Lanceolate slug - that's what this mysterious animal has been called for a long time. Now scientists are precisely aware of all the vital processes of the most primitive representative of the Chordate type. The appearance, internal structure of the lancelet and the features of its physiological processes will be considered in our article.
Discovery History 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 studies and the structure of the lancelet showed that this organism is an ancient chordate. All vertebrate animals originate from it .
In nature, the lancelet can be found at the bottom of the seas and oceans. He lives buried in sand at a depth of 25 meters. The larvae of this animal are found in plankton, a combination of plants and animals on the surface of the water. If the sand is too loose, the lancelet burrow deep into it, exposing only a small part of the front end of the body. If the bottom surface consists of silt, they simply lie on its surface. Lancelet can even move between particles of wet sand.
These animals prefer to settle in colonies, the number of individuals in which reaches thousands of individuals. Through seasonal migrations, together they cover distances of several kilometers.
The 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 is called a lancet. The body of the animal is flattened from the sides. The front end is pointed and the rear end is obliquely cut. On the ventral and dorsal sides, the integuments form folds that merge in the rear of the body into a lanceolate caudal fin. The dimensions of this animal are small - up to 8 cm.
Integument
The external structure of the lancelet is, first of all, the cover of the body. It is represented by integumentary tissue - a single-layer epithelium. 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 quite primitively. The skeleton is represented by a chord, which runs along the entire body from the front end to the rear. The muscles have the appearance of two cords. They stretch on both sides of the axial strand. This structure allows the lancelet to carry out only uniform movements. With the help of muscles, he bends the body in one direction. The chord also acts as a counterweight - straightens the lancelet.
Features of the internal structure of the lancelet
Its internal structure is the most primitive among the chordates. Their type of food is passive. These animals are filtrators. The digestive system is pass-through. It consists of an oral opening, pharynx, and tubular intestine with a hepatic outgrowth. Lancelet feeds are small crustaceans, ciliates, various types of algae, and larvae of other chordates.
Water filtration is closely interlinked with the breathing process. On the walls of the pharynx are many cells with cilia. Their action creates a constant flow of water, which passes through the pharynx and gill slits. Gas exchange is also carried out here. After that, water is released out 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 perigastric cavity.
The circulatory system is open. It consists of two vessels - the abdominal and dorsal. The heart is missing. Its function is performed by the abdominal vessel, due to pulsation of which blood circulation occurs. It mixes with the abdominal fluid, washing all the internal organs and thus carrying out gas exchange.
The nervous system is represented by a tube located above the chord. It does not form thickening, so the brain of the lancelet is absent. Such a primitive structure of the nervous system determines the weak development of the senses. They are represented by the olfactory fossa located on the front end of the body. It is able to perceive chemicals in water in a dissolved state. Tentacles are also located here, which serve as the organ of touch. Photosensitive cells are located along the neural tube.
Reproduction and development
The internal structure of the lancelet determines the type of reproductive system. These are dioecious animals with external fertilization. The development is indirect, as larvae develop from the egg, which initially swim in the water and look like fish fry. They feed, grow, and after some time sink to the bottom, burrowing in the sand with one end of the body. Lancelet life expectancy is 3-4 years.
The value of the lancelet in nature and human life
In the countries of Southeast Asia, the lancelet is eaten. Moreover, in this region they have been the subject of fishing for several hundred years. Fishermen catch them directly from boats in the period from August to 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. The first dishes are prepared from it, it can be fried, boiled or dried for export. The meat of this animal is very nutritious, rich in protein and fats.
Lancelet larvae are primitive marine chordates belonging to the class Cephalic chord subtypes Cranial. They lead a sedentary lifestyle and feed on by filtration. Currently, they are not only the subject of fishing, but are also used for scientific research, since the study of their origin and systematic position in the animal kingdom system made it possible to determine patterns in the evolution of chordates.