Gills - Respiratory Organs
Gills are the specialised respiratory organs of several aquatic animals. They are found in mollusis and as well in many crustaceans. Typically gills are filamentous structures richly supplied with blood. The gills found inside the body are considered to as internal gills whereas those found outside the body are referred to like external gills. Both are meant for respiration. In crustaceans, gills are generally related with appendages. Though, they may vary in form, origin and location. In crayfish, lobster, crab etc, the gills are filamentous, outgrowths from specific thoracic appendages, namely of epipodites and are enclosed in a chamber, and covered by carapace. These gills are generally ventilated by the paddle like movements of special appendages like the scaphognathite. Many aquatic insects have gills. These are usually abdominal or caudal and are called tracheal gills. These take place in nymphs of Odonata, Trichoptera and in larvae of some beetles. These gills are supplied with fine tracheae in place of blood capillaries. The tracheal gills of the nymphs of the dragonfly Aeshna lie in the rectum.