Jellyfish are animals of the phylum Cnidaria (class Medusae, Crustacea, and Cubojellyfish). They existed as early as 650 million years ago, even before the dinosaurs appeared. There are more than 250 species of jellyfish in waters around the world, and they are distributed in waters around the world. All live in the ocean.

Jellyfish are usually single and live a floating or swimming life. A few species live in groups, and some groups can live a fixed life. The jellyfish-shaped body is bell-shaped or inverted bowl-shaped, or umbrella-shaped. The protruding side is called the outer canopy or upper canopy, the concave side is called the lower canopy, and there is a drooping tube in the center of the lower canopy. The free end of the hanging lip is the mouth, and there is a circle of tentacles on the edge of the umbrella. The edge of the lower umbrella of the jellyfish extends inward to form a narrow membrane-like structure called the limbal membrane. The limbal membrane is a characteristic of the jellyfish of the class Hydrozoa. . All jellyfish in the jellyfish class do not have membranes. The body wall structure of the jellyfish type is basically similar to that of the polyp type. It is also composed of two layers of epithelial muscle cells with a mesoglea layer sandwiched between them. However, the mesoglea layer of the jellyfish type is much more developed than that of the polyp type. The gastric circulation cavity surrounded by the body wall is also relatively developed. It is either a simple sac, or it is divided into 4 gastric sacs by the membrane, and 4 (Hydrozoa) or more (Boss) extend from the gastric sac to the fimbria. Jellyfish) radiate tubes and are connected to the ring tubes parallel to the umbrella edge. Small centrifugal tubes can also be extended from the ring tubes into the tentacles and reach the end of the tentacles. There are sensory organs at the edge of the fimbria or at the base of the tentacles, such as eyespots or balance sacs.

The radial tubes, tentacles and sense organs of most jellyfish have four-radial symmetry. Tentacles are one of the important structures of cnidarians, although some polyps and jellyfish lack tentacles entirely. Tentacles can basically be divided into two shapes. One is capitate and short, with stinging cells concentrated at the end of the tentacle to form a cap-like structure. The other is filamentous and slender, with stinging cells forming a ring along the entire length of the tentacle. Distributed like or tumor-like, these two kinds of tentacles either exist alone in different species or exist together. The number, structure, and arrangement of tentacles vary in different species. The tentacles are either hollow due to the extension of the gastric cavity, or they are filled with gastric cavity cells and have a solid structure. The number of tentacles often increases with the age of the animal, and the base of the tentacles is often enlarged, which is the result of the concentration of sensory cells or stinging cells.

The jellyfish generation alternates, but the polyp type degenerates or disappears, and the jellyfish type generation is dominant. Single, mostly large jellyfish, with thick mesogelae and complex structure. The nerve senses are relatively developed, with tentacle sacs, eye spots, balance stones, olfactory fossa and other structures, and functions such as photoreception, balance and chemical receptors. The lack of membrane is the main difference between jellyfish and jellyfish. The digestive cycle cavity is complex, the radial tubes are well developed, and the gastric filaments originating from the endoderm have stinging cells, so there are stinging cells in both the endoderm and the ectoderm of the jellyfish. Gonads originate from the endoderm.

Different jellyfish have different shapes. The main component of the jellyfish body is water. The water content in the body can generally reach more than 98%. It is composed of two germ layers, the inner and outer germ layers. There is a thick middle layer between the two layers. The glue layer is not only transparent, but also has a floating effect. Others are made of proteins and lipids, so the jellyfish's body is transparent. They have no heart, blood, gills, or bones. The umbrella body of ordinary jellyfish is not very large, only 20-30 cm long. When they move, they use the water jet reflex in their bodies to move forward, just like a round umbrella floating rapidly in the water.

There is a special gland in the jellyfish's umbrella body that can emit carbon monoxide and cause the umbrella body to expand. There is a small ball on the thin handle in the middle of the jellyfish tentacle, with a small listening stone inside, which is the "ear" of the jellyfish. The infrasound waves generated by the friction between waves and air impact the hearing stones and stimulate the surrounding nerve receptors, allowing the jellyfish to obtain information more than ten hours before the storm arrives, and all of them disappear from the sea surface.

The jellyfish of the class Medusa are relatively large in size. The diameter of the umbrella edge is generally between 2-40 cm. The diameter of some large species can reach 1-2 meters, such as a kind of jellyfish. The umbrella is disc-shaped, like a moon jellyfish, or cone-shaped, semicircular, butterfly-shaped, etc. depending on the species. Because the gonads or other stomach pouches and other structures in the body have color, the body appears partially pink, orange, etc. in the transparency.

The body is also divided into an upper umbrella surface and a lower umbrella surface. The edgeless membrane and umbrella edge have a circle of tentacles. Different species have different numbers of tentacles. The tentacles may be solid or hollow, long or short. There are also a few species without tentacles, such as Genshui. parent class. The umbrella edge has sensory organs, also called tentacle sacs. The number is 4 or a multiple of 4. The moon jellyfish has 8, which are notched, so the umbrella edge is divided into 8 pieces. The end of the vertical lip in the center of the lower umbrella extends outward to form 4 or 8 orifices, and there is a groove on one side of the orifices toward the center. The mouth and wrist have the function of catching prey. When feeding, tiny food can enter the moon jellyfish along the mouth and wrist groove. The mouth and wrist of the root mouthworm heal, and the mouth at the end of the hanging lip is closed, forming many new small suction mouths for sucking. Smoking. There are generally stinging cells distributed on the outer surfaces of the tentacles, labial lips, oral wrists and umbrellas. The gonad area of ??some species is sunken inward to form the hypogenital fossa. Its function is unknown and may be related to the animal's respiration.

Boll jellyfish have a well-developed mesoglea, which is also a gel formed of proteins and mucopolysaccharides and contains collagen fibers. Different from polyps, there are free deformed cells of ectodermal origin in the mesoglea. These deformed cells play an important role in animal regeneration and tissue repair. The mesoglossum layer is also very elastic. Because it can maintain and adjust the composition and concentration of ions, it allows the body to maintain a certain buoyancy in seawater.

The muscles and movements of jellyfish are similar to those of polyps. There are circular muscles formed by ectoderm around the lower edge of the umbrella, and there are longitudinal muscle fibers on the tentacles. Their contraction causes the movement of the jellyfish.

Jellyfish are carnivorous and feed on plankton, small crustaceans, polychaetes and even small fish. Due to the mechanical stimulation and chemical stimulation of food, hydrozoans extend their tentacles and release nematocysts to wrap, paralyze, and poison the prey, and then bring the food into their mouths. The mucus secreted by the glandular cells in the oral area is beneficial to the swallowing of food. After the food enters the gastric cavity, the glandular cells in the gastric layer begin to secrete proteases to decompose and digest the food to form many polypeptides. At the same time, due to the flagellar movement of the nutritional muscle cells in the gastric cavity, the food be mixed and promoted. After this extracellular digestion, the intracellular digestion process begins. The pseudopodia that nourish muscle cells engulf food particles and form a large number of food bubbles within the cells. After acidic and alkaline chemical processes, nutrients diffuse from the cells. Effects are delivered throughout the body.