Vermicompost 101 — Worm Anatomy

Early drawing of Eisenia fetida (source).

Red wrigglers are complex composting machines! They’re built to live underground and eat their way through decaying material, leaving behind worm castings, otherwise known as “compost gold.”

External Anatomy

Red wrigglers are tube-shaped animals made up of up to 120 segments. The name of their phylum — Annelida — comes from the Latin phrase “little rings.” The front is easy to find as it is marked by a lighter-colored band a little ways down called the clitellum (see the reproduction discussion on the post about the worm’s lifecycle). Worms have two primary openings: a mouth in front and anus in rear, but most of their interaction with their environment happens through the skin. The outer layer — the cuticle — protects the worm. Each segment has a set of setae, which are bristles that jut out and help the worm grip the earth. Just below the cuticle is the epidermis, which contains nerves that inform the primitive brain (see below) about surrounding conditions. Below the epidermis is a layer of nerves that provide tactile information. And below all of this is a series of longitudinal and ringed muscles that allow the worm to scrunch and extend its body.

Internal Anatomy

Internal anatomy of an earthworm (source).

It’s a little evolutionary joke to say that all animals are just digesting tubes with bodies built around them, so let’s start with the digestive tract, from the point of view of a chunk of old mango. After passing through the small mouth (“buccal cavity”) at the tip of the front of the body, the small piece of food passes through the pharynx (the membrane-lined cavity behind the mouth) and down the esophagus into the crop. In the mouth, the worm detects mineral levels and edibility, seeking more edible bits. In the esophagus, glands excrete excess calcium into the food. The crop is a primary stomach that stores food and continues the digestive process. The gizzard follows, and is a muscle-lined sack that contains sand and grit swallowed by the worm. Food bits are ground up by the rocks in the gizzard before passing into the intestines, where secreted fluids digest it. Intestines are the rear two-thirds of the worm. Blood vessels on the intestinal walls absorb the nutrients into the worm’s body. Worms, like us, have a gut microbiome (organisms, fungi, bacteria, etc.) that aid them in digestion. Finally, the waste material is excreted out of the anus. Liquid waste is excreted through the skin.

Worms have a closed circulatory system with blood vessels and pumping organs. A ventral blood vessel (along the bottom) brings blood to the rear of the body. It rises up through capillaries into the dorsal blood vessel before heading back to the front of the body. The whole system is driven by five simple, parallel hearts called aortic arches. Oxygen is taken in and carbon dioxide is expelled through the skin. Worms need moist skin to allow the diffusion of gasses — they suffocate when dry and drown when under water.

Worms have a simple nervous system. Their brain, known as the cerebral ganglia, located at what humans would call the “head,” is a bundle of neurons that process moisture levels, temperature, vibrations, and light. They sense the first three through their skin and the fourth through a primitive eye, that can only detect light and shadow. The worm responds to these stimuli by digging deeper or surfacing. They must also be able to sense other worms at least to mate, but I could not find much written about this.


Uncle Jim’s Worm Farm

This is one article in a series of posts about vermicompost, or using worms in compost systems. Follow this link to see all posts in this series.

Check out these other resources:
Sustainable Table
Red Worm Composting
Composting Junkie
DIY Natural
Worm Composting HQ
City of Euless, Texas


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