Distinguishing Features
Further information: Sponge, Ctenophore, and BilateriaCnidarians form an animal phylum that more complex than sponges, about as complex as ctenophores (comb jellies), and less complex than bilaterians, which include almost all other animals. However, both cnidarians and ctenophores are more complex than sponges as they have: cells bound by inter-cell connections and carpet-like basement membranes; muscles; nervous systems; and some have sensory organs. Cnidarians are distinguished from all other animals by having cnidocytes that fire like harpoons and are used mainly to capture prey but also as anchors in some species.
Like sponges and ctenophores, cnidarians have two main layers of cells that sandwich a middle layer of jelly-like material, which is called the mesoglea in cnidarians; more complex animals have three main cell layers and no intermediate jelly-like layer. Hence, cnidarians and ctenophores have traditionally been labelled diploblastic, along with sponges. However, both cnidarians and ctenophores have a type of muscle that, in more complex animals, arises from the middle cell layer. As a result some recent text books classify ctenophores as triploblastic, and it has been suggested that cnidarians evolved from triploblastic ancestors.
Sponges | Cnidarians | Ctenophores | Bilateria | |
---|---|---|---|---|
Cnidocytes | No | Yes | No | |
Colloblasts | No | Yes | No | |
Digestive and circulatory organs | No | Yes | ||
Number of main cell layers | Two, with jelly-like layer between them | Two or Three | Three | |
Cells in each layer bound together | No, except that Homoscleromorpha have basement membranes. | Yes: inter-cell connections; basement membranes | ||
Sensory organs | No | Yes | ||
Number of cells in middle "jelly" layer | Many | Few | (Not applicable) | |
Cells in outer layers can move inwards and change functions | Yes | No | (Not applicable) | |
Nervous system | No | Yes, simple | Simple to complex | |
Muscles | None | Mostly epitheliomuscular | Mostly myoepithelial | Mostly myocytes |
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