Squids
The water and oxygen enter the squid's body through the mantle. The respiratory exchange between the carbon dioxide and the oxygen occurs in the gills. The oxygen is then carried through the tissues by the blood and the carbon dioxide and the water are expelled from the body through the siphon.
Snails
The respiratory system of gastropods which is the class the contains the snails, varies greatly in form. These variations were once used as a basis for dividing the group into subclasses. The majority of marine gastropods breathe through a single gill, supplied with oxygen by a current of water through the mantle cavity. This current is U-shaped, so that it also flushes waste products away from the anus, which is located above the animal's head, and would otherwise cause a problem with fouling.
Clams
In the primitive bivalves, which is the class the clams belong to, the paired gills are small and located near the rear. The gills in all other bivalves are greatly enlarged and possess a huge surface area. While the gills are thought to serve a respiratory function, respiratory demands are low in these mostly inactive animals, and, since the body and mantle are both bathed in water, respiration probably takes place across these surfaces as well. . For most bivalves that are exposed and are wet, respiration all but ceases during the drying phase.