All representatives of the Chordata type are conventionally 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, a subtype of Cranial (Acrania). A distinctive feature of this group is the presence of a chord at all stages of the life cycle.
Only one class is included in the Subcranial Subtype - Cephalohordata (Cephalohordata). This taxonomic group includes various species of lancelet.
Systematic position
In the direction from the highest systematic category to the lowest, inconspicuous ones have the following position in taxonomy: empire — cellular, supra-kingdom — nuclear, sub-kingdom — true multicellular, division — three-layered, sub-division — secondary-rotory; type - chordates, subtype - incoherent.
The last systematic group includes the cephalothorax class, consisting of three families of lancelettes: Branchiostomidae, Epigonichtidae and Amhpioxididae.
General characteristics of the subtype
All skullless are small marine animals with a fish-like body shape. The subtype includes about 35 species of lancelet. Along with shells, the skullless are considered a very primitive group of the Chordate type.
Characteristic of the Subcranial Subtype includes the following features:
- preservation of chords throughout life;
- lack of anatomical differentiation of the neural tube into the spinal cord and brain;
- primitiveness of the sense organs and behavior;
- lack of paired limbs;
- the presence of only one circle of blood circulation;
- colorless blood;
- breathing through gill slits and skin piercing the pharynx;
- symmetrical body structure.
The latter trait is characteristic only of typical representatives of the Subcranial Subtype, the Lancelet family of the Branchiostomidae family. Using their example, it is most convenient to consider the structure of Acrania.
Integument
The body of the skullless is covered with two-layer skin:
- single-layer epithelium (epidermis);
- Corium - a thin layer of gelatinous connective tissue lying under the epidermis.
The cuticle covers the top of the epithelium - 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
Lancelet food is passive. Food particles enter the body along with a constant current 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 digestive system of the skullless consists of three sections:
- mouth opening;
- pharynx;
- relatively short bowel ending with anus.
The mouth of the lancelet is located in the pre-oral funnel, to which the tentacle-forming corolla is attached. It is surrounded by a special muscular septum called the sail. On the front side of this formation is a ciliated organ with thin ribbon-like outgrowths, and short tentacles that do not allow too large particles of food to go inward.
The pharynx of the lancelet is much longer and thicker than the intestines. A groove passes through its bottom - the endostyle, which is lined with two types of epithelium:
- ciliated - runs in the form of two strips of strips extending from the anterior end of the endostyle and converging at the suprajugular sulcus, simultaneously passing around the mouth opening;
- glandular.
The glandular epithelium secretes mucus, which envelops the food particles, causing them to move upward towards the suprajugular sulcus. The movement of mucus in this direction is provided by the beating of the cilia of the endothelium. Having reached the gill sulcus, food particles are redirected by its ciliary epithelium back, thus entering the intestine. Upon transition to this section, the pharynx sharply narrows.
At the very beginning of the intestine, the hepatic outgrowth, which produces digestive enzymes, is directed away 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 cranial is played by a chord, which, unlike all other representatives of the Chordata type, is present at all stages of the life cycle. In a lancelet, this structure is present in the form of a special formation, which is called a 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 cranial nervous system is formed by a neural tube that lies above the chord, not reaching its anterior end. For this reason, the only class of the Subcranial subtype was called the cephalic chordates.
Despite the fact that there is no external separation of the neural tube into the head and dorsal sections, it can be traced functionally, since it is the front 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 myomer, providing regulation of muscle contractions. The spinal nerve innervates not only the muscles, but also the skin, providing its sensory sensitivity.
Sensory organs
The sense organ of the representatives of the subtype. Scabulous is very simple and primitively arranged. Thanks to them, lancelets are able to respond only to 3 types of irritants:
- mechanical (otherwise tactile);
- chemical;
- visual.
The perception of tactile signals is possible due to the presence of nerve endings in the skin. Encapsulated nerve cells that capture chemical signals are also located there. A large number of these cells are concentrated in the Kelliker fossa.
The organs of visual perception of the lancelet are the eyes of Hesse. They are located in the neural tube and trap the light penetrating through the translucent body. The main purpose of the Hessian eyes is to determine what part of the animal is in the ground. These organs consist of only two cells: photosensitive and pigmented.
Circulatory system
The Subcranial Subtype is characterized by a closed circulatory system. This means that blood flows exclusively inside the vessels, not spilling into the cavity.
The structure of the circulatory system resembles that of aquatic vertebrates. But, unlike the latter, the disembodied have no heart. His work is performed by the pulsating walls of the following vessels contracting in rhythm: abdominal aorta of the bases of the gill arteries.
The abdominal aorta is located below the pharynx of the animal. Venous blood flows through this vessel to the front of the body. Gill arteries extend from the aorta, the number of which is equivalent to the number of gill partitions (more than 100). Here, the blood is enriched with oxygen and enters the paired roots of the spinal aorta. Two short vessels depart from the latter towards the head part - the carotid arteries. They are responsible for saturating the anterior half of the body with blood.
After the pharynx passes into the intestine, the paired roots are connected into a common vessel - the spinal aorta, which runs under the chord and extends to the tail itself. Arteries departing from this vessel, passing into the capillary network, which feeds 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, branching into the capillaries again occurs, thus forming the portal system of the liver.
Then the capillaries again converge into one vessel - a short hepatic vein flowing into the venous sinus. Blood is sent to the same reservoir from the front and back of the body, which first collects in the corresponding cardiac veins. The latter, connecting, form the Cuvier ducts flowing into the sinus, from which the abdominal aorta originates.
Based on the circulatory system described above, the skulls 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 flow of oxygen through the skin.
Excretory organs
The excretory system of the cranial is represented by nephridia - short curved tubes that open holes in the atrial cavity. These formations are located above the pharynx in an amount of about 100 pairs.
The tubes of the organs of excretion are almost completely located in the coelom (this cavity in the cranium has remained intact in the form of several cavities), where decay products are filtered through the glomeruli of capillaries, which are then removed by the nephridia into the atrium cavity and are removed from the body with water.
Reproductive system
All representatives of the Subcranial Subtype are dioecious animals. The development of the testes or ovaries occurs on the wall of the body, which is adjacent to the atrial cavity. Due to the absence in the reproductive system of the cranial excretory ducts, the products of the sex glands leave the body through tears in the walls of the latter, from where the cells enter the atrial cavity and, together with the fluid flow, exit.