Internal Gills

Internal Gills:

In active animals, gills have become internal to avoid the injury and attack of predators and parasites. The internal gills are located inside a cavity and usually contain a common protective flap, the operculum. The respiratory current continuously washes the gills to facilitate gaseous exchange. The three major groups of aquatic animals possessing such protected gills are mollusks, crustaceans, and fishes. Although the three groups possess gills that accomplish the same functions, there is a marked difference in their morphology.

In mollusks, the gills lie in the mantle cavity and water is circulated through them by means of cilia lining the gills and mantle cavity. In higher molluscan classes like cephalopods, water is drawn inside and expelled out of the body by the movements of the muscular walls. This mechanism has enabled efficient respiration essential for an active life.

In crayfish, lobsters, and many smaller crustaceans, gills are relatively large and lie on the lateral sides of the thoracic region. The delicate gill filaments are protected by the chitinous exoskeleton.

Among the vertebrates, fishes constitute the most dominant class inhabiting the aquatic environment. The gills are located on the edges of a series of lateral openings from the pharynx. There are often 4 to 5 openings or gill slits on each side of the pharynx. Anteriorly there may also be a pair of modified slits, the spiracles, one on each side, often specialized for water intake in bottom dwellers like rays. In sharks and rays, the gill slits open directly to the outside, whereas in most of the bony fish, several gill slits open into an external chamber on each side lateral to the pharynx. The chamber in turn is protected by the operculum.

The rate of water current flowing constantly over the internal gills depends on the oxygen needs of the body. In these, the flow of water-current and the circulation of blood is in opposite directions (counter flow principle) and thus a maximum gradient is developed between the inner and outer environments.