Echiura (scientific name: Echiura), a small phylum of the animal kingdom, is a kind of benthos in the living sea, mainly living in the sediment, rock crevices in shallow sea, and burrowing in coral reefs or shells.There are about 150 known species.Urechis and stellate insects live in similar environments, but are mainly distributed along the coast of warm waters.
Echiura is similar to starworm in habitat, but mainly distributed along the coast of warm waters, and a few species are also found in polar waters.Body length varies from several millimeters to 40~50cm.There are about 140 species of echiurus, whose body is worm like, not segmented, and the snout cannot stretch.Closed pipe circulation system。Spiral cleavage, withTrochlear larvaPeriod.embryonic developmentIt is similar to annelids in early stage.Therefore, it is generally believed that the flatworm is a branch evolved from the ancestors of annelids and polychaetes[1]。
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There are about 140 species of Echiura, and a few species of Echiura are also found in polar waters, ranging in length from several millimeters to 40~50cm.The body is worm like, not segmented, and the kiss cannot stretch.The closed tube circulatory system has 1-3 pairs of metanephric tubes (a few species have many pairs), 1 abdominal nerve cord and 1 nerve ring.There is a pair of rectal sacs at the end of the rectum, which open in the body cavity and may be related to respiration and excretion.Spiral cleavage, with trochlear larval stage.Early embryonic development is similar to annelids.Therefore, it is generally believed that Urechis is an evolutionary branch of annelid polychaete ancestors[2]。
Urechis
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Urechis are allMarine benthos, distributed in various sea areas, ranging from intertidal zone to several thousand meters deep sea, but mainly in shallow sea bottom sediment, rock crevices andCoral reefMiddle, or gastropods or sea urchins live in caves in empty shells.About 100 species have been reported.
Urechis signs
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Urechis are 15mm-500mm long, most of them are not more than 10cm long, cylindrical or long cystic, and the body is composed of snout and trunk, such as Echiurus. The snout is a flat projection at the front of the body, actually its head leaf, which is homologous with the preoral leaf of annelids.The snout cannot be retracted into the trunk, and its edge curls to the ventral surface, forming a gutter in the center, with cilia on the surface.The base edge of the snout healed and formed a funnel shape around the mouth.The tip of the snout is shovel shaped, but some species have a bifurcate tip, such asBonellia （Bonellia）。The length of the snout varies greatly, generally shorter than 1/2 of the trunk, but some species have extremely long snouts, such as Urechis furnacalis, whose trunk is 8cm long, and its snout can be 1m longUrechis japonica(Ikeda) is 40cm long, and its snout is 1.5m long, which is used for feeding.
The body is generally light gray or brown, and some species are green, rose, or transparent.The trunk is cylindrical with smooth surface, or a large number of mastoids are scattered, or the mastoids areCircular arrangement。The mucus secreted by the mastoid process can form acupoints.There are two large curved bristles on the front and ventral surface of the trunk, and some species also have 1-2 circles of small bristles at the end. These bristles also come from the rigid hair follicle, and there are muscles to control its movement. These bristles are used to fix the body and clean the acupoints.
The Body Wall Structure of Urechis
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The body wall structure of Urechis is similar to that of annelids, with thin cuticle on the body surface (not in the snout ditch),epithelial cellsThere arePigment granuleThe muscles are arranged in sheets or bundles, and the longitudinal muscle fibers are developed.
The body cavity is developed. It is located in the trunk to form a wide cavity. There is a body cavity membrane around it, and there are some tubes or cavities at the snout. There is a partition between the body cavity and the trunk, but the body cavity fluid is connected.From the perspective of genesis, the muzzle cavity comes from the blastocyst cavity at the embryonic stage, while the trunk cavity comes from the true body cavity formed by the mesoderm.The coelom fluid contains spherical coelom cells, and the cells containhemoglobinOr pigment particles.
Feeding mode of Urechis
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Most Urechis are sediment feeders, and there are roughly two feeding modes: one is that when feeding, the snout protrudes from the acupoint, the ventral surface is upward, and the mucus on its surface sticks to food particles, then enters the oral canal, and is sent to the entrance along the ciliary groove;Another way of feeding is similar to the piloptera of polychaetes, such asUrechis spinosus(Urechis), with a U-shaped acupoint, its kiss is very short. When feeding, a circle of mucus papillae behind the bristles at the front of the trunk contacts the tube wall, constantly secretes mucus, and the body constantly moves backward. As a result, a mucus funnel net is formed around the front of the body. The swing of the trunk causes water to flow through the acupoints, and food particles in the water adhere to the funnel net,When the food has accumulated to a certain amount, the net will break away, and the food will be wrapped into a food ball with a kiss before swallowing.The digestive tract is very long. The mouth is located at the base of the snout. After passing through the esophagus and stomach, it is the intestine. The intestine is highly convoluted. The anus is opened at the end of the body through the rectum.
Except Urechis, Urechis all have a closed circulatory system. There is an abdominal blood vessel in the center of the body's ventral surface. It goes forward to the base of the snout and starts to bifurcate, then converges into a dorsal blood vessel, and then goes through the perienteric blood vessels to connect with the abdominal blood vessels.The blood is colorless and contains a few deformed cells.The coelomic fluid contains many coelomic cells, and some coelomic cells contain hemoglobin, which may help complete the circulation.
Urechis exchange gas with body surface.The excretory organ is the metakidney, which is cystic.The number of kidneys varies from species to species, with one pair of Urechis furnacalis, two pairs of Urechis furnacalis and up to 100 pairs of Urechis japonicus.If only 1-2 pairs, the opening of the renal pore is behind the ventral seta.Urechis has an anal sac at the end of the rectum, with many ciliated funnels in bundles on its surface. One end opens to the body cavity, similar to the renal orifice, and the other end enters the rectum.It is generally believed that it is a changed metakidney with excretory function, but its metabolites are discharged through the anus rather than through the renal pores.
The nervous system is mainly composed of the ventral nerve cord, which forms a ring after entering the snout and has no brain. The lateral nerve is given by the nerve cord along the way. The lateral nerve goes back to form a ring and enters the body wall. Only the epithelial cells, especially the snout, have tactile and chemical sensory cells.
Reproductive mode of Urechis
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Urechis are hermaphroditic. The germ cells come from the coelom and mature in the coelom. The gametes are discharged from the body through the renal capsule. Generally, the eggs of Urechis furnacalis are fertilized in seawater, but the eggs of Urechis furnacalis are fertilized in the female renal capsule.Urechis furnacalis is hermaphroditic, such asUrechis viridis(Bonellia Viridis) The female trunk is 8cm long, while the male trunk is only 1-3mm long. The male lives in the female kidney sac or body cavity.The male body is covered with cilia and has no digestive and circulatory system, leaving only the reproductive structure.Its sex is determined by the living environment of the larvae.If the newly hatched larvae contact adult females, they will develop into males under the influence of their estrogen.The larva first contacts the snout of the female, and then enters the renal capsule from the mouth a few days later. After 1-2 weeks of development, it develops into a male. Each renal capsule can contain about 20.If the larva does not contact the adult female, it will develop into a female and become sexually mature one year later.
After fertilization, the egg undergoes spiral cleavage and develops into a free swimming trochlear larva. During its development, there are segmented embryonic bands and 10 pairs of coelomic cysts arranged in segments. Accordingly, the nerves, circulation and metanephridium also show temporary segmentation, and form a similar body wall structure of annelids.All these indicate the relationship between Urechis and annelids, so it is generally believed that Urechis is from the early ancestor polychaetes, or that Urechis can be divided into this branch after the appearance of segmentation.
