Chord type: structure and development of lancelet

The development of the lancelet and its systematic position remained a mystery for a long time. Now scientists are reliably aware that this representative of the Chordovye type has an indirect development.

General characteristic of the Chord type

Fish, amphibians, reptiles, birds, mammals - all these animals are representatives of the Chordovye type. What unites such different organisms? It turns out that they all have a common building plan.

At the base of their body is the axial skeleton, which is called the chord. The lancelet retains it throughout life. A nerve tube is located above the chord. In the course of metamorphosis, in most representatives of the type from it, the spinal cord and the brain are formed. Under the axial skeleton is the intestine, which looks like a tube. In the throat of the chordates there are gill slits. In the species that live in water, this feature persists, while in terrestrial ones it is characteristic only for embryonic development.

History of the discovery of the lancelet

Why was the development of the lancelet for a long time a lot of controversy and questions? The fact is that for a long time it was considered a mollusk. The lancet (the photo below illustrates its external structure) really reminds these animals. It has a soft translucent body and lives in an aquatic environment - in the shallows of the seas and oceans. But the peculiarities of internal organization made it possible to separate them into a separate systematic unit.

In addition, thanks to the works of Peter Pallas and Alexander Kovalevsky it was established that these animals are the ancestors of modern vertebrates. Scientists call these organisms living fossils. It is believed that the lancelet did not evolve, since it has perfectly adapted to the habitat and lifestyle in the absence of competitors.

Features of external structure

Due to the shape of the body, this animal has an unusual name - a lancelet. The photo demonstrates that this organism resembles an old surgical instrument, which is sharpened on both sides. It is called a lancet. This similarity perfectly illustrates the features of the external structure.

The body of the lancelet reaches a maximum of 8 cm in length. It is squeezed from the sides and pointed at the ends. On the one hand, the longitudinal crease of the body forms fins - dorsal and caudal. At the rear end of the body, the lancelet is buried in the sand. On the front there is a pre-funnel, surrounded by tentacles.

Skeleton and musculature

The development of the lancelet is characterized by the preservation of the chord throughout his life. In the form of a strand it stretches along the entire body from the front to the back end. On both sides of the chord are located on a number of muscles. This structure of the locomotor system allows the lancet to move monotonously. The contractions of the muscles lead to bending of the body, and with the help of the chord it is straightened.

Internal structure

The organs of the lancelet form all the physiological systems. Digestive is represented by a mouth opening, pharynx and through tubular intestine with hepatic outgrowth, which serves as a gland. According to the type of food, lancelets are heterotrophic filterers. This process is closely related to breathing, which is carried out through the gills and the entire surface of the body.

The excretory organs are also open in the okolocal cavity. They are represented by numerous twin tubes - nephridia. The blood system of the lancelet is unclosed. It consists of the abdominal and spinal vessels.

The reproductive organs of the lancelet are called gonads. These are paired glands, the number of which can reach up to 25. Lancets are dioecious animals. Therefore, they develop ovaries or testes. These animals do not have genital ducts. Therefore, the cells enter the perigastric cavity when the gonads or body walls break.

Reproduction and development

The reproductive organs of lancelets provide their external fertilization. The gametes are released into the water, where they merge. Females spawn after sunset at all seasons, except in winter. Their sex cells contain very little yolk and are characterized by small dimensions - about 100 microns.

Even before the comminution begins, the contents of the eggs of the lancelet are differentiated into three germinal sheets: ecto-, meso-, and endoderm. In the course of subsequent divisions, each of them forms the corresponding system of organs.

The development of the lancelet gives an idea of the features of this process in chordates. It consists of a number of successive processes: fertilization, crushing, gastro- and neurulation, organogenesis. The lancelet variety, as well as their further development, are closely related to water. A larva develops from the fertilized egg within 4-5 days. It has a size of up to 5 mm and freely floats in the water column due to numerous cilia. The larval stage lasts about 3 months. At night, it rises to the surface of the water, and in the day it sinks to the bottom.

Amphi-oxides are the so-called giant lancet larvae, which are a phenomenon of the animal world. At first they were taken for adults. But in the course of numerous studies it was established that they live only on the water surface in the plankton. Amphi-oxides, which can reach 11 mm, retain all the features of the larval structure. Their body is covered with cilia, the oral tentacles, the circumbranchial cavity and the sex glands are practically not developed.

So, lancelets are primitive sea chordates. They belong to the subtype of the Cranial, Class. Lancets are characterized by a sedentary lifestyle, being dioecious animals with external fertilization and an indirect type of development.