gastrodermis; between both layers they have the mesoglea, which is a connective layer.
Cnidarians carry out extracellular digestion, where enzymes break down the food particles and cells lining the gastrovascular cavity absorb the nutrients.
Cnidarians have an incomplete digestive system with only one opening; the gastrovascular cavity serves as both a mouth and an anus.
The nervous system of cnidarians, responsible for tentacle movement, drawing of captured prey to the mouth, digestion of food, and expulsion of waste, is composed of nerve cells scattered across the body.
Anthozoa, Scyphozoa, Cubozoa, and Hydrozoa make up the four different classes of Cnidarians.