13.3.2 Excretion in planaria
In planaria a clearly defined excretory system exists. It consists of two or more highly branched longitudinal tubules, some side branches of the tubules end in bulb-like cells into which long flagella project. These flagella, beating constantly assume the appearance of a flame and give the system its name- flame cell system. These flagella create continuous currents within the flame cell system and the currents carry
A tuft of flagella present in the flame cell moves in a way similar to that of a flickering candle’s flame, thus given the name “flame”
Fluid and waste materials out of the body through the excretory pores present on the general body surface (fig 13.3). In planaria, ammonia and extra water are eliminated through this system. CO2 is removed through the general body surface by diffusion.
13.3.3 Excretion in earthworm
In the earthworm the excretory system is more advanced and complex than that of planaria. Excretion is carried out by nephredia which are closely associated with its closed circulatory system. Each segment of the earthworm’s body contains a pair or in some species a cluster of nephredia. The body fluids from the body cavity enter a nephridium through the nephridium.
The nephridium then continues into a coiled tubule which is closely associated with blood capillaries. This allows a selective reabsorption of materials. The nephridium ends into a large bladder, which opens to the exterior by means of a nephridiopore. (Fig. 13.4). Water, urea and CO2 are the waste products in earthworm. Most of the CO2 diffuses out in dissolved state through general body surface.
13.3.4 Excretion in cockroach
The nitrogenous wastes, inorganic salts and CO2 are the major excretory products in insects like cockroach, grasshopper etc. these unwanted and poisonous substances are removed through different organs e.g. Malpighian tubules and tracheal tubules. The CO2 is eliminated from the body by means of tracheal system. Water, salts and nitrogenous wastes, chiefly in the form of salts of uric acid, are extracted by Malpighian tubules from the haeemolymph and pass them in the lumen of the tubules. From here the extracted materials move in the solution form called urine towards the ileum.
Water and certain inorganic salts are absorbed from the urine into the proximal part of the tubules. This action of reabsorption leads to the precipitation of the uric acid in the lumen of the tubules. As the urine moves on to the hinder part of the digestive tract, more water is absorbed from the precipitated mass so that solid uric acid is finally thrown out with the faeces.
The ability to conserve water in the excretory process in insects is highly adaptive and is one of the reasons for these insects for successfully invading dryland.
Malpighian tubules are very fine yellowish thread-like processes which open into the anterior end of the ileum. They are arranged in six groups each having about fifteen blind ended tubules which float freely in the haemolymph. Some uric acid is stored in special cells of the fat body and remain their throughout the life of the cockroach.
This is excretion by storage.