Lancelet is an animal of the Chordate type (group Cranious). We invite you to get to know him better. We will consider the structure, features and signs of the lancelet. This species is found in the seas of the temperate and tropical zones. In particular, lancelet live in the Black Sea.
Body shape
The body of the lancelet is oval, tapering to the tail. The length is from 4 to 8 cm. The body is covered with a single-layer epithelium and cuticles secreted by epithelial cells. Under the epithelium is a thin layer of connective tissue - the corium. The dorsal side of the body and tail are bordered by a skin fin fold. The body is flattened laterally; paired metapleural folds are connected along the ventral side, connecting with the under-caudal part of the fin fold. Such a structure facilitates stabilization of the position of the body in the aquatic environment and increases the area of contact with it, which is important when moving with the help of body bends.
Musculoskeletal system
The base of the support system is a chord, located along the body from the head to the end of the caudal sections. The chord that the lancelet has is a formation enclosed in a connective tissue cover, the processes of which form the supporting elements of the fin folds, which look like cartilaginous columns. It also penetrates between individual portions of muscles (myomers), forming myosepts separating them. The myomers, separated by myosepts, are adjacent to the chord on both sides.
Musculature
The muscles are striated. Individual myomers have a cone-shaped shape and are, as it were, pushed one into the other, while their arrangement is asymmetric: opposite the middle of each of them on the right side lies the myosept on the left and vice versa. Sequential contraction of muscle segments causes lateral bends of the body, moving in waves from head to tail. The regular nature of muscle contractions is regulated by the central nervous system.
The lancelet chord at the anterior end extends further than the neural tube, hence the name of the only class of this subtype is the Golovokhordovye. It is composed of a large number of transverse muscle plates surrounded by a connective tissue membrane. Separate plates are separated by cavities, but are in contact with neighboring ones using short outgrowths.
Circulatory system
It is believed that the circulatory system of this animal is closed. Indeed, there are no blood lacunae here, but there are no real capillaries (the thinnest vessels with single-layer walls): from small arterial vessels, blood enters the intercellular spaces (they functionally correspond to capillaries), and from there it collects into small veins.
Reproductive system
The reproductive system of the lancelet is represented by a series of paired reproductive glands (gonads). They are located at the walls of the atrial cavity. Cranious (lancelet including) dioecious. The gonads of males and females are externally similar and differ only in the size of germ cells (ovules are larger). Ripen germ cells through rupture of the gonad wall fall into the atrial cavity, from where they are discharged outward through the atriopore by a current of water. Development occurs in the aquatic environment and is characterized by the fact that there is a larval stage.
Larva structure
The structure of the larva is very peculiar: in the early stages, it differs in an asymmetric arrangement of the mouth (on the left side) and gill slits (on the right side), the number of which is initially small. Only later, one row of gill slits moves to the abdominal, and then to the left side, and the mouth moves down. The number of gill openings increases by breaking through additional cracks and forming longitudinal partitions in existing ones. The larva actively moves with the help of cilia covering the body, and later with the lateral bends of the body, like adult forms. She is characterized by active nutrition of small planktonic animals. A near-mouth funnel with tentacles forms only towards the end of larval development, when the larva, like an adult organism, passes to life at the bottom.
Atrial cavity
Lancelet is an animal with an atrial cavity. In it openings of gill slits and metanephrilles open. Ripe eggs and sperm come here too. All this indicates the connection of the atrial cavity with the external environment. Anatomically, this connection is represented by a hole called an atriopore, and opens outward on the ventral side in the back of the body, in front of the anus. The connection between the atriral cavity and the external environment is most clearly manifested in the nature of its formation.
In the lancelet larva, longitudinal metapleural folds form in the lateral walls of the body above the gill openings. They gradually grow in the transverse direction and down, and then grow together. The space between these folds is an atrial cavity. In other words, the atrial cavity that the lancelet has is a part of the external environment captured by metapleural folds. It covers the pharynx, part of the intestine, most of the surface of the gonads, occupying a vast space and essentially displacing the whole, which is preserved only in the form of paired cavities along the upper edge of the pharynx, as well as in the endostyle and metapleural folds.
The biological significance of the atrial cavity is associated with the nature of the lancelet lifestyle. When digging into the ground, the exit of water from the gill slots is difficult, soil particles can enter the gill holes and clog them. The formation of the atrial cavity, the walls of which separate the gill openings from contact with the ground, removes this obstacle. Water, with force coming out of the atriopore, erodes the soil in this place, providing free breathing and excretion of metabolic products. As already mentioned, the lancelet larvae leading a pelagic lifestyle have no atrial cavity. Several species of the Amphioxididae family, leading a pelagic lifestyle in adulthood, do not have it either.
These are the main features of the lancelet - one of the representatives of the chordates.