EXCRETION IN HIGHER TERRESTRIAL INVERTEBRATES

INTRODUCTION

• Excretory system regulates the amount of nitrogenous material, inorganic salts and water content in the blood. Both cockroach and earthworm are terrestrial in habitat their excretion is discuss in detail.
• Nephridia act as primary excretory organs in case earthworm. On the basis of where the waste is removed they can be of two types enteronephric and exonephric. Enteronephric remove their waste in alimentary canal while exonephric eliminates waste outside the body.
• In cockroach the waste is removed from the hindgut. Their excretory organs are present at the junction of midgut and hindgut. Their excretory organ is highly branched malphighian tubules. Hence it can be said that malphighian tubules act as excretory organ in case of cockroach and other insects. Other than malphighian tubules fat body cells, uricose gland and cuticles also act as excretory structure in case of cockroach.

ANNELIDS

Pheretima posthuma :- Indian earthworm

In earthworm excretion is performed by nephridia. Nephridia are segmentally arranged, microscopic, coiled tubes, they are unbranched and their inner end open into coelom by ciliated funnel called metanephridium. Except the first three segments they occur in all segments of the body.

They are distinguished into three types :-

1. Pharyngeal nephridia
2. Integument nephridia
3. Septal nephridia

PHARYNGEAL NEPHRIDIA

• They occur as paired tuft on either sides of pharynx and oesophagus in the 4^th, 5^th and 6^th segments. Each tuft consist of hundred of coiled, branched tubules without nephrostomes. In each tuft the terminal tuft of each tubules joins together to form single thick walled common duct. Thus there are three pairs of common pharyngeal nephridial ducts, which runs anteriorly parallel to the ventral nerve cord. Ducts of 4^th and 5^th segments opens into buccal chamber. Pharyngeal nephridia are thus enteronephric.

INTEGUMENT NEPHRIDIA

• On the entire inner parietal surface of body wall except the first 2 segments this nephridia are found scattered. There terminal duct opens on body surface independently through minute opening known as nephridiopores. They are thus exonephric. There are about 200-250 nephridia.

SEPTAL NEPHRIDIA

• Behind the 15^th segment they attached to both the faces of each intersegment septum these are the largest nephridia in case of earthworm
• It consist of three main parts nephrostomes,  body, and terminal duct.
• Nephrostome is ciliated funnel communicating with the coelom. It posses the epithelial pores bound by upper and lower lips. Upper lip is formed of large central cell and 8^th or 9^th marginal cells. Where as lower lip is formed of 4 or 5 compact cells. All the cells are ciliated.
• Nephrostomes leads into the main body of nephridium through the short, narrow and ciliated body like neck. Body consist of two parts, a short straight lobe and a long twisted lobe with narrow apical part. Straight lobe is one half of the twisted lobe. Twisted lobe consist of proximal and distal limb, which are spirally arranged upon each other. Proximal limb is joined to the neck.
• Distal limb of body of nephridium end in a short and narrow duct called terminal duct.

NEPHRIDIAL TUBULES

• Glandular mass traversed by a coiled tubules is present in nephridium, they posses four ciliated tracts in its course, one in neck, two in body and one in terminal duct.
• There are four parallel tubules in the straight lobe.

ARRANGEMENT

• Each septum behind the 15^th segment bears a 4 rows of septal nephridia, two on the anterior and two on the posterior face. There are about 80-100 septal nephridia hence each row consist of 20- 25 nephridia. Nephridia remains suspended freely in coelom of each segment, while their terminal duct opens freely into the pair of septal excretory canals,  these canal discharge their content dorsally just above the intestine into the pair of supra- intestinal excretory ducts situated side by side mid dorsally just above the intestine, but beneath the dorsal blood vessel  and extending from 15^th to last body segment. These ducts opens into the intestine in each segment through the narrow ductules. Septal nephridia are thus also enteronephric.

PHYSIOLOGY

• Nephridia are abundantly supplied with blood vessels. Water and nitrogenous waste is excreted by their glands. Septal nephridia also eliminate excretory material from the coelomic fluid. Through the nephridipore, integument nephridia also eliminate discharge excretory material to outer body surface. Paryngeal and septal nephridia discharge excretory material to the gut lumen from where they are eliminated with faeces.

ARTHROPODA

Periplaneta americana :- The common cockroach

Main function of excretion is performed by following structures :-

1. Malphighian tubules
2. Fat body cells
3. Uricose gland
4. Cuticle

MALPHIGIAN TUBULES

• Present on the extremely anterior of hindgut attach to the alimentary canal. This are fine, long, unbranch, yellowish and blind tubules lying freely into the haemolymph. These are 60-150 in numbers  and are arranged in 6-8 bundles. They are lined by glandular epithelium with characteristic brush border.
• They have two functional parts. Distal secretory part of glandular cell, extract nitrogenous waste which mostly in the form of salt of uric acid and water from haemolymph forming a solution called urine. Proximal absorptive part of tubules which reabsorbs certain substances such as salts, and some water result in precipitation of uric acid and also allow to pass urine. Uric acid already present in haemolymph combines with reabsorbed potassium bicarbonate and water to form relatively soluble potassium urate which again become available to be actively transported from haemolymph into the lumen of distal portion of malphighian tubules. By a gentla peristaltic move uric acid moves into ileum from malphighian tubules. More water is reabsorbed from colon and rectum.

FAT BODY CELLS

• They are present in a greater part in haemocoel. It consist of different types of cells
• For example trophocytes are present for storing reserve fat, glycogen and protein.
• Urate cells accumulates, produce and store uric acid, this mode of excretion is known as storage excretion.
• Oenocytes  are believed to produce lipoprotein that is wax for new epicuticle at moulting.
• Mycetocyes are filled with bacteroids for symbiosis intraction. It is in the opnion of many scientist that bacteroids are able to breakdown uric acid.

URICOSE GLAND

• The mushroom gland of male cockroach posses long, blind tubules at its peiphey called uricose gland or utriculi majors.  These tubules stores uric acid and discharge it over spermatophore during copulation.

CUTICLE

• With shedding of each moult deposited nitrogenous waste are eliminated.

CONCLUSION

• Earthworm and cockroach both are cosmopolitan species. Cockroach eliminates uric acid but in case of earthworm uric acid is not found. A well fed earthworm excretes predominately ammonia as nitrogenous excretory waste. Excretory waste is eliminated in the form urine in both cockroach as well as earthworm.
• Earthworm excretory fluid contains 40% urea, 20% ammonia and 40% amino acid and other nitrogenous compounds but not uric acid or urates.
• In cockroaches as a result of protein metabolism, nitrogen is produced in excess which is then excreted as uric acid. Their process of excretion is similar to the vertebrates that is first waste material is removed from the blood and then the removed material is again filtered for resorption of useful substances such as potassium bicarbonate.