Name Supplementary Text: Annelids and Arthropods Introduction to Annelids




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Supplementary Text: Annelids and Arthropods

Introduction to Annelids

Annelids are worms with segmented bodies. The phylum Annelida consists of earthworms and leaches and other organisms. The body of an annelid is divided into segments that are separated by septa, which are internal walls. Most segments are similar to one another. Some segments may be modified to perform special functions. In many annelids, bristles called setae are attached to each segment which give the annelid traction with the ground. Annelids have complex organ systems such as a nervous system, reproductive system, excretory system, circulatory system and digestive system.



Comparing the Earthworm to the Hydra

When an earthworm is compared to a hydra there are important differences in how feeding and nutrition occur because the earthworm’s cells are highly organized into a digestive system made up of specialized organs for mechanical and chemical digestion. The hydra’s cells are only at the tissue level of biological organization . In the hydra, once digested, nutrients slowly spread from cell to cell by slow diffusion while in the earthworm, nutrients are transported quickly to all cells in the body by a blood circulatory system. In the hydra food enters and leaves through the same opening (the mouth/anus) to the gastrovascular cavity. The more complex earthworms digests food in a tube called a digestive tract that contains two openings. Food enters the body through the mouth, and wastes leave through the anus. A one-way digestive tract allows there to be specialized regions that allow food to be processed more efficiently. This is important in the type of environment an earthworm lives in, the type of food of the earthworm, and its greater energy needs.

Another major difference between the hydra and the earthworm is that the hydra only uses chemical digestion in which enzymes turn on chemical reactions to break down food. This works well if the food is very small and the organism does not need as much energy but if only chemical digestion was available to the earthworm much of it’s food would pass through its body without being broken down chemically because the enzymes would not be able to penetrate into the large food particles it ingested. So in an earthworm digestion has two stages-the mechanical stage, where food is physically broken down into smaller pieces, and the chemical stage, where the food mash is broken down further by chemicals.

The evolutionary advantage of using mechanical digestion before chemical digestion, is that that once the food has been mechanically broken into smaller pieces, they have much more surface area that can be exposed to digestive enzymes, making chemical digestion more efficient.





The Earthworm Digestive System

Earthworms eat soil and receive the nutrition from the organic molecules it contains. Earthworms ingest their food by using the



muscles of their pharynx to suck it up, mix the food with saliva and swallow it. Some digestive enzymes are released into the pharynx to begin the process of chemical digestion. As the food passes through the esophagus calcium carbonate is secreted to reduce the acidity of the food before it moves into a storage sac called the crop. Then, food moves into the gizzard. In the gizzard, strong muscular contractions, along with the help of small bits of gravel grind the food into much smaller pieces. This type of digestion , where food in the gizzard is physically broken down into smaller pieces, is called mechanical digestion. More enzymes are also secreted which can now penetrate into the food which is mostly broken down to monomers at this stage. The intestine is where absorption of the nutrients from the broken down food takes place. The walls of the intestine are made of very thin membranes next to blood vessels. The nutrients diffuse through the membranes from the intestine into the blood stream to be transported to all cells of the body.



Introduction to Arthropods and the Grasshopper

Phylum Arthropoda includes animals such as crabs, spiders, and insects. Arthropods have a segmented body, a tough exoskeleton, and jointed appendages. The arthropod exoskeleton is made of a tough, water tight carbohydrate called chitin. The evolution of arthropods has led to fewer body segments and highly specialized appendages for feeding, movement and other functions. We study the grasshopper as a representative of the insects. Unlike the hydra, and earthworm who live in either an aquatic environment or very moist soil environment, the grasshopper lives in a dry open-air environment which causes a constant loss of water from it’s body. The exoskeleton of the grasshopper is waterproof, which is an adaptation to prevent water loss. The disadvantage of this, it that the grasshopper can’t breath through its skin like the worm, and has evolved a respiratory system that can enable it to breath without losing too much water. The grasshopper must get all its nutrients and water from its food.



The Digestive system of the Grasshopper

Like the earthworm, the grasshopper has a one-way digestive tract, and extracellular digestion. Similarly to the earthworm, the grasshoppers food passes through the esophagus, crop, and into the gizzard where it is chemically and mechanically broken down, however the food of the grasshopper is very different from that of the earthworm. The grasshopper eats a variety of plants with tough cell walls made of cellulose. Since the earthworm’s food of soil doesn’t vary much and has already been partially decomposed by bacteria, not as much action is needed to complete the process in the worm’s digestive tract. However more digestion is needed to break down the type of plant matter that the grasshopper ingests. More specialized structures have evolved to handle the challenge of the diverse foods a grasshopper eats.



Mandibles in the Grasshopper’s mouth are mouth parts with sharp edges that can cut the food into smaller pieces, beginning the process of mechanical digestion. Digestive enzymes are secreted by salivary glands into the mouth to also begin chemical digestion. The food then passes through the esophagus and into the crop where it is temporarily stored before moving into the gizzard. Teeth made of chitin, in the gizzard help to mechanically break down the food as the muscles contract. A one-way door between the gizzard and stomach called the pyloric valve prevents backflow of food from the stomach. Digestive glands called the digestive caeca covering the gizzard and surrounding the stomach secrete digestive juices and enzymes into the stomach so that chemical digestion can be completed. Nutrients are absorbed from the stomach into the blood. The undigested food contains a great deal of water which must be prevented from being lost. The large intestine has evolved to be able to reabsorb all the water from the waste, leaving it a dry solid.


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