All representatives of the Chordata type are conditionally divided into higher and lower. The first includes the Vertebrate subtype, which is characterized by the presence of a skeleton of bones and cartilage. A typical taxon is a representative of the lower chordates, subphylum Acrania. A distinctive feature of this group is the presence of a chord at all stages of the life cycle.
The Cranial subtype includes only one class - Cephalohordata. This taxonomic group includes various types of lancelets.
Systematic position
In the direction from the highest systematic category to the lowest, non-cranial have the following position in taxonomy: empire - cellular, superkingdom - nuclear, subkingdom - true multicellular, department - three-layered, subdivision - deuterostomes; type - chordates, subtype - non-cranial.
The last taxonomic group includes the class Cephalochordidae, consisting of three families of lancelets: Branchiostomidae, Epigonichtidae and Amhpioxididae.
General characteristics of the subtype Cranial
All non-cranial are small marine animals with a fish-like body shape. The subtype includes about 35 species of lancelets. Along with the tunicates, the non-cranial are considered a very primitive group of the Chordata type.
Characteristic of the subtype Cranial includes the following features:
- preservation of the chord throughout life;
- lack of anatomical differentiation of the neural tube into the spinal cord and brain;
- primitiveness of the senses and behavior;
- lack of paired limbs;
- the presence of only one circle of blood circulation;
- colorless blood;
- breathing through gill slits and skin penetrating the throat;
- symmetrical body structure.
The last feature is characteristic only for typical representatives of the subtype Cranial - lancelets of the family Branchiostomidae. On their example, it is most convenient to consider the structure of Acrania.
Body covers
The body of the skullless is covered with skin consisting of two layers:
- single-layered epithelium (epidermis);
- corium - a thin layer of gelatinous connective tissue lying under the epidermis.
The top of the epithelium covers the cuticle - a film of mucopolysaccharides secreted by the epidermal glands. It is designed to protect the skin from possible damage in contact with the ground.
Digestive system
Foodlancelet is passive. Food particles enter the body along with a constant flow of filtered water. The amount of the latter is very significant, which provides the lancelet with a level of nutrition sufficient for its life.
The non-cranial digestive system consists of three sections:
- mouth opening;
- throat;
- relatively short intestine ending in anus.
The mouth opening of the lancelet is located in the preoral funnel, to which the tentacles forming the corolla are attached. It is surrounded by a special muscular partition called a sail. On the front side of this formation is a ciliated organ with thin, ribbon-like outgrowths, and short tentacles are turned inward, which do not let food particles that are too large.
The pharynx of the lancelet is much longer and thicker than the intestines. A groove runs along its bottom - endostyle, which is lined by two types of epithelium:
- ciliated - runs in the form of two strips of a strip extending from the anterior end of the endostyle and converging at the suprabranchial groove, simultaneously skirting the oral opening;
- glandular.
Glandular epithelium secretes mucus, which envelops food particles, causing them to move upward towards the supragillary groove. The movement of mucus in this direction is provided by the beating of the cilia of the endothelium. Having reached the gill groove, food particles are redirected back by its ciliated epithelium, thus entering the intestine. Atthe transition to this section of the pharynx narrows sharply.
At the very beginning of the intestine, a hepatic outgrowth directed forward, which produces digestive enzymes, departs from it. Food processing is carried out both inside the outgrowth itself and in the intestinal cavity throughout its entire length.
Musculoskeletal system
The role of the axial skeleton in non-cranial is performed by the notochord, which, unlike all other representatives of the Chordata type, is present at all stages of the life cycle. In the lancelet, this structure is present in the form of a special formation called the notochord. The latter is a system of striated muscle plates covered with a layer of connective tissue.
Notochord simultaneously plays the role of a muscular structure and a hydrostatic skeleton.
Nervous system
The nervous system of the non-cranial is formed by the neural tube, which lies above the chord, slightly short of its anterior end. For this reason, the only class of the subtype Cranial was named Cephalothordates.
Despite the fact that there is no external division of the neural tube into the head and dorsal sections, it can be traced functionally, since it is the anterior end that is responsible for reflex behavior.
In the dorsal part, in the amount of two pairs, the spinal and abdominal nerves depart from the tube. The latter branch in the myomere, providing the regulation of muscle contractions. The spinal nerve innervates not only the muscles, but also the skin, providing its sensory sensitivity.
Organsfeelings
The sense organ of representatives of the Cranial subtype is very simple and primitive. Thanks to them, lancelets are able to respond only to 3 types of stimuli:
- mechanical (otherwise tactile);
- chemical;
- visual.
The perception of tactile signals is possible due to the presence of nerve endings in the skin. There are also encapsulated nerve cells that pick up chemical signals. A large number of these cells are concentrated in the fossa of Kelliker.
The organs of visual perception of the lancelet are the eyes of Hesse. They are located in the neural tube and capture light penetrating through the translucent body. The main purpose of the Hesse eyes is to determine what part of the animal is in the ground. These organs consist of only two cells: photosensitive and pigment.
Circulatory system
Subtype Cranial is characterized by a closed circulatory system. This means that the blood flows exclusively inside the vessels, not pouring into the cavity.
The structure of the circulatory system resembles that of aquatic vertebrates. But, unlike the latter, the skullless do not have a heart. Its work is performed by the walls of the following vessels contracting in the rhythm of pulsation: the abdominal aorta the bases of the branchial arteries.
The abdominal aorta is located below the animal's pharynx. This vessel carries venous blood to the front of the body. Branchial arteries depart from the aorta, the number of which is equivalent to the number of gill septa (more than 100). Here the blood is enriched with oxygen and enters the paired roots.dorsal aorta. Two short vessels, the carotid arteries, depart from the latter towards the head part. They are responsible for saturating the front half of the body with blood.
Behind the passage of the pharynx into the intestines, the paired roots join into a common vessel - the dorsal aorta, which lies under the chord and stretches to the very tail. Arteries depart from this vessel, passing into the capillary network, which nourishes all parts of the body. At the end of this process, blood from the capillaries of the intestinal walls flows into the unpaired intestinal vein and moves towards the hepatic outgrowth. At this point, the branching into capillaries again occurs, thus forming the portal system of the liver.
Then the capillaries again converge into one vessel - a short hepatic vein that flows into the venous sinus. Blood from the anterior and posterior parts of the body is sent to the same reservoir, which is first collected in the corresponding cardiac veins. The latter, connecting, form the Cuvier ducts, which flow into the sinus, from which the abdominal aorta originates.
Based on the above circulatory scheme, non-cranial are characterized by only one circle of blood circulation. At the same time, their blood has no color due to the lack of respiratory pigments, the absence of which is compensated by the small size of the body and the supply of oxygen through the skin.
Organs of excretion
The excretory system of the non-cranial is represented by nephridia - short curved tubes opening into the atrial cavity. These formations are located above the pharynx in an amount of about100 pairs
The tubes of the excretory organs are almost completely located in the coelom (this cavity in the non-cranial is preserved in the form of several cavities), where decay products are filtered through the glomeruli of capillaries, which are then excreted by nephridia into the atrium cavity and are removed from the body together with water.
Reproductive system
All representatives of the subtype Cranial are dioecious animals. The development of the testes or ovaries occurs on the body wall, which is adjacent to the atrial cavity. Due to the absence of non-cranial excretory ducts in the reproductive system, the products of the gonads leave the body through gaps in the walls of the latter, from where the cells enter the atrial cavity and, together with the fluid flow, go out.
