The digestive tract of an acornworm is a straight tube that runs through the body from front to back. The mouth is located on the ventral surface at the junction of the snout and collar; the oral cavity is located on the collar, and its dorsal wall extends forward to a short blind tube to the base of the snout cavity, called the stomachord.

Amphioxus feeds passively. The mouth is located in the center of an annular velum, surrounded by velar tentacles that prevent sand from entering. The inner wall of the vestibule has finger-like protrusions made of cilia, called wheel organs, which can stir water into the mouth. The tentacles and whorls ensure sufficient water flow to carry food into the mouth, while sediment is blocked from the mouth. Water flows through the mouth and pharynx, and the food is filtered and retained in the pharynx. The water passes through the gill slits in the pharyngeal wall to the gill cavity, and then is expelled from the body through the abdominal opening. The structure of the pharyngeal cavity is similar to that of Ascidia spp., which also has internal columns, suprapharyngeal grooves, and peripharyngeal grooves. The food particles in the pharynx are adhered into agglomerates by the secretions of the inner column cells, and then moved through the peripharyngeal groove to the superior pharyngeal groove by ciliary movement, and then enter the intestine. The intestine is a straight tube, with a hollow blind sac protruding forward from the beginning of the intestinal tube and protruding into the right side of the pharynx. It is called the hepatic diverticulum. It can secrete digestive juices and may be homologous to the liver of vertebrates. organ. Food is further digested and absorbed in the intestines behind the hepatic caeca. There is an ileocolon ring in the liver caecum, where the food mass mixed with digestive juices is vigorously stirred, which can make digestion more complete.

The digestive tract includes the mouth, pharynx, esophagus, stomach, intestines and anus. The anus opens into the gill cavity. There is a mouth at the bottom of the water inlet, which is connected to the pharynx. There is a membrane composed of tentacles around the mouth. Its function is to filter out large objects and only allow water flow and tiny food to enter the digestive tube. Below the oral membrane is a wide pharynx, which occupies almost half of the body (3/4). The pharyngeal wall is penetrated by many small gill slits. The water entering the pharynx from the mouth passes through the gill slits and reaches the gill cavity, and then is discharged through the outlet hole. The inner wall of the pharyngeal cavity is covered with cilia, and there is a groove-like structure in the center of the dorsal and ventral sides, which are called dorsal lamina or epipharyngeal groove and endostyle respectively. There are glandular cells and endostyle in the groove. ciliated cells. The dorsal plate and internal columns are connected by the peripharyngeal groove at the front end of the pharynx. Glandular cells secrete mucus, which binds food entering the pharynx into food bolus. Due to the swing of the cilia of the inner column, the food bolus is pushed forward from the inner column, passed through the peripharyngeal groove, and guided backward along the dorsal plate into the esophagus, stomach and intestine for digestion. Indigestible residues are discharged into the gill cavity through the anus, and are expelled from the body through the outlet hole with water.

The lamprey's digestive organs are specialized as they adapt to a semi-parasitic life. The mouth is located deep in the oral funnel. With the funnel adsorbed on the fish body, the horny teeth on the funnel wall and tongue are used to break the fish body and suck out the blood and flesh. Horny teeth can regenerate after being damaged and lost. The tongue is located at the bottom of the mouth and is composed of circular and longitudinal muscles. It can move like a piston. There is a pair of special glands in the oral cavity, which lead to the sublingual surface with thin tubes. Their secretions can prevent the blood from coagulating in the host wound. Behind the mouth is the pharynx, which is divided into two parts, dorsal and ventral, with the esophagus on the back and the breathing tube on the ventral surface. There is a velum at the mouth of the breathing tube, which can block the mouth of the breathing tube when food enters the pharynx. There is no differentiation of the stomach, and the esophagus is connected to the intestines. The intestine is a straight tube. There are spiral mucosal folds in the intestine, which can increase the absorption area of ​​the intestine and prolong the time for food to pass through the intestine. The end of the intestine is the anus. The liver is divided into two lobes, located behind the pericardial sac. Adults do not have a gallbladder. There is no independent pancreas, only groups of pancreatic cells scattered in the intestinal wall and at the junction of the esophagus and intestine.

