External and internal structure of lancelet

Lancet-like slug - this is the name of this mysterious animal for so long. Now scientists know exactly all the processes of vital activity of the most primitive representative of the Chordov type. Appearance, internal structure of the lancelet and features of its physiological processes and will be considered in our article.

History of discovery and habitat

Back in the 18th century, the famous Russian traveler and scholar Peter Simon Pallas discovered a translucent little creature in the waters of the Black Sea. Outwardly it looked like a clam. Further studies and the structure of the lancelet have shown that this organism is an ancient chordate. From it all vertebrates originate .

The nature of the lancelet can be found on the bottom of the seas and oceans. He lives buried in the sand, at a depth of 25 meters. Larvae of this animal are found in the plankton - a collection of plants and animals that are on the water surface. If the sand is too loose, the lancelets dig into it very deeply, exposing only a small part of the anterior end of the body. If the bottom surface consists of silt, they simply lie on its surface. Lancets even can move between particles of wet sand.

These animals prefer to settle in colonies, the number of individuals in which reaches thousands of individuals. Making seasonal migrations, together they overcome distances of several kilometers.

External structure of lancelet

The structure of the lancelet, or rather the shape of the body, determined its name. It looks very similar to a surgical instrument. It is called a lancet. The animal's body is flattened from the sides. The anterior end is pointed, and the posterior obliquely cut. On the ventral and dorsal sides, the integument forms folds that merge into the lanceolate caudal fin in the posterior part of the body. The size of this animal is small - up to 8 cm.

Covers

The external structure of the lancelet is, first of all, the covering of the body. It is represented by a cover cloth - a single-layered 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 cover cloth is a layer of connective tissue.

Skeleton and musculature

The features of the structure of the lancelet are determined by the system providing support and movement. It is rather primitive. The skeleton is represented by a chord that runs along the entire body from the front to the back end. The musculature has the appearance of two strands. They stretch on either side of the axial strand. This structure allows the lancet to carry out only monotonous movements. With the help of musculature, he bends the body in one direction. Chord acts as a counterweight - straightens the lancelet.

Features of the inner structure of the lancelet

Its internal structure is the most primitive among the chordates. Their type of food is passive. These animals are filterers. The digestive system is through. It consists of a mouth, a pharynx and a tubular intestine with a hepatic outgrowth. The source of nutrition for the lancelet is small crustaceans, infusoria, various species of algae, larvae of other chordates.

The filtration of water is closely interrelated with the process of breathing. On the walls of the pharynx there are many cells with cilia. Their action creates a constant current of water, which passes through the pharynx and gill slits. Gas exchange is also carried out here. After this, water is released through the branchial period. Additionally, the absorption of oxygen and the release of carbon dioxide occurs through the covers of the body.

The lancelet has specialized organs of excretion. They are called nephridia. These are numerous twin tubes. They completely penetrate the body, and one end opens outward into the perigutular cavity.

The circulatory system is not closed. It consists of two vessels - the abdominal and dorsal. The heart is missing. Its function is performed by the abdominal vessel, due to the pulsation of which blood circulation takes place. It mixes with the cavity fluid, washing all internal organs and thus performing gas exchange.

The nervous system is represented by a tube located above the chord. It does not form a thickening, so the brain does not have a lancelet. Such a primitive structure of the nervous system determines the weak development of the sense organs. They are represented by the olfactory fovea located at the anterior end of the body. It is capable of perceiving chemicals that are in the water in a dissolved state. Here are also the tentacles, which serve as the organ of touch. Along the neural tube there are photosensitive cells.

Reproduction and development

The internal structure of the lancelet determines the type of the reproductive system. These are dioecious animals with external fertilization. The development is indirect, as larvae develop from the egg, which initially float in water and outwardly resemble fish fry. They feed, grow, and after a while fall to the bottom, one end of the body burrowing into the sand. The life span of the lancelet is 3-4 years.

The significance of the lancelet in nature and human life

In the countries of South-East Asia, lancelets are used for food. And in this region they have been a target of the fishery 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 pole of bamboo. Within a year several dozen tons of lancelet are caught. 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.

Lancets are primitive sea chordates belonging to the class of Holochordless subtypes. They lead a sedentary lifestyle and feed by filtration. Currently, they are not only an object of fishing, but also used for scientific research, since the study of their origin and systematic position in the animal world allowed us to determine patterns in the evolution of chordates.