Ehiurids

According to numerous molecular studies, echiurids are part of the type of annelids and polychaete class ^[4] ; apparently their close relatives are the polychaetes of the Capitellidae family ^{[5] [6]} .

The body is soft, non-segmented, similar to a sausage; usually 3 to 25 cm long. Equipped with a long, non-retractable proboscis, which is covered with cilia, fitting food particles to the mouth. Echiurids are able to discard the proboscis or part of it, which grows back in a couple of weeks. The mouth is located on the ventral side at the base of the proboscis. Behind the mouth are two abdominal setae involved in digging holes; the posterior end of the body is surrounded by 1-2 rings of bristles, with the help of which the worm cleans its shelter. The presence of bristles brings echiuride closer to annelids . There are glands on the skin that secrete mucus.

In a large coelom, there is a tubular intestine ending in the anus, paired excretory organs - metanephridia and anal sacs, dotted with ciliary funnels - nephrostomes . Anal sacs are a characteristic sign of eheurid; they perform excretory and respiratory functions. The circulatory system is poorly developed; blood is colorless, contains amoebocytes . Coelomic fluid contains red blood cells and white blood cells . In adults, germ cells are also present in it. Coelomic fluid performs respiratory, nutritional and excretory functions. There is an abdominal nerve trunk, but there are no ganglia or specialized sensory organs. The nervous system consists of a peri-pharyngeal nerve ring and an abdominal trunk without ganglionic extensions.

Echiurids dioecious, fertilization is usually external. Worms of the genus Bonellia have the most pronounced sexual dimorphism in the animal kingdom. Dwarf males, greatly simplified in structure, parasitize in the excretory ducts of the female excretory organs, through which mature eggs are brought out; during the passage of the egg through the excretory ducts, fertilization occurs. A larva - trochophora comes out of the egg.

Echiurids are sea worms that often live in great depths. Keep protected places; most burrow in sand or silt, where they live in U-shaped minks. Clefts of rocks, empty shells, shells of sea ​​urchins, and other shelters also inhabit. They feed by filtering detritus with the help of a proboscis. Representatives of the Urechidae family capture bacteria and food particles using body-secreted mucus. Mucus composes a mesh tube that fills the worm's hole; echiurida drives water through it, peristaltically contracting the body. Then the mucous tube is eaten. It was found that the pore size in it is 36–40 angstroms , since the network passes egg albumin molecules, but retains serum globulin molecules.

About 230 species of echiurid have been described, in Russia - 4 species ^[7] . Many features combine them with annelids , while the features of the early development of embryos bring them closer to mollusks . The fossil remains of echiurid are few; the earliest sample dates from the Carboniferous period . However, U-shaped minks, similar to the shelters of modern echiurids, are already known from the Cambrian .

As of April 2018, the following taxa up to and including the family are distinguished in a subclass ^[3] :

• Squad Echiuroidea
  • Suborder Bonelliida
    • Family Bonelliidae Lacaze-Duthiers, 1858
    • Family Ikedidae Bock, 1942
  • Suborder Echiurida
    • Echiuridae Quatrefages Family, 1847
    • Family Thalassematidae Forbes & Goodsir, 1841
    • Family Urechidae Monro, 1927
• Childbirth incertae sedis

Bonelliidae

The Bonelliidae family is characterized by pronounced sexual dimorphism . Most species are green; Bonellin pigment staining them is chemically close to chlorophyll . It is highly toxic to other organisms and, apparently, has a protective value.

The genus Bonellia ( Bonellia ), especially the green Bonella ( Bonellia viridis ), living in the coastal Mediterranean Sea and the East Atlantic, females are hundreds of times larger than males. In female bonellia, the body is cylindrical in shape (up to 7 cm long) and has a long trunk bifurcated at the end, which can extend up to 1 m. Bonella larvae do not bear signs of a certain sex and can develop in both males and females. Larvae settled on a substrate, as a rule, turn into females; the larvae that have settled on the proboscis of adult females are fixed on their skin or proboscis and after a few days turn into males. The male in Bonella is no more than 1-3 mm long and lives like a parasite on or in the body of the female, usually in a special chamber in the excretory ducts of the reproductive system. Sometimes up to 80 males accumulate in them, so they were mistaken for parasites for a long time. The male bonellia lacks proboscis and through intestines; it probably absorbs nutrients through the skin. For sperm excretion, a seed bag develops from the middle intestine and nephridia leading out of it is used ^[8] .

• Urechis unicinctus