Lanceolate slug - this is how this mysterious animal was called for a long time. Now scientists know exactly all the life processes of the most primitive representative of the Chordata type. The appearance, internal structure of the lancelet and the features of its physiological processes will be discussed in our article.
History of discovery and habitat
Back in the 18th century, the famous Russian traveler and scientist Peter Simon Pallas discovered a translucent small creature in the waters of the Black Sea. Outwardly, it resembled a mollusk. Further research and structure of the lancelet showed that this organism is an ancient chordate. All vertebrates originate from it.
In nature, the lancelet can be found at the bottom of the seas and oceans. They live, buried in the sand, at a depth of up to 25 meters. The larvae of this animal are found as part of plankton - a collection of plants and animals that are on the surface of the water. If the sand is too loose, the lancelets burrow into it very deeply, exposing onlya small part of the anterior end of the body. If the bottom surface consists of silt, they simply lie on its surface. Lancelets can even move between wet sand particles.
These animals prefer to settle in colonies, the number of individuals in which reaches a thousand individuals. Making seasonal migrations, together they overcome distances of several kilometers.
External structure of the lancelet
The structure of the lancelet, or rather the shape of the body, determined its name. In appearance, it is very similar to a surgical instrument. It's called the lancet. The body of the animal is flattened laterally. The anterior end is pointed and the posterior end obliquely cut. On the ventral and dorsal sides, the covers form folds, which merge into a lanceolate caudal fin in the back of the body. The size of this animal is small - up to 8 cm.
Sheaths
The external structure of the lancelet is primarily the cover of the body. It is represented by an integumentary tissue - a single-layer epithelium. From above it is covered with a thin layer of cuticle. Like fish, epithelial cells secrete a lot of mucus that covers the entire body. Under the integumentary tissue is a layer of connective tissue.
Skeleton and musculature
The structural features of the lancelet are determined by the system that provides support and movement. It is arranged rather primitively. The skeleton is represented by a chord that runs along the entire body from the anterior end to the posterior. Musculature has the appearance of two strands. They stretch on both sides of the axial strand. This structure allows the lancelet to carry out only monotonous movements. With the help of muscles, he bends the body in one direction. The chord acts as a counterweight - it straightens the lancelet.
Features of the internal structure of the lancelet
The internal structure of its most primitive among the chordates. Their type of food is passive. These animals are filter feeders. Digestive system through. It consists of a mouth opening, pharynx and tubular intestine with a hepatic outgrowth. The food source for the lancelet is small crustaceans, ciliates, different types of algae, larvae of other chordates.
Water filtration is closely related to the process of respiration. On the walls of the pharynx there are many cells that have cilia. Their action creates a constant current of water that passes through the pharynx and gill slits. This is where gas exchange takes place. After that, water is released outside through the gill pore. Additionally, the absorption of oxygen and the release of carbon dioxide occurs through the integument of the body.
The lancelet has specialized excretory organs. They are called nephridia. These are numerous paired tubules. They completely penetrate the body, and at one end open outward into the peribranchial cavity.
The circulatory system is open. It consists of two vessels - abdominal and dorsal. The heart is missing. Its function is performed by the abdominal vessel, due to the pulsation of which blood circulation occurs. It mixes with the cavity fluid, washing all the internalorgans and thus exchanging gas.
The nervous system is represented by a tube located above the chord. It does not form a thickening, so the lancelet does not have a brain. Such a primitive structure of the nervous system also causes a weak development of the sense organs. They are represented by the olfactory fossa located at the anterior end of the body. It is able to perceive chemicals that are in water in a dissolved state. The tentacles are also located here, which serve as an organ of touch. Along the neural tube are light-sensitive cells.
Reproduction and development
The internal structure of the lancelet determines the type of the reproductive system. They are dioecious animals with external fertilization. Development is indirect, since larvae develop from the egg, which initially swim in the water and outwardly resemble fish fry. They feed, grow, and after a while they sink to the bottom, burrowing into the sand with one end of their body. The life span of the lancelet is 3-4 years.
The meaning of the lancelet in nature and human life
In the countries of Southeast Asia, lancelets are eaten. Moreover, in this region they have been an object of fishing for several hundred years. Fishermen catch them directly from their boats between August and January, a few hours after low tide. To do this, use a special device. It is a sieve on a bamboo pole. Several tens of tons of lancelet are caught during the year. First courses are prepared from it, it can be fried,boil or dry for export. The meat of this animal is very nutritious, rich in protein and fat.
Lancelets are primitive marine chordates belonging to the class Cephalochordidae of the subtype Cranial. They lead a sedentary lifestyle and feed by filtration. Currently, they are not only an object of fishing, but are also used for scientific research, since the study of their origin and systematic position in the animal world made it possible to determine patterns in the evolution of chordates.
