Outwardly, the lancelet looks like a small fish 4 – 8 cm long and resembles a surgical instrument in shape – a lancet (hence the name of the animal).
For the first time lancelet were discovered in the Black Sea and described by the Russian scientist Peter Simon Pallas in 1774. Later, another Russian scientist Alexander Onufrievich Kovalevsky, having studied the embryonic development of the lancelet, proved its closeness to vertebrates.
Throughout its life, the lancelet retains a notochord (it plays the role of an internal skeleton).
On both sides of the chord, muscles are located in the form of two ribbons, thanks to their work, the lancelet floats and buries itself in the ground.
Nervous system and senses
Above the chord is a neural tube (numerous nerves extend from it to the internal organs and the surface of the body).
The sensory organs are light-sensitive eyes (the lancelet distinguishes light from darkness). In the thin skin there are tactile cells, thanks to which the lancelet perceives various touches.
The digestive system is located under the notochord. At the front end of the body from below there is a mouth opening, through which small organisms that serve as food for the lancelet enter the pharynx with a stream of water. The pharynx has many openings – gill slits through which water is removed. Food particles are sent to the intestines, where food is digested. Undigested residues are removed to the outside through the anus.
Circulatory and respiratory systems
The lancelet has a closed circulatory system. It is formed by two large vessels – dorsal and abdominal, as well as their thin branches and many of the finest capillaries. Capillaries also penetrate the tissues of the pharynx around the gill slits.
Gas exchange in the lancelet takes place in the intergill septa. Oxygen enters the blood through the thin walls of the capillaries, and carbon dioxide is released from the blood. Blood enriched with oxygen is carried along the dorsal vessel, and saturated with carbon dioxide – flows through the abdominal vessel to the branchial capillaries.
The lancelet has no heart. Blood moves due to contractions in the walls of the abdominal vessel.
The excretory organs of the lancelet are in many ways similar to the excretory organs of annelids (thin, convoluted tubules that open outward).
All lancelets are dioecious animals. Sexual organs: in females – ovaries, in males – testes.
Fertilization is external. The male and the female sweep the reproductive products into the water – there fertilization takes place. From fertilized eggs, larvae develop, first floating in the water column, and then sinking to the bottom, where they turn into adult lancelets.