Lancet - what kind of animal? Lancet: description, structure, features and features

Lancetz is an animal belonging to the Chordovye type (the group of the Cranial). We suggest you get to know him better. We will consider the structure, features and features of the lancelet. This species is found in the temperate and tropical seas. In particular, lancelets live in the Black Sea.

Body shape

The body shape of the lancelet is oval, tapering towards the tail. The length is from 4 to 8 cm. The body is covered with a single-layered epithelium and cuticle, which is secreted by epithelial cells. Under the epithelium is a thin layer of connective tissue - corium. The dorsal side of the trunk and tail are bordered by the dorsal fin fold. The body is flattened laterally; Along the ventral side there are paired metapleural folds joining with the subarachal part of the fin fold. This structure facilitates the stabilization of the body position in the aquatic environment and increases the area of contact with it, which is important when moving with the help of bends of the body.

Musculoskeletal system

The basis of the support system is the chord, located along the body from the head to the tail end. The chord that the lancelet has is an education enclosed in a connective tissue cover, the processes of which form the supporting elements of the fin fold, which look like cartilaginous columns. It also penetrates between individual portions of the musculature (myomers), forming the separating myosepts. The myomers, separated by myosepts, adjoin the chord from both sides.

Musculature

Musculature is striated. Individual myomers have a conical shape and, as it were, are pushed one into the other, and their arrangement is asymmetric: opposite to the middle of each of them, on the right side there is a myosept on the left and vice versa. Consecutive reduction of muscle segments causes lateral bends of the body, wavyly moving from head to tail. The natural nature of muscle contractions is regulated by the central nervous system.

The chord of the lanceolate at the anterior end extends farther than the neural tube, hence the name of the only class of this subtype is the Holohovard. It is composed of a large number of transverse muscle plates surrounded by a connective tissue membrane. Individual plates are separated by cavities, but contact with adjacent ones by means of short outgrowths.

Circulatory system

It is generally believed that the circulatory system of this animal is closed. Indeed, there are no bloody lacunae here, but there are no real capillaries (the finest vessels with single-layered walls): from small arterial vessels blood enters the intercellular spaces (functionally they correspond to capillaries), and from here it gathers into small veins.

Sexual system

The sexual system of the lancelet is represented by a series of paired gonads (gonads). They are located at the walls of the atrial cavity. Cranial (lancelet including) dioecious. The gonads of males and females are outwardly similar and differ only in the size of the sex cells (the egg cells are larger). Ripe germ cells through the rupture of the wall of the gonad fall into the atrial cavity, from where the water flow outward through the atriopor. Development occurs in the aquatic environment and is characterized by the fact that there is a larval stage.

The structure of the larva

The structure of the larva is very peculiar: in the early stages it is distinguished by an asymmetric arrangement of the mouth (on the left side) and gill slits (on the right side), the number of which is initially small. Only later one row of gill slits moves to the abdominal, and then to the left side, and the mouth moves downward. The number of gill openings is increased by breaking through additional gaps and forming longitudinal baffles into existing gaps. The larva actively moves with the help of cilia covering the body, and later lateral bends of the trunk, like adult forms. It is characterized by active feeding of small plankton animals. A perirotal funnel with tentacles is formed only towards the end of larval development, when the larva, like an adult organism, passes to life on the bottom.

Atrial cavity

Lancet is an animal with an atrial cavity. It opens the opening of the gill slits and metanephrilia. Here, mature eggs and spermatozoa arrive. All this indicates the connection of the atrial cavity with the external environment. Anatomically, this connection is represented by an opening called an atriopora, and opens outward on the ventral side in the back of the body, in front of the anal opening. The connection between the atrial cavity and the external environment is most clearly manifested in the nature of its formation.

Longitudinal metapleural folds form in the lancet larva in the lateral walls of the body above the gill openings. They gradually expand in the transverse direction and down, and then grow together. The space between these folds and represents the atrial cavity. In other words, the atrial cavity that the lancelet has is part of the external environment captured by metapleural folds. It covers the pharynx, part of the intestine, most of the surface of the gonads, occupying vast space and substantially displacing the whole, which remains only in the form of paired cavities along the upper edge of the pharynx, as well as in the endostyle and metapleural folds.

The biological significance of the atrial cavity is related to the character of the lancelet lifestyle. When buried in the ground, the water out of the gill slits becomes difficult, the soil particles can enter the gill holes and clog them. Formation of the atrial cavity, the walls of which separate the gill openings from contact with the ground, removes this obstacle. Water, with a force coming out of the atriopor, blurs the soil in this place, providing free breathing and excretion of metabolic products. As already mentioned, the larvae of the lancelet, which lead the pelagic life, have no atrial cavity. There are also several species of the family Amphioxididae, leading the pelagic way of life and in the adult state.

These are the main features of the lancelet - one of the representatives of chordates.