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Morphology and Anatomy of Earthworm

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Introduction to Earthworm

An earthworm can be defined as a segmented worm that belongs to the phylum Annelida. They are terrestrial invertebrates and are usually found in moist soil and feed on organic matter. They are also called farmer’s friends because the faecal deposit of worms helps to increase the fertility of the soil and the burrowing helps the soil to aerate properly, increasing fertility and burrowing helps in the proper aeration of the soil.

The following diagram of the earthworm depicts its morphological features:-

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Morphology of Earthworm

Earthworms possess a segmented tube-shaped body that is reddish-brown in colour. The body is precisely divided into small segments. The dorsal side holds a dark line of blood vessels whereas the ventral side comprises genital openings. The interior end is distinguished by the mouth and the prostomium, which is the organ that helps in the process of burrowing.

A glandular tissue called clitellum is present in segments 14–16 of a matured earthworm. This helps us to distinguish the mouth and the tail ends. The body is said to be distinguished into three segments, clitellum- prepatellar, clitellar, and postclitellar.

The earthworm has the capability of carrying both males as well as female sex organs.  Four pairs of spermathecal apertures are accommodated in segments 5–9. The fourteenth segment accommodates Eid the female genital pore whereas a pair of male genitals is held in the 18th segment. Except in the first, last, and clitellum segments, an S-shaped Setae is present which contributes to the locomotion in the earthworm.

Anatomy of Earthworm 

The body of the earthworm is externally covered by a thin layer of non-cellular cuticle, two muscle layers, and the coelomic epithelium, which is the inner layer, is sheathed above which lies a layer of the epidermis, and on the top lies the cuticle. The epithelium comprises glandular columnar epithelium, which is present in a single layer. 

An earthworm's body is made up of a digestive tube enclosed within a thick cylindrical muscular tube. The body is separated into segments, with furrows on the surface indicating where each segment begins and ends. The mouth is enclosed by the first segment, which features a fleshy, muscular lobe on top.

The Reproductive System of an Earthworm

Earthworms are said to be monoecious or hermaphrodite, that is both male and female reproductive organs are present in the same individual, but self-fertilisation does not occur as the male and female reproductive organs do not mature at the same time. When the testis matures earlier than the ovaries, this condition is called protandry. Hence, cross-fertilization takes place which is followed by cocoon formation. Following Listed Below are the parts of the male reproductive system:-

  • Two pairs of the testis:- One pair of the testis is held each in the 10th and 11th segments. 

  • Vasa Deferentia:- Two pairs of vasa deferentia are present. Up to the 18th segment, each side of vasa deferentia runs very close to each other. In the 18th segment, both vasa differentia of each side are joined to the prostate duct coming from the prostate gland. 

  • Two Pairs of Accessory Glands:- These are situated internally in the 17th and the 19th segments. During copulation, the secretion of these glands helps to keep the two worms together.

The Female Reproductive System Consists of :

  • One pair of Ovaries and Oviduct:- Between the 12th and 13th segment to the posterior surface of the septum is present a pair of white minute masses of the ovary. Behind the ovaries, there are two short tubes each laying respectively.

  • Four Pairs of Spermathecae:- In the 6–9th segments where the spermatozoa are stored.

Following is the diagrammatic representation of the reproductive system of an earthworm:-

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The Circulatory System of an Earthworm 

The earthworms exhibit a closed type of blood vascular system; in this type of system, the blood is confined to the heart and blood vessels.

  • Heart:- There are four pairs of tubular hearts that are present in an earthworm. The lateral hearts which are the two anterior pairs of the heart are present in the 7th and 9th segments. They connect dorsal blood vessels with ventral blood vessels. In the 12th and 13th segments, the posterior two pairs of hearts known as latero-esophageal hearts are present.

  • Blood Vessels:- Following are the kind of blood vessels found in an earthworm:-

    • Dorsal Blood Vessel:- It is said to be the largest blood vessel of the body which runs mid-dorsally above the alimentary canal. The backflow of blood is prevented by the valves present in the vessel. It has the function of receiving blood from various parts of earthworms through the different connecting vessels.

    • Ventral Blood Vessels:- This doesn't consist of any valves. The blood flows from the anterior to the poster end of the body. It has the main function of distributing blood.

  • Blood:- The blood of an earthworm is red in colour, the red colour is obtained from a protein called haemoglobin which is present in the plasma and helps in the transportation of oxygen for respiration. The blood is composed of fluid plasma and colourless blood corpuscles. The only kind of blood corpuscles, i.e., leukocytes are present in the blood of an earthworm that is phagocytic in nature, which means that it enters the body. 

  • Blood Glands:- 4th–6th segments small red-colored follicular bodies are found which are called blood glands. They are responsible for the production of haemoglobin and blood corpuscles.

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FAQs on Morphology and Anatomy of Earthworm

1. What does the nervous system of an earthworm comprise?

The activities performed by an earthworm are under the control of the well-developed and concentrated nervous system. It is segmented just like the rest of its body. Following are its components:-

  • Nerve ring: A pair of cerebral ganglia that is present in the third segment lying in the depression between the buccal cavity and pharynx. It elongates laterally and therefore gives rise to a pair of thick connectives that encircle the pharynx and meet ventrally into a pair of ganglia. This makes a complete ring around the pharynx.

  • Nerve chord: A nerve cord runs backward in the midventral line from ganglia to the posterior of the body. The ventral nerve cord bears ganglion in each segment. The segmental ganglia give off nerves to various parts of the body. The sensory information and the messages are carried from various parts of the body to the nerve cord and vice versa through these nerves.

2. Why does an earthworm's circulatory system have a closed system?

Hemolymph is not contained in vessels in open systems and instead surrounds tissues and organs. Blood is contained within vessels (dorsal, ventral, and lateral) in an earthworm's circulatory system, making it a closed system.

3. How do earthworms exchange gases?

Earthworms lack specific respiratory organs; therefore, they take in oxygen and release carbon dioxide through their skin. The earthworm's body surface allows oxygen to pass inward to the network of capillaries just under the body surface.

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