The development of the lancelet and its systematic position have long been a mystery. Now scientists know for certain that this representative of the Chordata type has an indirect development.
General characteristics of the Chordata type
Fish, amphibians, reptiles, birds, mammals - all these animals are representatives of the Chordata type. What unites such different organisms? It turns out that they all have a common building plan.
At the base of their body is an axial skeleton called the notochord. In the lancelet, it persists throughout life. Above the notochord is the neural tube. During metamorphosis, in most representatives of the type, the spinal cord and brain are formed from it. Under the axial skeleton is the intestine, which looks like a tube. In the pharynx of chordates are gill slits. In species that live in water, this feature is preserved, while in terrestrial species it is characteristic only for embryonic development.
The story of the discovery of the lancelet
Why did the development of the lancelet cause a lot of controversy and questions for a long time? The fact is that for a long time it was considered a mollusk. Lancelet (the photo below illustrates its external structure) is reallyreminiscent of these animals. It has a soft translucent body and lives in the aquatic environment - in the shallow waters of the seas and oceans. But the peculiarities of the internal organization made it possible to single them out as a separate systematic unit.
In addition, thanks to the work 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, as it perfectly adapted to its habitat and lifestyle in the complete absence of competitors.
Features of the external structure
Due to the shape of the body, this animal has an unusual name - the lancelet. The photo shows that this organism resembles an old surgical instrument, which is sharpened on both sides. It's called the lancet. This similarity perfectly illustrates the features of the external structure.
The body of the lancelet reaches a maximum length of 8 cm. It is squeezed from the sides and pointed at the ends. On the one hand, the longitudinal fold of the body forms fins - dorsal and caudal. The rear end of the body of the lancelet is buried in the sand. On the front is a preoral funnel surrounded by tentacles.
Skeleton and musculature
The development of the lancelet is characterized by the preservation of the chord throughout its life. In the form of a strand, it stretches along the entire body from the anterior to the posterior end. On both sides of the chord are located along a number of muscles. This structure of the musculoskeletal system allows the lanceletmove in the same way. Muscle contractions lead to flexion of the body, and with the help of the chord, it is straightened.
Internal structure
The organs of the lancelet form all physiological systems. The digestive tract is represented by the mouth opening, pharynx and a through tubular intestine with a hepatic outgrowth, which performs the function of a gland. According to the type of nutrition, lancelets are heterotrophic filter feeders. This process is closely related to breathing, which is carried out through the gills and the entire surface of the body.
The excretory organs also open into the peribranchial cavity. They are represented by numerous paired tubules - nephridia. The circulatory system of lancelets is open. It consists of the abdominal and dorsal vessels.
The reproductive organs of the lancelet are called gonads. These are paired glands, the number of which can reach up to 25. Lancelets are dioecious animals. Therefore, they develop ovaries or testes. These animals do not have reproductive ducts. Therefore, cells enter the peribranchial cavity when the gonads or body walls rupture.
Reproduction and development
Reproductive organs of lancelets provide their external fertilization. The gametes enter the water, where their fusion takes place. Females spawn after sunset in all seasons except winter. Their germ cells contain very little yolk and are characterized by small sizes - about 100 microns.
Even before the start of crushing, the contents of the lancelet eggs are differentiated into three germ layers: ecto-, meso- andendoderm. In the course of subsequent divisions, each of them forms the corresponding organ systems.
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 reproduction of lancelets, as well as their further development, are closely related to water. A larva develops from a fertilized egg in 4-5 days. It has a size of up to 5 mm and floats freely in the water column thanks to numerous cilia. The larval stage lasts about 3 months. At night, it rises to the surface of the water, and during the day it sinks to the bottom.
Amphoxides - this is the name of the giant larvae of the lancelet, which are a phenomenon of the animal world. At first they were mistaken for adults. But in the course of numerous studies, it was found that they live only on the surface of the water as part of plankton. Amphioxides, which can reach 11 mm, retain all the features of the larval structure. Their body is covered with cilia, mouth tentacles, peribranchial cavity and gonads are practically not developed.
So, lancelets are primitive marine chordates. They belong to the subtype Cranial, class Cephalic. Lancelets are characterized by a sedentary lifestyle, being dioecious animals with external fertilization and an indirect type of development.
