| Cnidaria |
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The phylum Cnidaria is also called the phylum CoelenterataCoelenterata, because it has special Named after cnidocytes, the body is tubular or umbrella-shaped, with one end open and the other closed, like a hollow sac. The unique stinging cells of cnidarians are used to defend and capture poisonous prey, and they are also harmful to us humans. Cnidarians have different shapes, but their basic body structure can be divided into two types: the polyp type that lives a benthic and sessile life and the jellyfish type that lives a planktonic life. These two types often show the phenomenon of generational alternation in their life history. Common polyps, corals, and sea anemones all belong to the polyp type, while jellyfish, moon jellyfish, and jellyfish belong to the jellyfish type. Most species of cnidarians aremarine, from the intertidal zone to deep water10,000 Its members can be seen at the bottom of the ultra-deep abyss. About 10,000 species have been discovered in the world, and about 1,000 species have been recorded in my country. |
Main characteristics of the phylum Coelenterata
Coelenterates are the realbeginning of metazoa. They occupy an important position in the evolutionary process. All other metazoans passed through this developed in stages. They have advanced characteristics compared to sponges, primitive and low-end characteristics compared with other metazoans, and characteristics of adapting to the external environment.
1. Progressive Characteristics
Compared with sponges, its advanced features are:
(1) YuFree titleToFull shooting name ;
(2) From unorganized differentiation toorganized differentiation, skin muscle appears Cells, neural networks, etc.;
(3) The body changes from two layers of cells to two germ layers, and a digestive circulation cavity appears. The digestive circulation cavity is different from the central cavity of sponges and has digestion and circulation functions.
2. Original characteristics
Compared with other metazoans, coelenterates show many primitive aspects in morphological structure and physiological functions, such as:
(1) The radially symmetrical body shape of coelenterates is just a primitive low-level form of symmetry, because this symmetry can only be divided into upper and lower parts, without distinction between front and rear, dorsal and ventral, and is only suitable for a fixed or floating life in the water.
(2) The digestive system has a mouth but no anus. The mouth is the original mouth during embryonic development. This shows that coelenterates are equivalent to the gastrula stage.
(3) Epithelial muscle cells belong to both epithelium and muscle, indicating that epithelium and muscle are not separated, which is a primitive phenomenon.
(4) The neural network of coelenterates is the most primitive reticular nervous system because there is no Forming a nerve center, nerve conduction has no direction and the conduction speed is very slow.
3. Particular Characteristics
(1) Coelenterates have stinging cells, which are special cells unique to coelenterates and are found all over the body surface.
(2) Coelenterates have two basic forms, namely polyp type and jellyfish type, which are adapted to different lives. The polyp type is adapted to a fixed life, and the jellyfish type is adapted to a floating life. But their basic structure is the same, they are composed of cells of two germ layers, radially symmetrical, with tentacles, stinging cells, mouth, digestive and circulation cavity, oral surface and anti-oral surface.
Comparison between hydra type and jellyfish type
| Hydra type | Jellyfish type | |
| Appearance | Cylindrical type | Disc-shaped |
| lifestyle | fixed life | floating life |
| Reproductive mode | asexual budding | sexual reproduction |
| middle glue layer |
Thin, mostly acellular distribution |
Thick, with a few cells distributed |
| mouth | Upward, with hanging lips | down |
| nerve | tentacleless handbag | tentacles |
| skeleton | Some have calcareous bones | no bones |
| water pipe | none | have |