Digestive system includes

The innermost mucosa and its derivatives are gastrulated endoderm, and the rest are formed from mesoderm. Fish begin to appear upper and lower jaws, which are organs for feeding, attack and defense.

The structure of the oropharyngeal cavity of amphibians is relatively complex, reflecting the major differences between terrestrial animals and fish. The digestive tract and digestive glands are not essentially different from those of fish. Has a cloaca.

The oral glands of amphibians do not contain digestive enzymes and have no digestive function for food. The structure of the frog's tongue is quite special and is a specialization of its predatory organ.

Terrestrial species have well-developed oral glands, which moisten food and facilitate swallowing.

Oral glands = palatine glands, labial glands, lingual glands, sublingual glands

Reptiles have many types of teeth: lower species have terminal teeth, most lizards and snakes have lateral teeth, and crocodiles have alveolar teeth. Various teeth can be continuously renewed after falling off. Turtles and turtles have no teeth and instead have horny sheaths.

The basic structure of the digestive tract is not essentially different from that of tetrapods. The end of the large intestine opens into the cloaca. The large intestine and cloaca (and bladder) of reptiles have the function of reabsorbing water, which is of great significance for reducing water loss in the body and maintaining water and salt balance. The junction of the large and small intestines is the cecum. The cecum began to appear in reptiles and is related to the digestion of plant fibers.

The main characteristics of the digestive system of birds are: having horny beaks, the shape of which varies greatly due to different feeding habits and lifestyles. The tongues of most birds are covered with a keratinous sheath. The shape and structure of the tongues are related to their feeding habits and lifestyle: the tongues of nectar-feeding birds are sometimes straw-shaped or brush-shaped; the tongues of woodpeckers are barbed and can grasp nectar. Pests hook out from under the bark. The tongues of some woodpeckers and hummingbirds can extend far outside their mouths due to special structures, and the longest tongues can reach 2/3 of their body length.

There are salivary glands in the oral cavity, and their main secretion is mucus; only the salivary glands of grain-eating finches contain digestive enzymes. Among birds, the salivary glands of swifts are the most developed. They contain sticky glycoproteins and use saliva to bind seaweed to build nests. The nests of swiftlets are the traditional tonic "bird's nest".

In some birds, part of the esophagus is specialized into a sac, which has the function of storing and softening food. During the breeding period of female pigeons, the crop wall can secrete a liquid called "pigeon milk" to feed the young pigeons.

The stomach of birds is divided into two parts: the glandular stomach (front stomach) and the gizzard (gizzard). The glandular stomach wall is rich in glands, which secrete mucus containing hydrochloric acid and digestive juice containing pepsin; the outer wall of the gizzard is a strong muscle layer, and the inner wall is a hard leathery layer. The cavity also stores gravel that birds constantly peck at. Under the action of muscles, the leathery walls grind the food together with gravel. Grit is closely related to the digestion of seeds.

The gizzard of carnivorous birds is underdeveloped. The opening from the gizzard to the duodenum is called the pylorus. The pylorus of birds is close to the entrance of the forestomach into the gizzard, and the liquid digestion in the forestomach can directly enter the duodenum through the pylorus. The rectum of birds is extremely short, does not store feces, and has the function of absorbing water, helping to reduce water loss and load during flight. There is a pair of cecum at the junction of the small intestine and the large intestine, which is particularly developed in birds (such as chickens) that feed on plant fiber. The cecum absorbs water and digests coarse plant fibers with bacteria. Some people think that cecal fluid has a significant role in gathering vitamin B.

The anus opens into the cloaca, which still retains reptilian characteristics.

From the perspective of structure and function, the mammalian digestive system is mainly manifested in the high degree of differentiation of the digestive tube and the emergence of oral digestion, which further improves the digestive function. Associated with this is the highly developed digestive glands. From a behavioral perspective, mammals rely on various sensitive senses and powerful motor organs to actively seek food, which is beyond the reach of other animals.

