Morphology, Anatomy & Function of different systems of an Annelid, an Insect & an Amphibian
Indian Earthworms (Pheretima posthuma)
| Phylum ||: ||Annelida |
| Class ||: ||Oligochaeta |
| Genus ||: ||Pheretima |
| Species || ||Posthuma |
There are several types of earthworms. The most common genus of earthworm is Pheretima in India and Lumbricus in Europe. Pheretima has 500 species, 13 of them are found in India.
Habitat. Earthworm is terrestrial animal which inhabits moist soil where it lives inside burrows. Earthworm inhabits those soils which have abundant organic matter. An acre of good moist soil can have upto 50,000 animals. The burrows are verical or oblique. They 30-45 cm deep during moist season but may be as deep as 2m in summer. The burrows are lined by debris or mucus secreted by the animals. The burrow is wider as the base. During winter the animal drags organic debris into its burrow and plugs the mouth of the burrow. This keeps the burrow warm. Even other wise the mouth of the burrow is hidden from view by leaves and small stones. The area of the burrow can be recognised by faecal pellets called worm castings.
Habit. Earthworm is nocturnal because it is sensitive to higher light intensities. It partly creeps out of burrows during night in search of food. It is only during rainy season that the earthworm comes out of the burrow even during day time. After heavy rainfall they can be seen clawling on the ground in large number. If the burrow is left, the animal does not re-enter the same. It digs a new burrow by pushing the body through the soft soil as well as by eating its way through the soil. The worm keep its skin moist through mucus, coelomic oozings and from moisture of the soil. The animal respires through skin.
- The body of earthworm has about 100-120 segments. The first segment is called as 'buccal segment' or peristomium which bears a very small terminal opening the mouth.
- A small projection is also present which hangs over the cresent shaped mouth and is called as prostomium.
- The skin of earthworm is brown due to the presence of porphyrin pigment and protects the earthworm from UV radiations.
- In all the body segmwents, except the first, last and clitellum, there si a ring of S-shaped setae, embedded in the epidermal pit at the middle of each segment (perichaetine).
- Protoreceptors - Light receptors in the earthworm are more diffuse, they differentiate only between light and darkness. These receptors are called as Phaosome. Photoreceptors are found mostly on the prostomium and first segment, they occur in lesser number in other segments.
- Setae are chitinous structures and are not dissolved in KOH.
- In the intersegmental grooves of 5/6, 6/7, 7/8, 8/9 segments spermathecal pores are present which are the opening of spermathecae.
- A thick band like clitellum (cingulum) surrounds segments 14 to 16, forming a thick girdle, its glands secrete mucus and albumin, they also form the cocoon.
- On the ventral surface of 18th segment a pair of male genital apertures are present and on the ventral surface of 14th is a median female genital aperture is present.
- On the ventral side of each of the 17th and 19th segments circular raised pair of genital papillae which help in reproduction.
Along the mid - dorsal line in the inter segmental grooves are dorsal pores starting from 12/13 to the last but one segment, they are linked with coelom.