Nemathelminthes Formerly known as "Circularia", it is a phylum in the animal kingdom.Most of them are small vermiform animals, which are usually long cylindrical, thin at both ends, not segmented, with a primitive cavity, and the digestive tract is not curved. The front end is the mouth, and the rear end is the anus. They are hermaphrodite.Many representatives are parasites, such as clematis, or floating or benthic life in the sea or fresh water.The earliest fossil (Scorpiophagus baculiformis) occurs in the Lower Carboniferous.[1]
The linear animal body is linear, uniform in thickness, 0.5~1mm.IntegumentCuticle membrane；Alimentary canalDegenerate, often without mouth, absorb host nutrition by body wall;The protocoelom is filled with stroma;The male is smaller than the female.Adults live freely in rivers, ponds and other fresh water;Eggs are laid in spring, and the eggs are sticky in cord shape;Larvae live underwater, looking for opportunities to burrow into or be swallowed into insects and other host bodies, which is called parasitism.Common examples areClematisGordiacea，The adult is 10~30cm long, like a rusty iron wire, aquatic, drought resistant
The adult is generally about 30-40cm long, 0.5mm in diameter, and the longest can reach 1.5m;3mm diameter.Females are generally longer than males, with dark brown or black body surface.There is no obvious head in the body, with mouth in the front end and sharp and thin rear end, or bifurcated into two or three leaves.
Body wall and false body cavity
The structure of the body wall of the adult worm is similar to that of the ascaris lumbricoides. The outer surface has a thick cuticle. The epithelial cells are clearly demarcated. The epithelium protrudes in the ventral surface to form an upper leather cord, or there is an upper leather cord on the back, but there is no lateral cord. The muscle layer has only longitudinal muscles and acyclic muscles. Except for the well-developed body cavity of the nematode, other kinds of body cavities shrink, which contain more stroma and cells.
digestive system
Starting with linear animals,digestive tractAppearsanusThere are two openings: mouth and anus. Food passes through the mouth, pharynx, intestine and rectum, and then is discharged from the anus, so that the digested and absorbed food will not be mixed with the new foodIncomplete digestive systemMore perfect and highly differentiated, which is of great significance in evolution.However, the digestive system of linear animals is more degraded than that of nematode animals.
excretory system
Linear animals have no excretory organs[2]。
Nervous system and sensory organs
Linearcentral nervous systemAt the front end, there are peripharyngeal nerve rings surrounding the pharynx and those connected with themLateral gangliaAnd the ventral ganglia, which are cylindrical nervous systems.Six nerves are sent forward from the parapharyngeal nerve ring to the sensory mastoid at the front end, and six are sent backwardNerve cord1 dorsal nerve cordVentral nerve cord2 dorsal nerves and 2 ventral nerves, of which the dorsal and ventral nerves are the most developed.All nerve cords are embeddedepidermisThe dorsal nerve cord and the ventral nerve cord are embedded respectively in the dorsal and ventral lines of the epidermis protruding inward.
reproductive system
It is dioecious, with two gonads, extending to the full length of the body.The end of the male reproductive tube passes into the rectum or cloaca, and there are mating thorns.The ovary has a special structure, including two lateral caecum, in which the egg develops and matures.When the egg is mature, it is in the cloaca through the opening of the fallopian tube, and there is also a seminal vesicle opening in the cloaca.During mating, males surround females and produce sperm around female genital pores. The sperm actively swim into female seminal vesicles.During the mating season, multiple insect bodies are often intertwined, and the female is not good at movement. After mating, she lays eggs in the water.The eggs hatch in water, and the newly hatched larvae have a retractable snout with thorns on the snout.
The development of linear animals can be divided into three stages: egg, larva and adult.Most eggs are oval,EggshellThe structure of the egg is more complex. The degree of maturity of the newly discharged egg often varies with the species.Larvae can secrete mucus to form cysts. Larvae can be eaten by their hosts or invade the body through the host surface. Their hosts are mostly aquatic beetles or locusts, crickets, and even leeches living near the water.The larva of marine nematode passes through the body wall of decapod crustaceans with its snout and enters the blood cavity.The digestive enzymes secreted by the body wall of the larva can dissolve the host tissue and be directly absorbed through the body wall.Larvae peeled several times in the host, and when the host came to the water again, it drilled out from the host to form an adult, living free.Adults soon become sexually mature and begin to mate and lay eggs again.
life style
The living environment and life history of linear animals are complex and diverse. Generally speaking, they can be divided into free livingSaprophyticAnd parasitic.Free living and aquatic species are generally benthic,Marine productsThe largest number of, from the beach to the very deep seabed, sometimes numerous, become the seabedmulticellular The largest number of animals. Freshwater living species exist in various freshwater environments, even in rapids. The mucus secreted by their caudal glands is often fixed to objects in water;In the environment where the temperature of hot spring is up to 37 ℃, some species can also be seen sometimes.Terrestrial species often live under soil, tree roots and fallen leaves.Some nematodes can even live in aquatic and terrestrial life, belonging to an amphibious type.It is obvious that nematodes originate from the sea, and then enter the land through amphibious life. From the perspective of flora classification, the species of marine and freshwater products are almost completely different, so the species of freshwater products are likely to enter the land first from marine products, and then invade the freshwater waters.The species that live in parasitism can live in animals or plants, and some adults live in parasitismParasitic life, while larval camp lives freely;Some larvae live in parasitic camps, while adults live in free camps;Or the larvae and adults all live as parasites, and there are different kinds of them.as forSaprophyticLiving is a group of animals and plants living in corpses, or eating dead animals and plants.It can be considered that,SaprophyticLife is the intermediate stage of transition from free life to parasitic life, such as many species of Rhabditis, which feed on the dead bodies of animals and plants, while some canAnaerobic respirationTherefore, they can pass through the animal's digestive tract without harm to themselves, do not proliferate in the intestinal tract, but are not digested and will not cause death.