Oral cavity and pharynx

Digestive tube

digestive glands

Cephalochordates have typical chordate characteristics such as a developed notochord, dorsal neural tube, pharyngeal gill slits, and post-anal tail throughout their life, which are relatively evolved among protochordates. Its notochord runs throughout the body and extends to the front of the neural tube, so it is called a cephalochordate. No real brain or head, also known as headless.

The notochord and dorsal neural tube only exist in the tail of larvae and degenerate or disappear in adults. Gill slits persist throughout life

The lamprey brain has been divided into five parts, namely the cerebrum, diencephalon, midbrain, cerebellum and oblongata. But the 5 parts are arranged on the same plane, and there is no brain curvature yet. The cerebral hemisphere is underdeveloped and the olfactory lobe is large; there are no nerve cells on the top of the brain. There are pineal gland and parapineal body at the top of the diencephalon, and the infundibulum and pituitary gland at the bottom. There is a pair of slightly larger optic lobes on the dorsal side of the midbrain, with a choroid plexus on top. The cerebellum is just a narrow transverse band.

There are 10 pairs of cranial nerves. The optic nerve does not form an optic chiasm on the ventral surface of the diencephalon. The structure of the spinal nerve is basically the same as that of amphioxus, but there are already ganglia on the dorsal roots.

nervous system = central nervous system peripheral nervous system

cartilaginous fish

bony fish

Fish have unique sensory devices developed during the evolution process to adapt to aquatic life - lateral line organs, which can judge the dynamics of water waves, the direction of water flow, the activities of surrounding organisms and fixed obstacles during swimming.

The brains of amphibians are basically similar to fish: the midbrain has well-developed optic lobes, forming a high-level center. However, the differentiation of the cerebral hemispheres of amphibians is more obvious than that of fish, and some scattered nerve cells appear on the top wall, still responsible for the sense of smell. The underdeveloped cerebellum is related to the simple way of movement.

The spinal cord is not significantly different from that of fish, but has a tendency to shorten. In addition, due to the appearance of the limbs, the spinal nerves in the shoulders and waist gather into nerve plexuses. In addition, the sympathetic and parasympathetic nerves are more developed than fish.

Reptiles have more developed brains than amphibians. Thickening of the cerebral hemispheres is prominent, but mainly the basal wall (striatum). The neocortex, on the surface of the brain, begins to assemble into layers of neural brain cells. The optic lobe of the midbrain is still a high-level center, but a few nerve fibers have reached the brain from the thalamus. This was the beginning of concentrating neural activity in the brain, which reached its peak in mammals. The acrosome in the nuchal part of the diencephalon is well developed. Reptiles have begun to have 12 pairs of cranial nerves.

The cerebral cortex of birds is underdeveloped, and the surfaces of the brain and cerebellum are relatively smooth, unlike mammals which have many wrinkles. Compared with reptiles, the nervous system of birds is much more developed than that of reptiles, such as increased brain size, well-developed cerebral hemispheres and cerebellum, degeneration of the olfactory lobe, and enlarged optic lobes formed by the midbrain, which reflects the Its vision is well developed and its movement is flexible, which is related to the flight life of birds.

The striatum of birds is particularly developed and is the center of complex instinctive activities and imitation of birds. It has been confirmed that the superior striatum, neostriatum and protostriatum of birds are homologous to the mammalian cerebral cortex and have similar functions.

Compared with other vertebrates, the nervous system of mammals is highly developed.

The cavity volume increases, the cortex thickens, the surface area expands, it has the structure of grooves and gyri, and the structure of the nerve nuclei is highly differentiated. The functions of the sensory, motor and joint systems of the cerebral cortex further move from non-specialization to specialization. develop. This is the basis for the integrated functional activities of the brain, thus ensuring the body's high adaptability to external living conditions. The left and right cerebral hemispheres are connected to each other through many nerve fibers.

Nerve fiber pathways develop with the development of the cerebral cortex and are unique to mammals.

The autonomic nervous system of mammals is very developed, and its main function is to regulate visceral activity and metabolic processes and maintain a balance in the internal environment. The main differences between the autonomic nervous system and the general spinal nerves and cranial nerves are as follows: