Excavating Earthworms

Anusha De 4/9/14 Vu-PAP Bio- P3

Objective

The objective of this dissection is to learn the structures and functions of earthworms, with emphasis on the digestive system.

Taxonomy

  • Kingdom: Animalia - eukaryotic, multicellular
  • Phylum: Annelida - body made up of segments separated by a septum
  • Class: Clitellata - clitellum (secretes clitella or cocoon during reproduction)
  • Subclass: Oligochaeta - setae, no lateral appendages
  • Order: Opisthopora - terrestrial worms, paired testes
  • Family: Lumbricidae - largest earthworm family, contains 33 species of earthworms
  • Genus: Lumbricus
  • Species: Terrestris

Earthworm at a Glance

Lumbricus terrestris, otherwise known as earthworms, are reddish-brown organisms with cylinder-shaped bodies that can range from 7 to 35 centimeters in length. An earthworm’s body is divided into ring-like segments called annuli. An earthworm can be made up of anywhere from 100 to 150 annuli. Annuli are covered in setae, or small bristles that allow the earthworm to move and burrow through in the soil in which it lives. The earthworm’s streamlined body shape with no limbs also enables it to move through soil. Other adaptations of the earthworm include a bioluminescent mucus, sensitivity to vibrations and light, and circular and longitudinal muscles. Subsequently, earthworms can be found in a variety of habitats, but they are rare in cold or dry places. Although they are native to Europe, earthworms are abundant in North America and western Asia. Earthworms, are also called night crawlers because they are mostly nocturnal animals. During the day, they burrow into the soil, sometimes digging as deep as 6.5 feet. They emerge above ground at night to feed on nutrients from the decomposing organic matter in the soil. Predators of the earthworm include birds, rats, snakes, beetles, and toads. Earthworms are important because they are constantly moving and burrowing through the soil, which mixes it, and fertilizing it through their waste. Humans use earthworms as compost and as bait.

Evolution

Originally, non-segmented worms reproduced by producing a new worm from their posterior ends. The new worm would then separate from the original worm. It is thought that earthworms have evolved from such worms that have failed to separate after reproduction, resulting in segments. Although the two worms failed to separate, each individual worm retained its own brain and body systems. Consequently, each segment in an earthworm is a repetition of the other segments. Each segment contains its own muscular systems, nerve center, excretory organs, coelom, and segmented blood vessels. The segments are separated from one another by a muscular membrane called the septum. However, in addition to segmentation, the earthworm also underwent the processes of cephalization and some tagmatization over the course of evolution. Cephalization caused the first few segments of the earthworm to become more tightly fused together in a definite head, and tagmatization caused certain segments in the earthworm to become capable of reproductive functions.
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Anatomy and Dissection

Refer to the following videos and images for an overview of the external and internal anatomy of an earthworm, along with a guided dissection.
Earthworm Anatomy
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Earthworm Dissection
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Delving Into Digestion: The Digestive System

The earthworm's digestive system enables it to take in soil, absorb its nutrients, and to eliminate waste products. It consists of the mouth, pharynx, esophagus, crop, gizzard, intestine, and anus. Soil is ingested by the earthworm through the mouth. Soil particles and the earthworm's lips break the food down into smaller pieces. Then, the food is lubricated by mucus secretions in the pharynx before it reaches the esophagus. The pharynx is supported by pharyngeal muscles. Calcium carbonate is added to the food in the esophagus. This happens for two reasons. First, the earthworm's body needs to release excess calcium. Second, the calcium carbonate is needed to neutralize any acids that will be formed after the food decays. Then, the calcified food mixture is stored in the crop. The gizzard breaks down the food further by adding certain enzymes from glands in its walls. Eventually, the food becomes a thick paste that is transported to the intestine. All the nutrients the earthworm needs from food are absorbed in the intestine. Bacteria and blood vessels in the intestine absorb nutrients and transport them to the rest of the body. Finally, soil particles and any undigested material are ejected from the earthworm's body by the anus. Each organ in the digestive system performs a different function:

  • mouth - takes in food (soil)
  • pharynx - lubricates food and pumps it into esophagus
  • esophagus - calcifies food and transport it to crop
  • crop - stores food
  • gizzard - breaks down food and churns it to a thick paste
  • intestine - transports food and absorbs nutrients
  • anus - ejects undigested material

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Earthworm Digestive & Circulatory Systems

Earthworm Ecology

There are almost 6,000 different species of earthworms around the world. Earthworms can be found in almost any type of soil, although they are more abundant in warm, moist habitats. In the food chain below, the earthworm would be classified as a primary consumer because it gets its energy by consuming the leaf, a producer. However, earthworms are also classified as decomposers because they are heterotrophic organisms that break down dead or decayed organic matter and recycle soil back into the environment via their waste. This process also fertilizes the soil, increasing plant productivity. Earthworms burrow into the soil, which actually makes it less likely to be scattered by wind due to the stable-crumb structure that is created. Tunneling also makes it easier for roots to anchor themselves in the soil. Common predators of the earthworm include birds, rats, snakes, beetles, and toads, and its diet is composed of organic matter in soil. Earthworms undergo a little competition with introduced species.
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Human Impact

Fun Facts

  • Earthworms are both male and female because they produce sperm and eggs.
  • Earthworms breathe through their skin; they do not have any respiratory organs.
  • The largest earthworm ever found was 22 feet long, and located in South Africa.
  • Earthworm secretions contain nitrogen.
  • Charles Darwin spent over 39 years studying earthworms.
  • Earthworms can eat up to a third of their body weight each day.
  • There can be up to 1 million earthworms in a single acre of land.
  • When a worm is cut in half, the head end will regenerate into a new worm, but the tail will now.
  • Tiny stones in an earthworm's gizzard allow it to grind food.
  • Earthworms have existed for 120 million years.