Phylum: Mollusca
The organisms that pertain in the mollusca phylum range from snails and slugs to clams and oysters to squids and octopus, all which are invertebrates. The entire phylum consists of 85,000 extant species, making mollusks the largest marine phylum, amounting to about 23% of all marine organisms. But the mollusca phylum doesn't only contain marine animals. This phylum includes both land and underwater organism, with and without shells.
Garden Snail
Garden snails do breath in oxygen but only have one lung. When they breathe in, muscles in the mantle, the outer wall of a mollusk's body, would expand and compress the lung. Oxygen enters and carbon dioxide exits both from one opening called their pneumostome, located on the right side of the their body. The snail can control the opening and closing of the pneumostome, but will keep it close to keep moisture in.
Octopus
Octopuses have two gills and three hearts which are all used to breath. Two of these hearts are close to their gills, giving them the name of gill hearts. Octopuses take in oxygen by taking in water through a cavity in their mantle. The water will provide their gills with the oxygen they need. The water will then be pushed out of their body through the siphon,a tube-like structure where water would flow. This will provide the gill hearts with oxygen, which will pump the oxygen-rich blood the the third heart, the systematic heart.
Clams
Clams use their foot to breath. Live clams have a long brownish foot made of only muscles, that will stick out of the mouth of their shell and anchor them to an object. The foot will have a pair of large thin gills on opposite sides. The gills would take in water and take the oxygen from it. It will then release the water through its rear. When clams are in sand, their mantle edge will have out two tubular siphons, one for incoming water and one for outgoing water.