Invertebrate Adaptations To Soil Microhabitats Biology Essay

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Common earthworms, also known as Lumbricus Terrestris are about 12-30cm long and live in moist rich soil. They use peristaltic movement, via simultaneous contraction of longitudinal and circular muscles. Setas aid this locomotion. Earthworms lack definite eyes due to being nocturnal. As with many other organisms, earthworms possess a central nervous system, as well as digestive, circulatory, excretory and reproduction systems. Each was studied in detail. The main aim of this experiment was to study and understand the structure and function of each of the body systems of an earthworm.

Materials

Dissection kit

Blank A4 paper for drawings

Pencil and pencil sharpener

Methods

The external dorsal and ventral surfaces of an earthworm were drawn. The earthworm was then cut ventral surface down and dorsal surface up, then pinned into place to show the internal anatomy. After that the internal anatomy II was drawn after carefully removing the digestive system.

A Description of the Major Anatomical Components of an Earthworm

A CLITELLUM is a thickened, glandular, saddle-like region found in the epidermis of some annelids such as earthworms. It is located towards the anterior third of an organism. The clitellum is part of the reproductive system by which a cocoon is formed by the production of a viscous fluid. Eggs are stored here during sexual reproduction before they are hatched.

Blood moves towards the anterior of an animal through a DORSAL BLOOD VESSEL, which contains valves to prevent the backflow of blood. It is located above the gut. This contractile vessel is longitudinal and pumps blood into the ventral vessel by a series of aortic arches. In earthworms, oxygen is absorbed from the surrounding moist soil through their skin into the bloodstream via the dorsal blood vessel.

GENITAL SETAE are needle-like chitinous integuments.

SPERM GROOVES allow the movement of semen from one animal to the other during mating. For example in earthworms, sperm moves from one worm along the sperm groves to the seminal receptacles of the other worm in order to be stored.

SPERM DUCTS allow the passing of sperm from segments 10 and 11 to two genital openings on the bottom of segment 15 of earthworms.

FEMALE GENITAL PORES are located on segment 14, by which eggs produced in the ovaries pass out of the body in earthworms.

SEMINAL RECEPTACLES receive sperm during copulation.

The INTESTINE is the part of the alimentary canal extending from the stomach to the anus, and in humans, consists of the small and the large intestine respectively. In the small intestine, food is absorbed into the bloodstream. In the large intestine the remaining bile acids and undigested material accumulate, water returns into the body and bacterial digestion occurs. The intestine is usually folded with villus which increases the surface area to volume ratio in order to maximise possible digestion and absorption. In an earthworm, the intestine is the primary organ which is located posterior to the gizzard.

A GIZZARD is a muscular digestive organ present in some animals by which food is broken down into smaller pieces. It's a thick, muscular pouch which is used to grind food before digestion, often with the aid of ingested stones or grit. Chitinous plates are also within a gizzard in some insects and molluscs. The gizzard is located posterior to the crop in earthworms.

A CROP is a thin walled region of the oesophagus used for the storage of food before it is passed into the gizzard. In invertebrates the crop is a development of the anterior part of the gut system where food is stored or digested.

SEMINAL VESSICLES are tubular organs which secrete seminal fluid in both vertebrate and invertebrates.

SEMINAL RECEPTACLES are used to store sperm from another animal until they are required for fertilization. The two pairs of seminal receptacles are found between segments nine and ten in earthworms.

AORTIC ARCHES connect the ventral arterial system to the dorsal arterial system and pump blood throughout the body. They are arranged in six pairs of embryological vascular structures, giving rise to several major arteries.

ESOPHAGUS is an organ which is present in vertebrates. It is a muscular tube by which food, liquids, and saliva from the mouth passes from the pharynx to the stomach via peristalsis. In earthworms, the oesophagus's function is to carry food ladened soil between the pharynx and the crop.

The PHARYNX is part of the digestive tract between the mouth cavity and the oesophagus. In earthworms the pharynx draws the soil, containing nutrients into the digestive tract.

SUPRAPHARYNGEAL GANGLIA coordinate nervous activity and serves as the human equivalent of a brain.

CHLORAGOGUE CELLS are yellow in colour and line the intestine of an earthworm in order to absorb wastes from the coelom. Fat synthesis also occurs from these types of cells.

The CALCIFEROUS GLAND secretes calcium carbonate into the oesophagus to help to regulate the calcium concentration and overall pH.

The NERVE CORD is present in most vertebrates and invertebrates. In vertebrates it is comprised of the central nervous system, which develops into the brain and spinal cord. In invertebrates, the nerve cord is a hard double strand of nerve fibres throughout their body, joining with a pair of nerve ganglia at each body segment.

VENTRAL NERVE CHORD WITH SEGMENTAL GANGLION coordinates nerve impulses within the body. The ventral nerve cord travels from the anterior towards the posterior.

A VENTRAL BLOOD VESSEL leads blood towards the posterior end of an organism.

NEPHRIDIA are organs of excretion and regulation of water content in many invertebrates. It consists of coiled tubules lined with cilia which waft metabolic wastes towards the exterior of the body. Blood filtration occurs across the tubule wall due to the close organisation of the nephridium with the capillaries.

SEPTA hold an annelid's body wall in place by separating their segments.

The Gizzard of an Earthworm

Earthworms have a complex digestive system even though they do not have teeth. Instead they have a thick, muscular gizzard to serve the function of breaking down large food particles into smaller ones and the gizzard is the main region of mechanical digestion. It receives food which was stored in the crop and grinds it up with the aid of tiny stones, releasing organic matter. The septa connect the crop-gizzard to the body wall.The muscles of the gizzard have a rich blood supply, and the transverse folds are predominant. After the food is thoroughly grinded, the gizzard passes the food into the intestine to be further digested before it is absorbed. The gizzard, along with the crop, esophagus, pharynx and buccal cavity comprise the foregut.

The gizzard is of endodermal origin and its epithelium secretes chitin, which is not arranged in layers. The thickness of the gizzard cuticle varies between species. In Lumbricus terrestris, the gizzard cuticle is around 10 and 90 μm, growing back at a high rate. The cuticle of the gizzard contains a non-fibrillar elastin.

In experiments, it has been found that there are two types of gizzard muscle cells in an earthworm; slow moving muscle cells; and fast moving muscle cells. Each type can be distinguished from its homodimeric myosin isoform. The myosin isoform in slow moving muscle cells is regulated by Ca2+ ions. ATPase is high and succinic dehydrogenase is low in the main part of the gizzard's body wall compared to all the muscle cells. ATPase activity is three times lower in slow type muscle cells than in the body wall.

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