Useful facts on external morphology of earthworm

Shape and Size:

Earhworm has elongated body with pointed ends. It is bilaterally symmetrical. It grows upto 15cm in length, 4 mm in diameter. It is deep brown dorsally and pale ventrally.

Segmentation:

Externally the matameres are marked by circular grooves called annuli. The external segmentation corresponds to the internal segmentation of the body. This type of segmentation is called metamerism. The coelom of earthworm is partitioned by septa (internal segmentation) but the digestive tract, longitudinal blood vessel nerve cords penetrate the septa and run the length of the animal. However, four anterior segments lack internal septa.

The first segment is called peristomium. It has a crescentic opening called mouth. There is fleshy lobe over hanging the mouth called protostimium. The protostimium is olfactory in function. The last segment is called analsegment or pygidium.

Clitellum:

The clitellum is a prominent band of glandular tissue, which encircles 14th, 15th and 16th segments. The clitellum secrets the material for the formation of the egg case or cocoon. The clitellum divides the body of the worm into 3 regions. The pre-clitellar, the clitallar and the post clitellar.

Setae:

Except the first, clitellar and last segments, there is a ring of f-shaped chitinous chatae or setae, encircling the middle of each segment setae help in locomotion.

Female Genital Pore:

A single median female genital pore lies on the ventral side of the 14th segment.

Male Genital Aperture:

A pair of male genital aperture lies on the ventral side of the 18th segment.

Spermathecal opening: there are four pairs of Spermathecal openings located on the ventral side in the grooves between the segements 5/6, 6/7,7/8, and 8/9. One earthworm receives spermatozoa from other earthworm from other earthworm that these pores.

Nephridiospores:

The nephridia, the excretory organs of earthworm, open outside through minute pore or nephridiopores except the first two segments.

Dor sal pores:

These are minute openings lying on the mid-dorsal line in the grooves between the segments behind 10th segment. The first dorsal pore is located between the 10th and 11th segments. Dorsal pores are openings of coelom to the outside coelomic fluid come out through these pores to keep the body moist.