Invertebrate Respiratory System:
In single celled Protists such a Protozoa diffusion across the Plasma membrane moves gases into and out of the organism. In that worms all body cells are relatively close to the body surface or are thin walled and hallow. (e.g. Hydra). Again gases diffuse into and out of the animal. Forth worms live in moist environments and have capillary network just under their integument and they exchange gases with the air spaces among soil particles. Most aquatic invertebrates carry out gas exchange with gills e.g. sea stars. Marine and annelid worms have parapodia that are richly supplied with blood vessels and function as gills.
Crustaceans and Molluscs have gills that are compact and protected with hard covering devices. Such gills divide into highly branched structures to minimize the gas exchange. Some terrestrial invertebrate like insects, centipedes and some mites, ticks and spiders have tracheal systems consisting of highly branched chitin lined tubes called tracheae. Trachea opens to the outside of the body through spiracles; most gas exchange occurs by diffusion though preumostome. Arachnids possess tracheae, book lungs or both. Book lungs are paired invaginations of ventral body wall. Air enters the book lung through a spiracle and circulates between lamellae. Respiratory gases diffuse between the hemolymph moving along the lamellae and the air in the air chamber.
Some ventilations also result from the contraction of a muscle attached to the dorsal side of a muscle attached to the dorsal side of the air chamber. This contraction dilates the chamber and opens the spiracle. Most gas exchange is still by diffusion. In the snails and slugs gas exchange organ is a pulmonate lung that opens to the outside via preumostome. Thus lung is derived from the mantle cavity. Primitive pulmonate snails are aquatic and close the preumostome during submergence. When the snail surfaces to breathe air, the preumostome opens. Most of the higher pulmonaters are terrestrial and rely on their lungs for gas exchange. The lung may be ventilated by arching and then flat.
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