Organ System Overview

7.3 Organ and Organ System

Sponges have less complicated body design but in higher organisms a complex pattern of organisation has evolved. Cells specialised in certain function and grouped together to form tissues. In organs like stomach, lung, heart and kidney, two or more tissues are organised in specific proportions and patterns. When two or more organs perform a common function by their physical and/or chemical interaction, they together form an organ system.

Each organ in our body is made of one or more type of tissues. For example, our heart consists of all the four types of tissues — epithelial, connective, muscular and neural. The complexity in organ and organ systems displays a discernable trend called the evolutionary trend — from simple to complex.

Some chordates (frog, lizard, rat, human) are open for detailed study because their organ-system organisation is fundamentally similar. NCERT introduces three model invertebrates/vertebrate trios:

• Earthworm — Pheretima (Phylum Annelida)
• Cockroach — Periplaneta (Phylum Arthropoda)
• Frog — Rana tigrina (Phylum Chordata)

In Part 2 we cover the earthworm. The cockroach is covered in Part 3 and the frog in Part 4.

7.4 Earthworm — Pheretima

Earthworms are reddish-brown terrestrial invertebrates that inhabit the upper layer of the moist soil. During day-time, they live in burrows made by boring and swallowing the soil. In the gardens, they can be traced by their faecal deposits known as worm castings. The common Indian earthworms are Pheretima and Lumbricus.

7.4.1 Morphology (External Features)

Earthworms have a long cylindrical body. The body is divided into more than a hundred short, similar segments called metameres or segments (about 100-120 in number) which are present externally as well as internally. The dorsal surface of the body is marked by a dark median mid-dorsal line (dorsal blood vessel) along the longitudinal axis of the body. The ventral surface is distinguished by the presence of genital openings (pores).

The anterior end consists of the mouth and the prostomium, a lobe which serves as a covering for the mouth and as a wedge to force open cracks in the soil into which the earthworm may crawl. The prostomium is sensory in function. The first body segment is called the peristomium (buccal segment) which contains the mouth.

In a mature earthworm, segments 14-16 are covered by a prominent dark band of glandular tissue called clitellum. Thus the body is divisible into three prominent regions — pre-clitellar, clitellar and post-clitellar segments. Four pairs of spermathecal apertures are situated on the ventrolateral sides of the inter-segmental grooves, between segments 5/6, 6/7, 7/8 and 8/9. A single pair of female genital pore is present on the ventral side of segment 14. A pair of male genital pores is located on the ventrolateral sides of segment 18.

In each body segment, except the first, last and clitellum, there are rows of S-shaped setae, embedded in the epidermal pits in the middle of each segment. Setae can be extended or retracted. Their principal role is in locomotion.

7.4.2 Anatomy — Digestive System

The body wall of earthworm is covered externally by a thin non-cellular cuticle below which is the epidermis, two muscle layers (circular and longitudinal) and an innermost coelomic epithelium. The epidermis is made up of a single layer of columnar epithelial cells, which contain secretory gland cells.

The alimentary canal is a straight tube running between the first to the last segment of the body. The mouth opens into the buccal cavity (segments 1-3) which leads into the muscular pharynx. A short, narrow, tubular oesophagus (segments 5-7) is followed by a muscular gizzard (segment 8-9). It helps in grinding the soil particles and decaying leaves, etc. The stomach extends from segments 9-14. The calciferous glands present in the stomach neutralise the humic acid present in the humus.

Food of the earthworm is the decaying leaves and organic matter mixed with soil. Intestine starts from the 15th segment onwards and continues till the last segment. A pair of short and conical intestinal caecae project from the intestine on the 26th segment. The characteristic feature of the intestine between 26-35 segments is the presence of internal median fold of dorsal wall called typhlosole. This increases the effective area of absorption in the intestine. The alimentary canal opens to the exterior by a small rounded aperture, the anus. The ingested organic-rich soil passes through the digestive tract where digestive enzymes break down complex food into smaller absorbable units.

7.4.3 Circulatory System

Earthworm has a closed type of circulatory system, consisting of blood vessels, capillaries and heart. Due to closed circulatory system, blood is confined to the heart and the blood vessels. Contractions keep blood circulating in one direction. Smaller blood vessels supply the gut, nerve cord, and the body wall. Blood glands are present on the 4th, 5th and 6th segments. They produce blood cells and haemoglobin which is dissolved in blood plasma. Blood cells are phagocytic in nature.

Earthworms lack specialised breathing devices. Respiratory exchange occurs through moist body surface into their blood stream.

7.4.4 Excretory System

The excretory organs occur as segmentally arranged coiled tubules called nephridia (sing.: nephridium). They are of three types — (i) septal nephridia, present on both sides of inter-segmental septa of segment 15 to the last, opening into intestine; (ii) integumentary nephridia, attached to lining of the body wall of segment 3 to the last, opening on the body surface; and (iii) pharyngeal nephridia, present as three paired tufts in the 4th, 5th and 6th segments. The nephridia regulate the volume and composition of the body fluids. A nephridium starts out as a funnel that collects excess fluid from coelomic chamber. The funnel connects with a tubular part that delivers the wastes through a pore to the surface in the body wall or into the digestive tube.

7.4.5 Nervous System

The nervous system of earthworm is represented by ganglia arranged segmentwise on the ventral paired nerve cord. The nerve cord in the anterior region (3rd and 4th segments) bifurcates, laterally encircling the pharynx and joins the cerebral ganglia dorsally to form a nerve ring. The cerebral ganglia along with other nerves in the ring integrate sensory input as well as command muscular responses of the body.

Sensory system: The sensory system does not have eyes, but does possess light and touch sensitive organs (receptor cells) to distinguish the light intensities and to feel the vibrations in the ground. The receptors are responsible for taste and respond to chemical stimuli. These receptors are located in the anterior part of the worm.

7.4.6 Reproductive System

Earthworm is a hermaphrodite (bisexual), i.e., testes and ovaries are present in the same individual. Two pairs of testes are present in the 10th and 11th segments. Their vasa deferentia run up to the 18th segment where they join the prostatic duct. Two pairs of accessory glands are present one each in the 17th and 19th segments. The common prostate and spermatic duct (vasa deferentia) opens to the exterior by a pair of male genital pores on the ventrolateral side of the 18th segment. Four pairs of spermathecae are located in 6th-9th segments (one pair in each segment). They receive and store spermatozoa during copulation. One pair of ovaries is attached at the inter-segmental septum of the 12th and 13th segments. Ovarian funnels are present beneath the ovaries which continue into oviduct, join together and open on the ventral side as a single median female genital pore on the 14th segment.

A mutual exchange of sperm occurs between two worms during mating. One worm has to find another worm and they mate juxtaposing opposite gonadal openings exchanging packets of sperms called spermatophores. Mature sperm and egg cells and nutritive fluid are deposited in cocoons produced by the gland cells of clitellum. Fertilisation and development occur within the cocoons which are deposited in soil. The ova (eggs) are fertilised by the sperm cells within the cocoon which then slips off the worm and is deposited in or near the soil. Embryonic development occurs within the cocoon and after about 3 weeks each cocoon produces 2-20 baby worms with an average of 4. The development of earthworms is direct, i.e., without a larva.

7.4.7 Economic Importance

Earthworms have a great economic importance because of the following:

• They make burrows in the soil and make it porous, thus aiding water and air to reach plant roots.
• Activities of the earthworms increase fertility of the soil. Hence, they are known as 'friends of farmers'.
• The process of decomposition of organic matter to produce nutrient-rich manure by earthworms is called vermicomposting.
• Earthworms are used as bait in game fishing.

7.5 Worked Examples