Structural Organisation in Animals Test 3

Segmentation—Soft and naked body of earthworm is divided into 100 to 120 similar segments, called meta-meres or somites. The body segmentation in earthworm is called 'metameric segmentation (metamerism)', due to the presence of similar body segments along the longitudinal axis of the body. The segments are without parapodia, and segments are separated from each other by distinct ling-like grooves. External segmentation corresponds with the internal segmentation of body.

The clitellum is a thickened glandular and non-segmented section of the body wall near the head in earthworms and leeches, that secretes a viscid sac in which the eggs are deposited. It is located near the anterior end of the body (around the 14th, 15th and 16th segments).

In earthworm, the excretory system is made up of many segmentally arranged microscopic coiled tubes called nephridia. These are ectodermal in origin and analogous to the vertebrate kidney.
In Pheretima, following three types of nephridia are seen.

(i) Pharyngeal nephridia - Situated on either side of pharynx and oesophagus in 4, 5 and 6 segments.

(ii) Integumentary nephridia - Scattered in the body wall, seen from 7th segment onwards.

(iii) Septal nephridia - Largest nephridia attached to both faces of each intersegment septum behind 15th segment.

An earthworm belongs to phylum Annelida. The body of an earthworm is made up of 100-120 segments. The segmented body provides various structural function, as earthworm moves with the help of them. Each segment has muscles and bristles, known as setae. Hence, option B is correct.

Hermaphrodite means to have both sexes.
Direct development means that their is no larval stage present in life cycle.
Chemoreceptors are those receptors which are sensitive to chemicals

Oesophagus in Eathworm - The pharynx leads into the oesophagus. It extends from the fifth segment to the eighth segment. Oesophagus is a short and narrow tube. In the eighth segment it is modified to form gizzard. The gizzard is an oval shaped, glandular region with thick walls of circular muscle fibers and is lined by internal cuticle. With the contraction of the muscles, the food is ground into fine particles in gizzard. Hence for this reason, gizzard is also known as the grinding mill.

A nerve cord originates from the sub-pharyngeal ganglia and runs on the mid-ventral side beneath the ventral vessel but above suhneural vessel extending up to the posterior end of the body. The nerve cord is double and is made of two longitudinal cords fused together. In each segment form 5th to the last the nerve cord has a pair of ganglia fused together as swellings. representing the segmental ganglia. The nerve cells are distributed all over the Ilene cord and are not confined to ganglionic swellings. 

There are four pairs of hearts with valves in the earthworm. Two pairs of lateral hearts, one pair in 7th and one pair in 9th segment. Two pairs of lateral oesophageal hearts in 12th and 13th segments. There are two pairs of lateral loops valves absent. One pair in 10th and one pair in 11th segment. Blood flows in upward direction.

The body cavity (coelom) of an earthworm is filled with an alkaline, colourless or milky coelomic fluid containing water, salts, some proteins and four types of coelomic corpuscles, ie, phagocytes, mucocytes, circular nucleated cells and chloragogen cells. The coelomic fluid becomes turgid and acts as a hydraulic skeleton during burrowing.

Earthworm (Pheretime posthuma) living in burrows which are made in moist earth. It makes its burrow which is made in the moist earth. it makes its burrow partly by boring with its pointed anterior end and partly sucking and swallowing the earth. The body shows metameric segmentation. About the middle of each segment, there is a ring of tiny curved bristles called setae or chaetae, formed of any nitrogenous organic substance known as chitin. The setae and musculature serve for locomotion as well as for anchoring body firmly in the burrow.

The blood of earthworm is composed of a fluid plasma and colourless corpuscles, physiologically comparable to the leucocytes of vertebrates.

The body cavity of an earthworm is true coelom (schizocoelous) as it is formed by the division of mesoderm. The coelom fluid, which contains different types of corpuscles. Thus, if the alive earthworm is prickled with a needle on its outer surface, the coelomic fluid will come out.