Viktor Rydberg Gymnasium

Biology B; Animal Physiology

Mikaëla Lind N3

Fall semester 1997

An organism has to be well adopted to its surroundings in order to survive. How this can be seen will be shown here, basing the facts in an example about the earthworm. After a short introduction of this animal, the physiological features of this worm, concerning the digestion and the respiration will be discussed, relating the structures of the different organs to their functions. To start with the respiration system will be dealt with and thereafter a discussion about the feeding will follow. With these two processes it is going to be shown how an animal can be built up in order to suit its environment.

The earthworm is an annelid with a round, long body being divided into several sections, so called segments. Earthworms are hermaphrodites, which means that they are both male and female. Yet they have to mate to multiply, and when mating one of the worms works as a female and the other as a male. The earthworms lack eyes, but their skin has got light sensible cells which help the worm to know when the surrounding is too light and it has to go down deeper in the ground. Its sensation is well developed, but as far as one can know it cannot hear. The earthworm lives in moist and nourishing soil, where it digs canals in order to move forwards. It lives of the nutrition in leaves and other plants, which it pulls down in its network of canals, where they decay and are eaten up buy the worm.

The worm is like all other animals in a constant need of oxygen, for the aerobic cellular respiration to go on. This process releases energy from nutrients, such as glucose. During the cellular respiration carbon dioxide is produced and the animal has to get rid of this excess. Gas exchange is a process by which organisms obtain oxygen from the surroundings and release excess carbon dioxide. The earthworm does not have any special respiratory organs, and therefore this breathing process works through the skin. The skin is very thin and it is kept moist by special cells that secrete mucus. This is why the earthworm has to live in a damp environment. If it is exposed to air, the skin will soon dry out, and the earthworm suffocate. A large number of thin blood vessels, capillaries, are to be found just below the skin. Oxygen diffuses from the air to the blood through these vessels, that is why they have to be so thin. The haemoglobin in the blood picks up the oxygen and carries it to all the cells of the body. As the blood gives up the oxygen it picks up carbon dioxide, which is transported back to the capillaries in the skin. The carbon dioxide diffuses through the skin out into the air.

Thanks to the damp soil where the earthworm lives, the respiratory system works well. If the weather is too dry, the worms have to burrow down deeper into the soil, until they reach a moist, convenient area. Too wet environment also causes problems to the earthworm, since rain floods its burrows and it has to escape to avoid drowning in the water that is deficient in oxygen.

The earthworm also has a well-developed digestive system. The animal consists of two tubes. The inner-tube is the digestion system, while the outer-tube is the body wall. The digestive-tube is also called the alimentary canal and it has got two openings, one in each end. The mouth in the frontal opening lets the food into the body and the anus at the back lets waste products out of the worm. The food goes only in one direction inside the worm, from the mouth to the anus. The food is broken down both mechanically and chemically. Mechanical breakdown means that the pieces of food are cut or crushed into smaller particles without being changed chemically. Chemical digestion occurs by help from some enzymes, which act on the surface of the particles. Since the worms digestive system is both mechanical and chemical, the procedure goes faster. The mechanical breakdown increases the surface area of the food molecules, which means that more chemical reactions can occur on the same time. The particles are broken down step by step into smaller and smaller particles until the usable nutrients are gotten and thereby absorbed into the body cells.

The earthworm ingests a lot of soil as it excavates its canals. It also eats leaves and other decaying plant matter. Inside the worm there is a kind of muscle called pharynx, which helps the worm to suck up the food, and which makes the food pass through the digestive tube since it contracts and relaxes. From the pharynx the food passes via the esophagus to the crop, a round organ with very thin walls, working as a storage room. From here the food is gradually released into the gizzard, where it is crushed. The walls of the gizzard are very thick so that the food could be grinded and broken down mechanically here. When the worm eats soil it gets a lot of sand into the body, and it helps out in the mechanical breakdown to crush the organic material. The next step is that the food mass passes into the long intestine, where the chemical digestion takes place. The wall of the intestine is folded and forms a fold called typhlosole so that the chemical digestion should be more effective. Cells lining the intestine secrete enzymes which break down large molecules into smaller ones, and thanks to the fold and thereby the larger surface area these are numerous. A larger number of chemical digestive reactions can occur on the same time. Blood vessels surround the alimentary canal, so that the nutrition can be picked up easily. The walls of these vessels have to be thin to let the nutrition through. Via the blood the food is then apportioned to the whole body. Waste products that the worm does not need, such as soil, pass out of the body through the anus, in the other end of the animal.

The earthworm is a good example of an organism being well adapted to its surroundings. It has a respiratory system depending on moisture, as the worm lives in dump soil. The thin skin and the numerous capillaries are examples of adaptation to the situation, as the worm breaths through its skin. Moreover the worm has got an advanced digestive system, which is important for an animal being forced to take in soil to move forward. The excess from the soil, that is the particles that do not contain nutrients, are being sorted out, and they even help out in the digestive process before they leave the body.